Materials Transactions, Vol. 47, No. 5 (2006) pp. 1322 to 1327 #2006 The Japan Institute of Metals
The Effect of Forging on Microstructure and Mechanical Properties of In Situ TiC/Ti Composite Feng-cang Ma, Wei-jie Lu, Ji-ning Qin and Di Zhang* State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, P. R. China In this paper, 1 vol% TiC/Ti-1100 composite was prepared, and the transus temperature of the composite was measured using metallographic techniques. The composite was forged at 1173, 1333 and 1423 K, respectively. Microstructure of forged samples was studied by optical microscopy (OM) and transmission electron microscopy (TEM). The forging in the upper phase field was investigated especially. Mechanical properties of the composite after hot forging were tested by tensile tests at ambient temperature. It was found that there is an about 100 K increase in transus temperature compared to monolithic Ti-1100 alloy. Widmanstatten microstructure, bimodal microstructure and microstructure containing aligned plate were obtained after forging at various temperatures. Compared to Widmanstatten microstructure or bimodal microstructure, there is an increase about 200 MPa in ultimate tensile strength for the microstructure containing aligned plate. The increase in tensile strength for the microstructure containing aligned plate may result from two factors: one is the work hardening during the forging process, and the other is the interaction between = interface and dislocations in phase during tensile process. [doi:10.2320/matertrans.47.1322] (Received September 8, 2005; Accepted March 3, 2006; Published May 15, 2006) Keywords: in situ titanium matrix composite, forging, microstructure, mechanical properties
1.
Introduction
Ti-1100 is a near Ti alloy. Because of their superior microstructure stability, toughness and excellent specific strength, near Ti alloys and its composites (TMCs) are used very popular for structural applications at high temperature. Near Ti alloys offer considerable scope for obtaining a variety of microstructures by thermomechanical processing (TMP).1) Alloy specific studies have demonstrated significant benefits of altering the thermomechanical processing sequence to obtain specific microstructural goals. In near Ti alloys, microstructures after thermomechanical processing range from transformed (acicular ) type with good creep resistance to equiaxial þ phase with excellent low cycle fatigue properties. Earlier studies2,3) have indicated that the creep properties in these alloys are largely dependent upon the microstructure. In bimodal microstructure containing equiaxial grains and Widmanstatten ( þ ) lath grains, Bania,4) Seagle5) and Thiehsen6) have reported that both primary and secondary creep rates increase with the amount of equiaxial phase in the microstructure. On the other hand, the Widmanstatten microstructure, containing multiple colonies of and laths within a prior grain, is more creep resistant than the bimodal microstructure.7) Higher cooling rates usually result in finer lath spacing, and improved creep properties.2,3,7) Improvement in mechanical properties in finer lath structures has been attributed to shorter slip lengths within the phase.2,8,9) For fracture critical applications in aircraft structures, a beta-processed microstructure was developed to optimize fracture toughness and fatigue crack growth resistance.10–12) Generally speaking, near Ti alloys and its composites are usually forged in the phase field or in the ( þ ) phase field. But the reports about microstructure and mechanical properties of Ti alloys and its composites after thermomechanical processing in the upper phase field are deficient. *Corresponding
author, E-mail:
[email protected]
In this paper, in situ Ti-1100 composite reinforced with 1 vol% (TiC) was prepared and forged at 1173, 1333 and 1423 K, respectively. Microstructure of samples after forging was observed by optical microscopy (OM) and transmission electron microscopy (TEM). The mechanical properties of samples after forging were tested by tensile tests at ambient temperature. 2.
Experimental Procedure
For preparing TiC/Ti-1100 TMCs, the raw materials were grade I sponge titanium, graphite powder (99.8%, average particle size, 5–7 mm), aluminum thread (98%), crystal silicon (99.5%), sponge zirconium (98.8%) and master alloys of other alloying elements such as Ti–Sn, Al–Mo. The nominal alloy composition of Ti-1100 was Ti–6Al–2.75Sn– 4Zr–0.4Mo–0.45Si and the amount of graphite powder added in the present material was 0.25 mass%. Firstly, the stoichiometric amounts of sponge titanium, graphite powders, Al thread, sponge Zr, and intermetallic compounds of Ti-Sn and Al-Mo were blended thoroughly, and then they were compacted into pellets. The pellets were melted homogeneously in a consumable vacuum electrode arc remelting (VAR) furnace. In order to ensure the chemical homogeneity of the composite, the ingots were melted at least three times. The reinforcements were synthesized utilized the reaction between Ti and C as following reaction: Ti þ C ! TiC
ð1Þ
The alloying element contents of the material in cast are measured by chemical method, and the results are presented in Table 1.
Table 1
Contents of alloying elements in the composite (as-cast).
Element
Al
Sn
Zr
Mo
Si
C
O
Content (mass%)
6.13
2.72
4.02
0.41
0.46
0.23
0.08
The Effect of Forging on Microstructure and Mechanical Properties of In Situ TiC/Ti Composite
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Fig. 1 Bright field TEM images of TiC (in (a)) and selected area diffraction patterns (in (b)).
The t temperature (the temperature þ ! ) of the material was measured by metallographic method. The samples used in this test were a cube with a gauge size of 10 mm. The samples were heated in a box furnace with a temperature accuracy of 2 K, after hold for 30 min at a temperature, and then quenched into water. In the final tests to measure the t temperature, the tests were performed with a temperature interval of 10 K. After casting, the ingots were machined to remove the defects on surface. During hot forging process, firstly, the ingots were heated in a box furnace with resistance heater and hold temperature for 1.5 h. The forging was performed using a hydraulic pressure machine with the molds heated to 1073 K, and the temperature of samples during forging was measured using infrared ray techniques. The total reduction in cross section in the forging process was 4 (The total reduction in cross section is A0 =Ai , where A0 is the cross sectional area before hot forging and Ai is the cross sectional area after hot forging). The samples after forging were quenched into water. The tensile samples were a plate 1.5 mm in thickness and 40 mm in length. They were machined from hot-forging rods with the specimen axis parallel to the hot-forging direction. Three specimens were tested at ambient temperature using an MTS-810 servohydraulic test machine equipped with extensometer. The average strain rate was 1:0 103 s1 . The data during tensile tests were collected by a computer system, which was linked to the test machine. Samples for optical microscopy (OM) were cut from hotforging rods. Then the samples were prepared using conventional techniques of grinding and mechanical polishing. The samples were etched in Kroll’s reagent (composition: 1–3 ml HF, 2–6 ml HNO3 , 100 ml water). The microstructure of samples was characterized by a LECO2000 optical microscopy. Thin foils for transmission electron microscopy (TEM) were prepared by twin-jet electro-polishing technique using an electrolyte composed of 10 pct HclO4 and 90 pct CH3 COOH at temperature 293–303 K and a potential of
30–50 V. The foils were examined using a JEM-2010 electron microscope operating at 200 kV. 3.
Results and Discussion
3.1 phase identification Because the volume fraction of TiC in composite was so small that it cannot be identified by normal XRD method. The phase identification was utilized TEM equipped with energy dispersion X-ray spectronmeter (EDS). The results of EDS show that TiC particles have formed in the composite. The crystal structure of TiC is the NaCl type structure. The bright TEM image of TiC and selected area diffraction patterns are presented in Fig. 1. The volume fraction of TiC particles of the composite prepared in this experiment is measured to be 0.98% using metallographic techniques. 3.2 the transus temperature of the composite Because microstructure evolution is quite different for Ti alloys and its composites when deformation is performed whether in the ( þ ) phase field or in the phase field, the ð þ Þ ! temperature is a very important parameter for the thermomechanical processing of Ti alloys and its composites. The transus temperature of the composite was measured using metallographic techniques, and the results are presented in Fig. 2. From Fig. 2, the transus temperature of the present composite was identified as 1388 K. There is an increase about 100 K in transus temperature compared to monolithic Ti-1100 (the transus temperature is about 1288 K, which was also measured by these authors in this work using same method). To interpret the obtained results, it is helpful to examine the Ti-Al-C ternary alloy phase diagram. The present matrix alloys contain stabilizers Al, Sn, Zr, and O, the effect of which can be represented in terms of an Al equivalent content. According to Rosenberg,13) the equivalent of Al content can be expressed as ½Aleq ¼ [Al] þ [sn]/3 þ [Zr]/6 þ 10[O]. From Table 1, the present matrix alloys [Al]eq is calculated to be 8.37. Thus we can appropriately
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F. Ma, W. Lu, J. Qin and D. Zhang
Fig. 2
1400
Temperature(T/K)
1300
Optical micrographs of sample after quenched (a) at 1383 K, (b) at 1393 K.
β+Carbide α+β+Carbide
β
0.28%C
α+β
1200 α
1100 1000 900
AlTi3+α
α+Carbide
Composition of the composite
800
0.00 C Weight percentage 1.01 C 8.02 Al 8.0 Al 91.98 Ti 90.99 Ti Fig. 3
Vertical section of Ti-8Al-C equilibrium alloy phase diagram.
simplify discussion of the ð þ Þ= transus temperature of the matrix alloys under study and refer to the Ti-8Al-C phase system. A vertical section of the Ti-8Al-C alloy phase diagram14) is shown in Fig. 3, and the present composition of the material in this experiment is presented in Fig. 3. According to this vertical section, the addition of carbon increase the ð þ Þ= transus temperature rapidly when the C concentration of matrix alloy is blow 0.28 wt%, and the transus temperature measured in this experiment is slightly lower than that presented in the Ti-Al-C ternary phase diagram, and this maybe the effects of Mo and Si, which are stabilizers. 3.3 Microstructure of samples after various forgings The composite was forged at 1423, 1333 and 1173 K, respectively. From the results about transus temperature measured in this experiment and Fig. 3, it can be known that the three forging temperatures in this work are in the phase
Fig. 4
Optical microstructure of the composite before forging.
field, the ( þ ) phase field and the upper phase field, respectively. The optical micrographs of the samples before and after forging are presented in Figs. 4 and 5. From Fig. 5, it can be seen that different microstructures are obtained after the samples were forged at different temperatures. Figure 5(a) presents the Widmanstatten microstructure obtained after the sample was forged at 1423 K, and Fig. 5(b) presents the bimodal microstructure obtained after the sample was forged at 1333 K. Figures 5(c) and (d) present the microstructure containing aligned plate obtained after the sample was forged at 1173 K. From Fig. 3, it can be known that the material is in the upper phase field at 1173 K. During the forging in the upper phase field, the microstructure before forging is made of all plates and residual phase, and the character of this microstructure is that all plates in a colony have the same orientation (Fig. 4). This character enables that a bundle of plates deform harmoniously as a whole grain. During this forging process, plates are bended and stretched along the flowing orientation of material. With the increasing of deformation amount, all plates intend to align along the flowing orientation (Fig. 5(d)). -titanium deforms by slip on basal and prismatic planes,15) which give rise to substructures and dislocations piling up. The TEM of dislocations in phase is presented in Fig. 6. On the other hand, the TiC particles can impede the movement of dislocations in phase, which give rise to
The Effect of Forging on Microstructure and Mechanical Properties of In Situ TiC/Ti Composite
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Fig. 5 Optical microstructure of samples after forging (a) at 1423 K (transverse), (b) at 1333 K (transverse), (c) at 1173 K (transverse), (d) at 1173 K (longitudinal).
Fig. 6 Bright field TEM of dislocation in phase after the sample was forged in the upper phase field.
substructures too. There were many substructures in grains after deformation in the upper phase field, and the TEMs of substructures in grains are presented in Fig. 7. 3.4 Mechanical properties The results of tensile tests of the composite at ambient temperature are presented in Fig. 8. These results indicate
that the best ductility is obtained in the bimodal microstructure, and the highest ultimate tensile strength (U.T.S) and yield strength (Y.S) are obtained in the microstructure containing aligned plates. Compared to the bimodal microstructure or the Widmanstatten microstructure, there is a 200 MPa increase in strength for the microstructure containing aligned plates. In other mechanical properties tests by these authors, the material before forging was quenched from various temperatures, and the results showed that the ultimate strength of the samples with various volume fraction of primary phase is almost the same, which indicates that the volume fraction of primary phase has little effect on the ultimate strength of samples. From the stress-strain curves of Ti alloys during hot deformation,15) it can be known that there is work hardening during thermomechanical processing. The lower of temperature at which the forging is performed, the more obvious is the effect of work hardening. Generally, work hardening results from the generation and piling up of dislocations. And the dislocation image after forging has been presented in Fig. 6. Suri16) have studied the mechanical properties of singlecolony crystals of monolithic near- Ti alloy at ambient temperature, and there is a significant anisotropy in strength and strain hardening rate along different orientations. They attribute this to the different interaction between the = interface and dislocations in phase along different orientations. Suri16) proposed a model, which explained the critical resolved shear stress (CRSS) and work hardening behavior of prism slip through the = interface for dislocations in the phase. As mentioned above, titanium
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F. Ma, W. Lu, J. Qin and D. Zhang
Fig. 7 Bright field TEM images of substructures in phase after the sample forged in the upper phase field (a) showing substructures in phase, (b) showing substructures in phase near TiC particle.
1600 Forged at 1173K Ture Stress, σ/MPa
1400 1200 1000
Forged at 1333K Forged at 1423K
800 600 400 200 0 0 1 2 3 4 5 6 7 8 9 10 11 12 True Strain (%)
Fig. 8 True stress-true strain curves of the composite after forged at various temperatures.
improvement in mechanical properties for the microstructure containing aligned plates may result from shorter slip lengths for dislocations in phase, and the result has some agreement with prior reports.2,8,9) 4.
Conclusions
From this work, some conclusions can be drawn. Some C element dissolve in Ti matrix during the preparation process of in situ TiC/Ti composites, which increase the transus temperature of the matrix alloy obviously. The microstructure containing aligned plate is obtained after the composite was forged in the upper phase field, and many substructures have formed in phase after such treatment. Compared to the samples with the bimodal microstructure or the Widmanstatten microstructure, there is an obvious increase in U.T.S for the sample with the microstructure containing aligned plates when the axe of plate in length is parallel to tensile direction. Acknowledgments
deform by slip on basal and prismatic planes. It is well known that the highest shear stress orientation makes an angle of 45 with the tensile axes in tensile tests, and the slip systems along this direction start to move firstly. According to above theory, due to resistance of = interface to dislocations movement, when the direction of plates in length are parallel to tensile direction, the pffiffiffi maximum free slip distance for dislocations in plates is 2d, where d is the thickness of plate. In other two microstructures (bimodal microstructure or Widmanstatten microstructure), because the average aspect p ffiffiffi ratio (length/thickness) of laths is much lager than 2 and the orientation of laths is random (Figs. 5(a) and (b)), the effect of resistance of = interface to dislocations movement is not so obvious as the microstructure containing aligned plates (the axe of plates in length is parallel to tensile direction during tensile tests). In general, the
We would like to acknowledge a financial support provided by A Foundation for the Author of National Excellent Doctoral Dissertation of P R China under Grant No: 200332, the Research Fund of Science and Technology Commission of Shanghai Municipality under Grant No: 04DZ14002, 0452nm045, 03ZR14063, and the ItoYama Foundation. REFERENCES 1) R. Boyer, E. W. Collings and G. E. Welsch (Eds.): Materials Properties Handbook: Titanium Alloys, (Materials Park, OH: ASM International, 1994) pp. 488–502. 2) G. Luetjering, J. Albrecht and A. Gysler: Titanium’92 Science and Technology, (In: Froes FH, Caplan I, editors. Warrendale:TMS, 1993) pp. 163–165.
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