Effect of Grain Refinement on Tensile and Fracture Behaviour of ZE41 Alloy Raviraj Verma1, R. Jayaganthan2, S. K. Nath3 1,2,3
Department of Metallurgical and Materials Engineering & Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee-247667, India 2 Department of Engineering Design, Indian Institute of Technology Madras, Chennai – 600036, India Presenting authors Email: 1
[email protected] Other Author’s Email(s):
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Abstract The multiaxial forging (MAF) at 723K was used to produce the ultrafine grained (UFG) microstructure in ZE41 magnesium alloy. Prior to MAF, ZE41 was subjected to two stage solution heat treatment at 603K and 453K for 2hrs and 16hrs, respectively. The cumulative strains of 2.1 and 4.2 were induced by MAF in 1st and 2nd cycles, respectively at 723K. The tensile and fracture properties were investigated for MAFed samples and compared with solutionized specimen. The ultrafine grained specimens have shown enhancement in yield strength (YS), ultimate tensile (UTS), elongation up to failure (%E), hardness (HV), and J-integral (J1C) values i.e. ~215MPa, ~282MPa, ~13.4%, ~68HV and ~16.5kJ/m2, respectively compared with solution treated specimen. The deformation behaviour of the processed alloy has been characterized through optical, field emission secondary electron and transmission electron microscopy. The fracture behaviour has been tested by 3-point bend specimens and characterized through fractography. Keywords: ZE41 Mg alloy; Multiaxial forging (MAF); Tensile properties; Fracture properties.
1. Introduction The magnesium alloy with rare earth (RE) addition (ZE41) shows an excellent specific strength, higher ductility and higher creep properties and therefore used for light weight structural application [1]. Traditionally used magnesium alloys have limited ductility at room temperature (RT), low elastic modulus, poor cold workability and low corrosion property and therefore restrict its use for modern structural applications such as aviation and automobiles applications [2]. To overcome these limitations, continuous efforts were taken by many researchers to process the magnesium alloy by several thermo mechanical routes [3]. The ZE41 alloy is also one of those modern Mg alloys. The ZE41 is generally available in casting form and introduced first in the year 1940. The RE contained magnesium alloys have very good combination of structural and corrosion properties, which promotes its applicability in the aviation industry. Also, surface treatment studies have shown the potential use of Mg alloy in the field of bio-implantation due to better biocompatibility [4]. The limited availability of slip systems at room temperature (RT) causes poor ductility and formability in Mg alloy. The (0001) is the only available basal plane, which
facilitates deformation at RT in the highly closed packed direction [112̅0]. The slipping governed deformation generally occurs due to a reduction in critical resolved shear stress (CRSS) value, which is temperature dependent. As processing temperature increases, nonbasal planes are also activated and slip system becomes active in those planes. For example, at 498K and 573K, pyramidal {101̅1}〈112̅0〉 plane and prismatic {101̅0}〈112̅0〉 plane in Mg alloy are activated, respectively [5].There are various severe plastic deformation (SPD) techniques such as ECAP [6], rolling [7] and MAF [8] reported in the literature to achieve the ultrafine grained (UFG) microstructure, which imparts good combination of strength and ductility in magnesium alloy. Dynamic recrystallization in UFG ZE41 during ECAP at 603K has shown enhancement in both, strength and ductility at room temperature. The literature on the mechanical behaviour of SPD-processed ZE41 is limited. Hence, the present work is focussed to study the influence of ultrafine grains produced in ZE41 alloy by multiaxial forging. Due to lower CRSS at the elevated processing temperature (723K), better formability up to the higher strain value (4.2) has been achieved without edge or internal cracking in the MAFed block.
2. Experimental Procedure The chemical composition of procured as cast ZE41 alloy are given in Table 1. The ZE41 alloy was subjected to 2 stage solution treatment (ST) at 603K for 2hrs followed by 453K for 16hrs. Multiaxial forging (MAF) was performed to produce ultrafine grain or nanostructured grain in the alloy. MAF was performed up to 3 passes and 6 passes or corresponding true strains are 2.1 and 4.2, respectively at 723K. The following 3 passes of the forging are basically pressing along the 3 perpendicular respective planes (i.e. XY, YZ and ZX planes) on MAF block, which considered as 1 complete cycle of MAF as shown in Fig. 1. The induced strain during each step of forging pass was -0.7. The Pre-MAF block size was 202540 mm3 (Fig. 1) and the induced strain is evaluated by Eq. (1), where ℎ𝑓 is final height and ℎ𝑖 is initial height. ℎ𝑓 𝜀 = 𝑙𝑛 ( ) ℎ𝑖
(1)
Table 1 The chemical composition of procured ZE41 as cast Mg alloy Element
Zn
RE
Zr
Mn
Fe
Cu
Ni
Si
Mg
wt%
3.5
0.8
0.4
0.15
0.01
0.03
0.005
0.01
Balance
After severe deformation, tensile test and 3-point bend tests were conducted according to ASTM E8 and ASTM 1820E, respectively. Both tests were performed by using Tinius Olsen H25K-S. Micro-hardness was conducted on VMHL microhardness tester with 25mg force. Polishing was carried as per the standard metallographic procedures. Diamond polishing was performed by using diamond paste having particle size 0.25µm. Use of water at any stage of metallography is absolutely avoided. For preparing the TEM sample, fischione automatic twin-jet electropolisher was used. The Leica DMI 5000 optical microscope, TECHNAI TEM (200KeV) and FEI Quanta SEM equipment were used to characterize the processed Mg alloy.
Fig. 1. Schematic of Multiaxial Forging (MAF) showing each pass of forging steps.
3. Results and Discussion 3.1. Microstructure The optical micrographs of solution treated ZE41 Mg alloy are shown in Fig. 2(a), which exhibits the coarse and equiaxed grains after ST. Multiaxial forging was performed up to 1 and 2 cycles and the respective microstructures are demonstrated in Fig. 2(b,c), revealing the highly deformed microstructure. The strengthening in MAFed Mg alloy is governed by HallPetch relationship as shown by Eq. (2). From the Fig. 2(b), highly distorted grain boundaries are found after the 3rd pass of forging but still the coarser grains are observed. The reason behind the presence of coarser grains in the specimen are the following:3-passes are insufficient to induce the required value of strain to achieve the grain refinement. Hence, to achieve the UFG microstructure, further MAF cycle (3 passes) was performed and the micrograph for the same is shown in Fig. 2(c). 𝜎 = 𝜎0 +
𝑘 √𝑑
(2)
where 𝜎0 and k are the experimentally derived constants. Average grain size is defined by d.
Fig. 2. Optical micrographs of (a) solution treated, (b) 3-pass forged and (c) 6-pass forged ZE41 Mg alloy.
Optical micrographs have shown the highly deformed microstructure, especially after 6 pass forged samples but to understand the grain refinement mechanism and the size of UFG grain, TEM study was performed. Bright field TEM micrograph as shown in Fig. 3 has revealed the size of UFG grains, which has formed due to dynamic recrystallization (DRX) th
during 2nd cycle of MAF. Dislocations are generated due to MAF deformation process performed at elevated temperature. The MAFed generated dislocations are highly mobile and it is accumulated near the high angle grain boundaries. By the particle stimulated nucleation (PSN) process, dynamic recrystallization occurred where these dislocations get converted into the proper ultrafine sized grains. The average size of developed UFG grains after the 6th pass of forging are nearly 500nm, which is in the regime of UFG scale i.e. 100nm-to1000nm.
Fig. 3. BF-TEM micrograph of 6th pass forged specimen having dynamically recrystallized UFG grains marked by red arrows.
3.2. Mechanical and Fracture behaviour The microhardness and tensile properties were investigated to understand the effect of grain refinement on mechanical behaviour. The hardness improvement in ST, 3PF, and 6PF is attributed to higher dislocation density and pinning of its mobility near the low and high angle grain boundaries as shown in Fig. 3. These results are in good agreement with Dang et. al. [9]. The maximum hardness was observed in 3PF specimen i.e. 73HV, which has shown ~9% increment compared to ST specimen. The ultimate tensile strength of ST, 3PB, and 6PF is 160MPa, 282MPa, and 255MPa, respectively. However, elongation to failure for ST, 3PB, and 6PB is 4%, 11.7%, and 13.4%, respectively as shown in Fig. 4(a). It is clearly observed from the tensile data that the strength has enhanced up to 3PF and then reduced in 6PF specimen. However, the increment in ductility is continued due to grain softening after a certain point, near about 3PF conditions. This result is tandem with Roberto B. F. et al work on Mg alloy [10]. The elongation to failure enhances after 3rd passes of forging due to superplasticity, whereas strength decreases after 3rd pass. It is due to the formation of bimodal microstructure after 3PF and the similar observation is found in ZK60 alloy [11]. Homogenous fine grain structure is formed up to the 3rd passes only, hence strength is maximum. Whereas, grains became inhomogeneous when forging passes reached up to 6, leading to decrement in strength. The fracture energy (J-integral) for solution treated (ST), 3 passes forged (3PF), and 6-passes forged (6PF) were 8kJ/m2, 16.5kJ/m2, and 14.7kJ/m2, respectively calculated by using Eq. (3). Fracture toughness increases from AC to 3PF but then decreases in 6PF. The 3PF specimen has less dislocation density compared to 6PF.
Fracture toughness has reduced in 6th pass forged sample because of extensive grain refinement and formation of bimodal microstructure [12], which is the mixture of nano and ultrafine grains. Nano-sized grain boundaries can act as a crack initiation zone and higher dislocation density available in 6PF specimen, facilitates crack propagation easier than 3PF as shown in Fig. 4(b). 1+𝛼 2𝐴𝑡𝑜𝑡. 𝐽=( )×( ) 2 1+𝛼 𝐵𝑏 𝑎
(3) 𝑎
1
𝑎
1
Where b = (W-a) and α is the constant i.e.𝛼 = √(𝑏)2 + 𝑏 + 2 − 2(𝑏 + 2)
Fig. 4. Mechanical behaviour of ZE41 Mg alloy for ST, 3PF and 6PF conditions (a) YS, UTS and %E (b) Hardness and Fracture toughness behaviour
4. Summary and conclusions The Mg alloy was processed by MAF and their mechanical properties were investigated. The following conclusions are drawn based on the present study.
The MAFed triggered dislocations are pinned and accumulated near the low and high angle grain boundaries, which further stimulated through PSN, leading to the formation of dynamically recrystallized UFG grains. The average UFG gain size was 500nm. The 3-pass forged specimen has shown optimum yield (215MPa) and ultimate tensile strength (282MPa), whereas elongation to failure (13.4%) was found to be better in 6-passes forged specimen compared to as cast specimen. The bimodal microstructure is believed to be the cause behind the reduction of strength in 6th pass forged specimen. Maximum grain refinement was observed in 6-pass forged specimen, which facilitates easy crack initiation. Hence, fracture toughness was found to be better in 3-pass forged specimen, 16.5kJ/m2compared to 6-pass specimens.
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