Mechanical Properties Of Cast & Wrought Hybrid Disks

Superalloys 2016: Proceedings of the 13th International Symposium on Superalloys Edited by: Mark Hardy, Eric Huron, Uwe Glatzel, Brian Griffin, Beth Lewis, Cathie Rae, Venkat Seetharaman, and Sammy Tin TMS (The Minerals, Metals & Materials Society), 2016

MECHANICAL PROPERTIES OF CAST & WROUGHT HYBRID DISKS Hesser Taboada Michel1,2, Layla Sasaki Reda1,2, Georgia Effgen Santos1,2, Jonathan Cormier2, Christian Dumont1, Patrick Villechaise2, Philippe Bocher3, Damien Texier3, Eric Georges1, Florent Bridier3,*, Florence Hamon1, Alexandre Devaux1 1 Aubert & Duval, site des Ancizes, Research and Development Department, BP1, 63770 les Ancizes Cedex, France 2 Institut Pprime, UPR CNRS 3346, Physics and Mechanics of Materials Department, ISAE-ENSMA, 1 avenue Clément Ader, BP 40109, 86961 Futuroscope - Chasseneuil, France 3 Ecole de Technologie Supérieure, Department of Mechanical Engineering, 1100 Rue Notre-Dame Ouest, Montréal, QC H3C 1K3, Canada * Now at DCNS Research, Dynamique des Structures Department, CESMAN, 44620 La Montagne, France.

Keywords: Cast & WroughW superalloys, Creep, Fatigue, Tensile properties, Dual Properties Disks Hybrid disks with the so-called “dual-microstructure” have recently been manufactured using specific technologies to achieve a spatial optimization of the microstructure and resulting mechanical properties [4-16]. Most of these studies were devoted to powder metallurgy (PM) alloys, since the maximum reachable grain size is limited by the prior particles boundaries. DMHTs have resulted in an optimization of local mechanical properties considering both tensile, creep, and fatigue properties [4-6, 17]. Focus has especially been on the mechanical properties in the transition areas, especially in fatigue [5, 6]. However, up to now, very few characterizations have been reported in the open literature on the mechanical properties of cast & wrought (C&W) hybrid disks in which very coarse grain sizes can be expected in the rim sections heat treated above the ′-solvus during DMHTs [18, 19]. Moreover, to the authors’ very best knowledge, the gradient in creep properties in the transition areas of hybrid disks has never been characterized systematically. These will be two main motivations of the present article.

Abstract This paper summarizes five years of joint efforts by Aubert & Duval, Ecole de Technologie Supérieure, and ISAEENSMA/Institut Pprime in developing microstructure graded turbine disks. It is mainly focused on the characterization of the mechanical properties of cast & wrought hybrid disks made of UDIMETTM 720Li and the newly developed AD730TM alloys. In such disks, a coarse grain structure (grain size greater than 100 μm) has been introduced in the rim sections, where time dependent damage processes (creep, dwell-fatigue crack growth) are the main life limiting factors, while bore sections keep a fine grain structure (grain size between 5 to 15 μm). Smooth disks with a various diameters and height were processed and investigated. Tensile properties at room temperature, 550 °C, and 700 °C, creep properties at 700 °C/750 MPa, 770 °C/540 MPa, and 850 °C/300 MPa together with low cycle fatigue properties at 550 °C were investigated as a function of radial position. Moreover, the effect of an aging heat treatment after the dual microstructure heat treatment (DMHT) has also been investigated. From this study, it is shown that tensile and low cycle fatigue (LCF) properties, as well as creep properties at 770 °C/540 MPa and 850 °C/300 MPa, are mainly controlled by the grain size, whatever the ′ precipitation state. Moreover, a ′-subsolvus solution heat treatment is recommended after the DMHT to homogenize the intragranular microstructure through the disks and optimize tensile and LCF properties in the bore sections. Finally, creep and tensile properties in the grain size transition areas are shown to be highly dependent to the fraction of coarse grains.

Experimental Procedures Materials and disks C&W disks made of UDIMETTM 720Li (U720Li) and AD730TM were supplied by Aubert & Duval, les Ancizes, France. Their chemical compositions are given in Table I. Different disk sizes were investigated in this study: 80, 126, and 224 mm in diameter for height varying between 10 mm up to 60 mm. These disks, having an initial fine grain structure (ASTM 9-11/Grain size between 5 and 20 μm), were then heat treated using a newly developed process aimed at achieving a grain size variation along radial direction. To reach a coarse grain structure in the rim sections, disks were locally heat treated above the ′-solvus temperatures of the alloys, which are ~ 1154 °C and ~ 1110 °C for U720Li [19, 21] and AD730 TM [18, 20], respectively. Parts were rapidly cooled down after the heat treatments to achieve a fine ′ precipitation. Typical grain structures obtained in different size disks are shown in Fig. 1. The thicknesses along radial direction of the coarse grain and transition areas are presented in Table II. It has to be noted that an arbitrary 100 μm grain size criteria has been chosen to define the coarse grain area. For a given disk dimension (Fig. 1a and 1b), a wider coarse grain area, as well as a more progressive transition area for AD730 TM alloy is noted compared to U720Li, due to its larger heat treatment window (the difference between the ′-solvus temperature and the incipient melting temperature).

Introduction Optimization of the performances and/or the durability of turbine disks used in the hottest stages of aeroengines or turboshaft engines for helicopters can be achieved by adjusting locally the microstructure. Indeed, the main design criteria of gas turbine disks are the resistance to disk burst [1] and the low cycle fatigue durability in notched areas close to the bore [2], as well as creep and dwell crack growth resistance in the rim sections due to higher operating temperatures [3]. Hence, choosing a homogeneous microstructure in terms of grain size and precipitation state for a high pressure turbine disks with a large spatial variations in temperature and mechanical fields results in severe trade-offs in the design and/or components service life.

539

Table I. Chemical compositions of U720Li and AD730TM in wt % Alloy

Ni

Cr

Co

W

Mo

Al

Ti

Nb

Fe

C

Zr

B

U720Li

Base

16.0

14.5

1.25

3.0

2.5

5.0

-

-

0.025

0.03

0.02

AD730 TM

Base

16.0

8.5

2.7

3.0

2.3

3.5

1.1

4.0

< 0.02

0.03

0.01

Figure 1. Grain size variation at mid-height in 126 mm disks made of U720Li (a) and AD730 TM (b) and in a 224 mm disk made of AD730TM (c). (a) is an SEM observation in backscattered imaging mode while (b) and (c) are EBSD characterizations with the color code along the radial direction (horizontal axis here).

Disk type

Coarse grain

Transition zone

Fig. 1). Such a choice has been made since hoop stresses have a much higher magnitude compared to radial ones in the rim sections for a rotating component and also, to better investigate the mechanical properties in the transition areas.

80 mm (U720Li)

0  6.5 mm

6.5  11.5 mm

Table III. Mechanical properties investigated in each hybrid disk

Table II. Radial extent of the coarse grain and transition zone in the investigated hybrid disks (External radius used as a reference)

126 mm (U720Li)

0  6.5 mm

6.5  10 mm

126 mm (AD730 TM)

0  9.5 mm

9.5  14 mm

Disk

80 mm (U720Li)

126 mm (U720Li & AD730TM)

224 mm (AD730 TM)

0  16 mm

16  24 mm

Tension

RT

550 °C*

Creep

-

700 °C/750 MPa** 770 °C/540 MPa 850 °C/300 MPa

Samples, heat treatments and mechanical testing Table III summarizes the mechanical characterizations performed in this study on the different disks. While tensile properties were evaluated in each of the disk geometries, creep properties were taken from 126 mm disks and LCF ones from 224 mm disks. All the samples were machined along tangential direction (i.e. perpendicular to the microstructure characterizations presented in

LCF *Only for U720Li, **Only for AD730TM

540

224 mm (AD730TM) 550 °C 700 °C 550 °C

All mechanical specimens were machined from the disks at specific radial and height positions by electron discharge machining. Before turning and subsequent polishing, most of the samples were heat treated to target different mechanical properties.

45 V, using a solution made of 10% perchloric acid in methanol. Finally, /′ microstructures were observed using a Jeol JSM 7000F FEG-SEM operating at 25 kV. These characterizations were performed after selective dissolution of the ′ phase by chemical etching of the samples in a solution made of 1/3 HNO3 + 2/3 HCl (vol. parts) at 4°C. Further stereological analyzes were performed using the VisilogTM software and specific algorithms developed at Institut Pprime [19].

U720Li samples have been heat treated for 24 hours at 700 °C, followed by an air quench (AQ). This heat treatment has been chosen according to Jackson and Reed [21] and was applied for all specimens whatever their position in the disks. In the following sections of the paper, a comparison of the tensile and creep properties between the as-DMHT state and the DMHT + aged state will be performed for U720Li.

Microstructure characterizations The evolutions of the primary ′ area fraction and of the secondary (intragranular) ′ average diameter are reported Fig. 2 in a 126 mm AD730 TM disk after the 8 h/730 °C/AQ heat treatment. These characterizations were performed at different heights in the thickness of the disk, starting from the mid plane of the disk (H=0 mm) up to the surface (H = 10 mm). As expected, a decrease of the primary ′ content is observed towards the external diameter of the disk, whatever the position along the thickness (Fig. 1a). Interestingly, and in good agreement with past studies on RR1000 hybrid disks, it is observed that the grain growth (Fig. 1b and Table II) starts before the total dissolution of primary ’ particles [4, 14]. Large variations in secondary ′ size are also observed along both radial and axial directions. These variations results both from the differences in cooling rates after the DMHT in the different positions of the disk, but also from the remaining primary ʹ volume fraction at the end of the DMHT.

AD730TM samples have all been aged following two different routes after the DMHT heat treatment. One of these heat treatments consists in a single aging (8 h/730 °C/AQ), as proposed by Devaux & al. to optimize the mechanical properties of fine grain AD730TM alloy. The other one contains a ′ subsolvus solution treatment followed by the same aging, to optimize the mechanical properties of a coarse grain microstructure [22, 23]: 4 h/1080 °C/AQ + 8 h/730 °C/AQ. Creep and some tension tests specimens had a 14 mm gage length, with a 4 mm gage diameter and a 40 mm total length. LCF and most of the tension test specimens had a 13 mm gage length, a 4.37 mm gage diameter and a 56 mm total length. Tensile test specimens machined in the 80 mm disk (see Table III) had a 10 mm gage length, a 2mm gage length and a 30 mm total length. Before mechanical tests, all the samples were low stress mechanically polished up to a SiC 4000 grade for tension and creep tests specimens, while LCF specimens were polished up to a mirror finish with a 1 μm diamond spray. Final polishing was performed along the gage length.

Primary gamma Prime Area Fraction (%)

7

Creep tests were performed using a dead load creep rig equipped with a resistive furnace. They were performed at 700 °C, 770 °C, and 850 °C, with a +/- 1 °C temperature control. The initial applied stresses were 750 MPa, 540 MPa, and 300 MPa, respectively. Creep elongation were followed with LVDT following samples heads relative displacement. A 3 hours soak time at maximum temperature was applied before starting the tests. Tension and LCF tests were performed using an electromechanic Instron 8862 machine. Heating was ensured by a radiant furnace. Tests were performed at either 550 °C or 700 °C with a +/- 2 °C accuracy. Specimen’s elongation was followed by a high temperature extensometer. Tension tests were conducted at a 10-3 s-1 strain rate up to failure. LCF tests were performed under total strain control mode, triangular waveform, with a strain ratio R = 0.05, a frequency of 0.5 Hz, a total strain amplitude of t = 0.78%.

H=0mm

H=2mm

H=4mm

H=6mm

H=8mm

H=10mm

6

5

4

3

2

(a)

1

0 0

2

4

6

8

10

12

14

12

14

Radial Position (mm)

Secondary Gamma-Prime Diameter (nm)

175

In the following of the paper, all the figures showing the evolution of the mechanical properties as a function of the radial position will use the external radius as a reference. Microstructure characterizations Fracture surfaces observations of tensile, creep, and LCF specimens were performed using a Jeol 6400 scanning electron microscope (SEM) operating at 25 kV. Grain size evolution in the disks as presented in Fig. 1 was characterized by EBSD. The OIM software from EDAX attached to a Jeol 6100 SEM was used. EBSD characterization was performed after mechanical polishing and subsequent electrochemical polishing of the samples at 4°C under

H=0mm H=6mm H=8mm H=10mm

165 155 145 135 125 115 105 95

(b)

85 75 0

2

4

6

8

10

Radial Position [mm]

Figure 2. Evolution of the primary ʹ content and secondary ʹ size in a 126 mm AD730TM disk as a function of the radial position and for different profile heights in the disks.

541

The typical /′ microstructures in AD730 TM specimens after the DMHT followed by the two different heat treatments are shown in Fig. 3. These pictures have been taken close to a transition zone in the hybrid disk in an area with a ~ 80-100 μm grain size. It is observed in Figs. 3a and 3c that using a two-steps heat treatment containing a subsolvus solution treatment after DMHT, a bimodal ʹ precipitation is obtained. It is composed of coarse ʹ particles, both at grain boundaries (GBs) and in the grain, with a cuboidal morphology, and a hyperfine spheroidal precipitation whose diameter is below 25 nm, respectively. In comparison, a monomodal intragranular ʹ precipitation is observed after DMHT + 8 h/730 °C/AQ, without any ’ particles at GBs (see Figs. 3b and 3d). Their size variation has already been presented in Fig. 2. A limited ′-depleted layer close to GBs is observed (see Fig. 3d) after the 8 h/730 °C/AQ heat treatment while no such ′-free layer close to GBs is observed with the two-steps heat treatment. /′ microstructure in U720Li disk is very similar to that of AD730TM with single step heat treatment, with a slightly coarser ′ size due to the higher ʹ volume fraction of U720Li compared to AD730TM [18]. All these observations are in good agreement with past studies using these alloys [18, 19, 22-24].

ductility. The higher YS of U720Li at 550 °C in the coarse grain areas (radial position < 12.5 mm in Fig. 5a) is a result of the higher ′ volume fraction of the alloy compared to AD730 TM [18]. In the fine grain areas (radial position > 15 mm in Fig. 5a), the lower YS of U720Li hybrid disks is a result of a slightly higher grain size in the investigated disk compared to the AD730TM one. Considering the effect of heat treatment, a pronounced increase in YS at room temperature and 550 °C is observed as in UTS at 550 °C for U720Li disks in both, coarse and fine grains areas. The results obtained in the 224 mm AD730 TM disk also show that an additional ′-subsolvus solution treatment after DMHT is even better in improving the tensile properties at 550 °C and 700 °C, especially in the coarse grain areas. The increase in tensile properties at 700 °C with such a two-steps heat treatment results probably from the precipitation of coarse ′ particles at GB, allowing a better GB strength and a lower sensitivity to oxidation at this temperature [25]. 1300

1200

Stress (MPa)

1100

Coarse Grains

1000

Transition Zone

Fine Grains

900 Y.S. 0,2% Dual Treatment + 700°C/24h/AQ

800 Y.S. 0,2% Dual Treatment

700 0

2

4

6

8

10

12

14

16

18

20

Radial position (mm)

Figure 4. YS evolution at room temperature in a 80 mm disk made of U720Li as a function of the radial position. Fatigue properties Following tensile properties characterizations include LCF tests performed at 550 °C/t = 0.78%. Figure 6 shows the evolution of the fatigue lives up to failure as a function of the radial position and the cyclic behavior, respectively. Higher fatigue life is observed in coarse grain areas for the specimen heat treated with a further subsolvus solution treatment. This is in good agreement with the better tensile properties for such a heat treatment. The fatigue lives in fine grain areas is controlled by heterogeneities in the form of some “coarse” grains (size ~ 30-40 μm) or coarse primary carbides. This is the reason why no clear analysis of the role of the heat treatment can be done in this disk zone. Typical examples of crack initiation sites were observed in the fine grain area close to the transition zone (points at a radial position of 24 mm in Fig. 6) and in the coarse grain area of the sample (Fig. 7). In both cases, fatal crack initiated at sub-surface coarse grains, along twin boundaries; in good agreement with recent studies in other polycrystalline disk alloys [2, 5, 6, 14, 26].

Figure 3. ′ precipitation at different magnification in AD730TM disks after a 4 h/1080 °C/AQ + 8 h/730 °C/AQ (a, c) and 8 h/730 °C/AQ (b, d) heat treatments after DMHT. The dotted white line in (d) highlights at a grain boundary. Mechanical properties Tensile properties Figures 4 and 5 show the tensile properties of both alloys in different disks, at room temperature, 550 °C, and 700 °C. As expected from the Hall & Petch relationship, a higher yield stress (YS), defined at 0.2 plastic deformation, is observed in the fine grain areas (see Figs. 4 and 5a). The ultimate tensile stress (UTS) follows a similar trend at all temperatures and for both alloys (Fig. 5b). All the mechanical properties obtained in the present study are in good agreement with homogeneously heat treated samples [18, 22]. The ductility is observed to be greater in the fine grain areas compared to the coarse grain results (Fig. 5c). Tensile properties at 550 °C of both alloys are really similar in terms of UTS and

The cyclic behavior is characterized by macroscopically opened loops in the first 5-10 cycles, and the rest of the tests consist of an (almost) elastic behavior (see Fig. 8) up to the macroscopic crack initiation. As expected from the lower YS in the coarse grain areas, a greater plastic deformation is observed during the first LCF loops (compare Figs. 8a and 8b). The main consequence of the difference in heat treatment after DMHT is observed on the maximum stress

542

relaxation, as shown in Fig. 9. Whatever the grain size, the 4 h/1080 °C/AQ + 8 h/730 °C/AQ always induces a greater stress relaxation, which is desirable from a crack propagation point of view.

1.E+06

Nf (cycles)

Supersolvus treated area AD730/D = 224 mm Supersolvus treated area U720Li/D = 126 mm

1.E+05

8 h/730 °C/AQ

1200

(a)

1150

4 h/1080 °C/AQ + 8 h/730 °C/AQ 1.E+04

1100

0

10

20

YS0.2% (MPa)

1050

30

40

50

60

Radial Position (mm)

1000

Figure 6. Evolution of the LCF life at 550 °C/t = 0.78% in a 224 mm disk made of AD730TM as a function of the radial position.

950 900 550 °C - AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ 550 °C - AD730/D = 224 mm - 8 h/730 °C/AQ 700 °C - AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ 700 °C - AD730/D = 224 mm - 8 h/730 °C/AQ 550 °C - U720LI/D = 126mm - No aging 550 °C - U720Li/D = 126 mm - 24 h/700 °C/AQ

850 800 750 0

5

10

15

20

25

30

35

40

45

50

Radial Position (mm) 1600

(b)

1500

UTS (MPa)

1400

Figure 7. Typical LCF crack initiation sites at 550 °C/t = 0.78% in AD730 TM in the near transition (a) and coarse grain (b) areas. Aging heat treatment: 4 h/1080 °C/AQ + 8 h/730 °C/AQ.

1300

1200

1100

550 °C 550 °C 700 °C 700 °C 550 °C 550 °C -

1000

900

AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ AD730/D = 224 mm - 8 h/730 °C/AQ AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ AD730/D = 224 mm - 8 h/730 °C/AQ U720Li/D = 126mm - No aging U720Li/D = 126 mm - 24 h/700 °C/AQ

950

750

800 5

10

15

20

25

30

35

40

45

50

Stress (MPa)

0

Radial Position (mm) 30

(c)

550

350

First cycle Mid life

150

25

Strain at failure (%)

-50 0

20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-250

0.8

0.9

(a)

-450

15

Strain (%) 10

950

550 °C - AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ 550 °C - AD730/D = 224 mm - 8 h/730 °C/AQ 700 °C - AD730/D = 224 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ 700 °C - AD730/D = 224 mm - 8 h/730 °C/AQ 550 °C - U720Li/D = 126mm - No aging 550 °C - U720Li/D = 126 mm - 24 h/700 °C/AQ

5

750

550

0

5

10

15

20

25

30

35

40

45

Stress (MPa)

0 50

Radial Position (mm)

Figure 5. YS0.2% (a), UTS (b) and Ductility (c) evolutions at 550 °C and 700 °C in 126 mm and 224 mm disks made of U720Li and AD730TM (respectively) as a function of the radial position. The extent of the supersolvus heat treated area in each type of disk during the DMHT process have been added in the top of the figure.

350

First cycle Mid life

150

-50 0

0.1

0.2

0.3

0.4

0.5

-250

0.6

0.7

0.8

0.9

(b)

-450

Strain(%)

Figure 8. Example of LCF loops at 550 °C/t = 0.78% in AD730 TM in the fine (a) and coarse (b) grain areas. Aging heat treatment: 8 h/730 °C/AQ.

543

The dependence of the creep life to the local grain size is even better illustrated in Fig. 11 where a good correlation between the creep life and grain size evolutions as measured by EBSD is observed. It has to be noted that this good correlation along radial direction is obtained both in the average plane of the disk (i.e. at midthickness), but also close to its external surfaces, where variation in intragranular ′ size were observed (see Fig. 2b). Hence, the creep properties for both alloys at 850 °C/300 MPa and 770 °C/540 MPa are primarily controlled by the grain size. These results are in good agreement with former studies on dual-properties disks [8, 10, 14].

1200

1100

1000

115 MPa

107 MPa

900

800

70 MPa

R=49 mm - 8 h/730 °C/AQ R=49 mm - 4 h/1080 °C/AQ+8 h/730 °C/AQ

700

The evolution of the creep properties at 700 °C/750 MPa in a 126 mm AD730 TM disk has, however, been observed to be more scattered (see Fig. 12). If the general trend is an increase in creep life moving toward the rim sections of the disks for radial positions between 10 mm and 30 mm, a huge variation in creep life is observed in the coarse grain sections. At this temperature, and according to recent results from L. Thébaud et al. [23], the minimum creep rate and creep life has been observed to mainly depend on the secondary ′ size. A rather weak dependence to the grain size has been observed at 700 °C in this alloy [23]. This is also in rather good agreement with the creep results in PM alloys below 700 °C from Gayda & al. [10] and Mitchell & al [14]. Hence, using a 8 h/730 °C/AQ aging heat treatment after DMHT, the creep properties at 700 °C and at lower temperatures are still very sensitive to the intragranular ′ size and then, to the cooling rate after the DMHT. This is the reason why applying a ′-subsolvus solution treatment after the DMHT is recommended to limit such scatter in creep properties at moderate temperatures.

R=10 mm - 8 h/730 °C/AQ R=10 mm - 4 h/1080 °C/AQ + 8 h/730 °C/AQ

600 1

200

400

700

1500

4000

7000

15000 50000

Cycle

Figure 9. Maximum stress evolution at 550 °C/t = 0.78% in AD730TM as a function of the number of cycles for different radial positions and heat treatments. Note that X-scale in not linear and black arrows point out at the fatal crack initiation. Creep properties The creep properties under three different temperature/stress conditions are presented in Figs. 10 to 13. In what follows, only the creep life will be analyzed, but similar analyzes could be performed on the minimum strain rate and time to reach, e.g., 0.5% or 1% creep strain. A special attention has been paid to the evolution of the creep properties in the transition areas. The typical creep curves obtained at 850 °C/300 MPa in different positions along the radial axis of a 126 mm disk are shown in Fig. 10 for AD730 TM. Similar creep curves are also obtained for U720Li, both at 850 °C/300 MPa and 770 °C/540 MPa. It is observed in this figure that a significant increase in creep life and decrease in strain rate is obtained when moving from the bore to the rim sections of the disks. This increase in creep life is also accompanied by a simultaneous decrease of the creep ductility. Focusing on the “Transition Zone” specimens, a large variation in creep properties is obtained when increasing the fraction of coarse grains (this fraction is lower in Transition Zone #1 than in Transition Zones #2 and #3). To our very best knowledge, this is the first time that such creep curves according to the location in a hybrid disk appear in the open literature.

450

120

Creep results - Outer Profile 100

Creep life (h)

80

400

Creep results - Central Profile

350

Grain size

300 250

60 200 40

150 100

20 50 0

0 0

35

Creep Strain (%)

25

5

10

15

20

25

30

Radial Position (mm)

Fine Grains #1 Fine Grains #2 Transition Zone #1 Transition Zone #2 Transition Zone #3 Coarse Grains

30

Grain size (m)

Maximum Stress (MPa)

122 MPa

Figure 11. Creep life at 850 °C/300 MPa and grain size evolutions as a function of the radial position in a 126 mm disk made of AD730 TM heat treated 8 h/730 °C/AQ.

20

A direct comparison of the creep lives at 850 °C/300 MPa in 126 mm disks made of U720Li and AD730 TM is performed in Fig. 13. It is observed very similar creep lives in both alloys/disks, except in the coarse grain areas. Indeed, U720Li performs better than AD730 TM in the coarse grain areas (Radial positions lower than ~ 7-8 mm in Fig. 13) by a factor of nearly 2.5-3. This difference in creep performance in the rim sections between the two alloys leads to a creep life benefit of a factor ~ 150 over the fine grain areas in U720Li hybrid disks, while this factor is of ~ 50 for AD730TM ones. Once again, these factors are in good agreement with past studies using PM alloys [8, 10, 14]. Since the grain sizes in the rim sections of both alloys are comparable (see Fig. 1), the advantage of U720Li over AD730 TM at this very high temperature creep conditions only results from the higher ′ content of U720Li. According the

15 10 5 0 0

20

40

60

Time (Hours)

80

100

Figure 10. Creep curves at 850 °C/300 MPa at various positions in a 126 mm disk made of AD730 TM heat treated 8 h/730 °C/AQ. Note that the increase in the transition zone curves index corresponds to larger coarse grains content in the specimen.

544

Thermo-Calc simulations [18], the remaining ′ contents at 850 °C are around 40% and 30% in U720Li and AD730TM, respectively. Such a difference in ′ content directly leads to a lower Orowan stress at 850 °C for AD730TM alloy, favoring easier dislocation bypassing mechanisms and faster creep strain rate.

sections. This means that a sample machined in the transition zone is a structure in itself due to the variation in creep strength associated to the variations of grain size and intragranular ′ content within the gage diameter. Creep properties in these areas hence deserve a special attention compared to the LCF ones since under fatigue loading, the weakest point (i.e. the largest crystalline facet/the larger grain) will be the life limiting factor (see e.g. Fig. 7).

Finally, the better creep properties at 770 °C/540 MPa and 850 °C/300 MPa of U720Li specimens in the coarse grain regions using a 24 h/700 °C/AQ aging heat treatment after the DMHT have been linked to a better GB strength, due to the precipitation of secondary carbides during this additional heat treatment (not shown here).

As a first comparison, the fracture surface after creep tests at 850 °C/300 MPa of AD730TM samples in different regions of a hybrid disk are shown in Fig. 14. Creep failure is fully intergranular in the coarse grain areas and fully ductile/transgranular in the fine grain areas. Interestingly, specimens in the transition areas all exhibit a mixed fracture mode, depending on the content of coarse grain in the gage diameter. Such a local intergranular fracture mode is highlighted in Fig. 14 by the red dotted line.

300 850 °C/300 MPa External Profile 850 °C/300 Mpa Central Profile 770 °C/540 Mpa External Profile 770 °C/540 Mpa Central Profile 700 °C/750 MPa External Profile 700 °C/750 Mpa Central Profile

250

Creep life (h)

200

Further analyzes along the gage length of a U720Li creep specimen from the transition zone are shown in Fig. 15. In this specimen, a quite large fraction of coarse grain is observed on the fracture surface (nearly 40% of the surface is occupied by grains having a size greater than 100 μm, see Fig. 15a). The creep damage development, in the form of intergranular cracks, has been observed to be mainly localized on the coarse grain side of the sample (Fig. 15b) while almost no intergranular cracking has been observed on the fine grain side (Fig. 15c), even at high magnifications under SEM. Moreover, observations along longitudinal cuts clearly evidence the presence of GB cracks only on the coarse grain side of the specimen (Fig. 15d). The crack propagation along grain boundaries was observed to be assisted by oxidation at 850 °C, in good agreement with a previous study on the creep behavior of a coarse grain C&W U720Li alloy [27]. Other evidences of intergranular cracks were observed at triple points, close to the fracture surface, without any noticeable oxidation influence.

150

100

50

0 0

5

10

15

20

25

30

Radial Position (mm)

Figure 12. Creep life evolutions at 850 °C/300 MPa, 770 °C/540 MPa and 700 °C/750 MPa as a function of the radial position in a 126 mm disk made of AD730 TM heat treated 8 h/730 °C/AQ. 350 U720Li - DMHT 300

U720Li - DMHT + 24 h/700 °C/AQ

Creep life (h)

250

According to these results, it seems that the creep properties at high to very high temperature are mainly controlled by the fraction of coarse grains in the load bearing sections of the specimens. Indeed, Fig. 16 shows a good correlation between the creep life and the area fraction of coarse grains (grains having a size larger than 100 μm) measured from the fracture surface observations. Hence, assuming that the load bearing capacity of creep samples machined in the transition areas of hybrid disks is only ensured by the coarse grains contained inside the volume, the creep life can be plotted as a function of the effective stress (Fig. 17). This effective stress is calculated by dividing the applied load by the cross section occupied by the coarse grain area in each transition zone specimen. A power-law dependence is clearly observed in Fig. 17. However, a significantly different stress exponent was obtained by Chateau & Rémy at 850 °C in air [27], using supersolvus treated U720Li specimens with a homogeneous ~ 150 μm average grain size. It means that our approach which consists in neglecting the load bearing capacity of the fine grain area is too simple to be able to predict accurately the creep durability of the alloy. Indeed, despite a minimum strain rate ratio of 100 up to 1000 observed at 850 °C between the fully fine grain and fully coarse grain specimens, the contribution of fine grain areas in the transition zone specimens cannot be fully ruled out. Moreover, the impact of oxidation on the creep properties at these extreme temperatures for polycrystalline disk alloys is very sensitive to the grain size [25].

AD730 - DMHT + 8 h/730 °C/AQ

200 150 100 50 0 0

5

10

15

20

25

30

35

Radial Postion (mm)

Figure 13. Comparison of the creep life evolutions at 850 °C/300 MPa as function of the radial position in a 126 mm disks made of U720Li and AD730TM. Creep properties in the transition areas According to the previous results, a special attention has been paid to the evolution of the creep properties in the transition areas of hybrid disks. Indeed, a very careful discretization of creep specimens machining in these region has been performed, as observed in Figs. 10 to 13. The investigation of the creep damage mechanisms in these areas is a challenging issue since each specimen contains a variation in grain size inside the load bearing

545

Figure 14. Fracture surfaces of AD730 TM samples crept at 850 °C/300 MPa showing the evolution of the failure mechanisms as a function of the radial position. A good understanding of the creep performances in the transition areas of hybrid disks could only be gained by finite element modelling, using a microstructure sensitive constitutive modelling approach, taking into account, e.g. the growth of ′ particles, the activity of oxidation and the dependence of the creep ductility to the grain size. Further studies are under progress in this way, in addition to the analysis of the notch sensitivity in creep. 1000

Effective stress (MPa)

y = 11152x-0.654 R² = 0.9739

y = 614.27x-0.113 R² = 0.9684

Hybrid disk - Transition Zone Chateau & Rémy, 2010

100

Figure 15. Failure mechanisms in a U720Li sample from the transition zone crept at 850 °C/300 MPa.

1

100

1000

Creep life (Hours)

350

Figure 17. Effective stress versus creep lifetime at 850 °C in the transition zone of U720Li hybrid disks compared to results obtained using homogeneous coarse grain specimens [27].

300 250

Creep life (h)

10

200

Summary and Conclusions

150

The mechanical properties of dual-microstructure disks made of cast and wrought AD730TM and UDIMETTM 720Li alloys have been investigated. Disks with sizes in the range of small and medium power gas turbines components have been investigated. According to the present investigation, it is shown that a spatial optimization of the tensile, fatigue, and creep properties is possible with such cast and wrought alloys. Indeed, the applied DMHT process succeeded in improving the creep lifetime in the rim sections by a factor of up to 150 compared to the bore areas, maintaining very high strength and fatigue life in the bore sections.

100 50 0 0

0.2

0.4

0.6

0.8

1

1.2

Fraction of grains > 100 μm

Figure 16. Evolution of the creep life at 850 °C/300 MPa in the transition zone of a U720Li disk as a function of the fraction of coarse grains (grains with a size greater than 100 μm).

546

According to this investigation, it is shown that a post-DMHT heat treatment is necessary to even improve mechanical properties (both tensile and creep, whatever the location in the disks). It is also recommended that a post-DMHT ′ subsolvus solution treatment heat treatment is used to increase the grain boundary ductility during high temperature creep and to homogenize the secondary ′ size as such treatment improved significantly the tensile strength in the bore sections of the disks.

9. J. Gayda, T.P. Gabb, and P.T. Kantzos, Heat treatment devices and method of operation thereof to produce dual microstructure superalloy disks. 2003, US Patent US006660110B1: USA.

Finally, the evolution of the creep properties in the transition zones has been investigated extensively. It is shown that the creep lifetime in such locations is controlled by the damage nucleation in the coarse grain areas of the specimens. Moreover, the creep life correlates well with the fraction of coarse grains in the load bearing sections. However, the prediction of the creep properties in these transition zones requires a more sophisticated approach than the simple consideration that the creep strength is only ensured by the coarse grain portion of the specimens.

11. J.M. Hyzak, C.A. Macintyre, and D.V. Sundberg. "Process development and microstructure and mechanical property evaluation of a dual microstructure heat treated advanced nickel disc alloy", (Paper presented at Superalloys 1988, Seven Springs, Champion, PA, USA, 1988), 121-130.

10. J. Gayda, T.P. Gabb, and P.T. Kantzos. "The effect of dual microstructure heat treatment on an advanced nickel-base disk alloy", (Paper presented at Superalloys 2004, Seven Springs, Champion, PA, USA, 2004), 323-329.

12. J. Lemsky, "Assessment of NASA Dual Microstructure Heat Treatment Method Utilizing Ladish SuperCooler™ Cooling Technology" (Report CR—2005-213574, NASA, 2005).

Acknowledgments

13. G.F. Mathey, Method of making superalloy turbine disks having graded coarse and fine grains. 1994, US Patent US005312497A: USA.

P.B. and F.B. would like to acknowledge the support from the MDEIE PSR-SIIRI-676 grant provided by Québec government. L. Jouvanneau is gratefully acknowledged for the careful machining of the specimens.

14. R. Mitchell et al. "Process development and microstructure and mechanical property evaluation of a dual microstructure heat treated advanced nickel disc alloy", (Paper presented at Superalloys 2008, Seven Springs, Champion, PA, USA, 2008), 347-356.

References

15. D.P. Mourer and J.L. Williams. "Dual heat treat process development for advanced disk applications", (Paper presented at Superalloys 2004, Seven Springs, Champion, PA, USA, 2004), 401-407.

1. M. Mazière et al., "Overspeed burst of elastoviscoplastic rotating disks - Part I: Analytical and numerical stability analyses", European Journal of Mechanics A/Solids, 28, (2009), 36-44. 2. B. Larrouy et al., "Grain boundary - Slip bands interactions: impact on the fatigue crack initiation in a polycrystalline forged Nibased superalloy", Acta Materialia, 99, (2015), 325-336.

16. Y. Ning et al., "Structural-gradient-materials produced by gradient temperature heat treatment for dual-property turbine disc", Journal of Alloys and Compounds, 557, (2013), 27-33.

3. J. Telesman et al. "Effect of Microstructure on Time Dependent Fatigue Crack Growth Behavior In a P/M Turbine Disk Alloy", (Paper presented at Superalloys 2008, Seven Springs, Champion, PA, USA, 2008), 807-816.

17. E. Galindo-Nava, L. Connor, and C. Rae, "On the prediction of the yield stress of unimodal and multimodal γ′ Nickel-base superalloys", Acta Materialia, 98, (2015), 377-390.

4. L.D. Connor, " The development of a dual microstructure heat treated Ni-base superalloy for turbine disc applications", (Ph.D. thesis, University of Cambridge, 2009).

18. A. Devaux et al. "Effect of aging of heat-treatment on mechanical properties of AD730TM superalloy", (Paper presented at 8th International Symposium on Superalloy 718 and Derivatives, Pittsburgh, PA, USA, 2014), 521-535.

5. T. Gabb et al., "Fatigue resistance of the grain size transition zone in a dual microstructure superalloy disk", International Journal of Fatigue, 33, (2011), 414-426.

19. J.R. Vaunois et al. "Influence of both ’ distribution and grain size on the tensile properties of UDIMET 720Li at room temperature", (Paper presented at 7th International Symposium on Superalloy 718 and Derivatives, Pittsburgh, PA, USA, 2010), 199213.

6. T.P. Gabb et al. "Fatigue failures modes of the grain size transition zone in a dual microstructure disk", (Paper presented at Superalloys 2012, Seven Springs, Champion, PA, USA, 2012), 6372.

20. F. Masoumi et al., "Coarsening and Dissolution of γ′ precipitates during solution treatment of AD730TM Ni-based Superalloy: Mechanisms and Kinetics Models", Journal of Alloys and Compounds, 658, (2016), 981-995.

7. S. Ganesh and R.G. Tolbert, Differentially heat treated article, and apparatus and process for the manufacture thereof. 1996, US Patent US005527020A: USA.

21. M.P. Jackson and R.C. Reed, "Heat treatment of UDIMET 720Li: the effect of microstructure on properties", Material Science and Engineering A, 259, (1999), 85-97.

8. J. Gayda and D. Furrer, "Dual-Micro Heat Treatment", Advanced Materials & Processes, July 2003, (2003), 36-39.

547

22. A. Devaux et al. "Mechanical properties and development of supersolvus heat treated new nickel base superalloy AD730TM", (Paper presented at Eurosuperalloys 2014, Presqu'île de Giens, France, 2014), 01004. 23. L. Thébaud et al., "Relationships between Microstructural Parameters and Time-Dependent Mechanical Properties of a New Nickel-Based Superalloy AD730™", Metals, 5 , (2015), 22362251. 24. A. Devaux et al. "AD730 TM - A new nickel-based superalloy for high temperature engine rotative parts", (Paper presented at Superalloys 2012, Seven Springs, Champion, PA, USA, 2012), 911-919. 25. L. Thébaud et al. "Relationships between microstructural paramaters and time-dependent mechanical properties of a new nickel based superalloy AD730TM", (Paper presented at Superalloys 2016, Seven Springs, Champion, PA, USA, 2016). 26. J. Stinville et al., "A combined grain scale elastic–plastic criterion for identification of fatigue crack initiation sites in a twin containing polycrystalline nickel-base superalloy", Acta Materialia, 103, (2016), 461-473. 27. E. Chateau and L. Rémy, "Oxidation - assisted creep damage in wrought nickel-based superalloy: experiments and modelling", Materials Science and Engineering A, 527, (2010), 1655-1664.

548