Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy

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1 Materials Transactions, Vol. 44, No. 4 (2003) pp. 490 to 495 Special Issue on Platform Science and Technologyfor Advanced Magnesium Alloys, II #2003 The Japan Institute of Metals Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy Mamoru Mabuchi 1, Yasumasa Chino 1 and Hajime Iwasaki 2 1 Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, Nagoya , Japan 2 Division of Materials Science and Engineering, Graduate School of Himeji Institute of Technology, Himeji , Japan Mechanical properties of an AZ31 Mg alloywith the grain sizes of 4, 12, 60 and 450 mm were investigated bytensile tests at K with 1: : s 1. The Mg alloyexhibited unique behaviors of low elongation of 17% at 473 K with 1: s 1 for the specimen with the grain size of 450 mm and large elongation of 234% at 673 K with 1: s 1 for the specimen with the grain size of 60 mm. These behaviors could not be explained from the viewpoint of the plastic stability. Microstructural observation revealed significant twin formation at 473 K with 1: s 1 for the specimen with the grain size of 450 mm and active grain boundarysliding at 673 K with 1: s 1 for the specimen with the grain size of 60 mm. Therefore, it is likelythat enhancement of twining and grain boundarysliding gave rise to the unique behaviors of the Mg alloythat could not be explained from the plastic stability. (Received September 19, 2002; Accepted December 3, 2002) Keywords: magnesium alloys, mechanical properties, grain size dependence, twining, grain boundary sliding 1. Introduction Magnesium alloys are promising structural materials because of their low densities. In magnesium, the critical resolved shear stresses for the non-basal slips as the prismatic and pyramidal slips are much larger than that for the basal slip at room temperature, 1,2) and hence, the non-basal slips are hardlyoperative at room temperature. Five independent slip systems are necessary for a polycrystalline material to be able to undergo a general homogeneous deformation without producing cracks. However, the number of independent mode for the basal slip is only2 for the hexagonal crystals such as Mg. 3) This gives rise to poor formabilityat room temperature for polycrystalline Mg and its alloys. However, the ductilityincreases with increasing temperature because the difference in critical resolved shear stress between the basal slip and the non-basal slips decreases with increasing temperature. Furthermore, grain refinement leads to a significant increase in elongation at elevated temperatures because superplasticityis attained for the fine-grained alloys. 4 6) The grain boundarydiffusion coefficient is large for Mg, 7) and grain boundarysliding occurs even in a Mg alloywith a relativelylarge grain size. 8) In addition, twining easilyoccurs in Mg due to the lack of slip systems. 9) Twining is affected by the grain size. 10,11) These suggest that mechanical properties of Mg and its alloyare uniquelyaffected bythe grain size. The previous studies 12,13) revealed that superplastic properties are stronglyaffected bythe grain size in Mg alloys. However, there are few studies on the grain size dependence of mechanical properties in a wide grain size range. 14) In the present paper, influence of the grain size on elongation at K in an AZ31 Mg alloyis investigated in the grain size range of mm. In particular, elongation in a nonsuperplastic region is focused bycomparing between the specimens with the grain sizes of 60 and 450 mm. 2. Experimental Procedure An AZ31 (Mg 3 mass%al 0.9 mass%zn 0.15 mass%mn) alloyblock was prepared. The block was extruded at 573 K with the extrusion ratio of 100 : 1. The grain size after the extrusion was 4.3 mm (Fig. 1). The extruded rod was annealed at 623 K for 28 h. The grain size after the annealing was 12 mm (Fig. 1). Furthermore, the extruded rod was annealed at 693 K for 72 h to produce the specimen with the grain size of 60 mm (Fig. 1(c)). The specimen with the grain size of 450 mm (Fig. 1(d)) was obtained byremelting the extruded rod and annealing at 693 K for 48 h. Tensile specimens with 10 mm in gage length and 2.5 mm in gage diameter were machined. Tensile tests were carried out at 473, 573 and 673 K with 1: : s 1, where the angle between the tensile direction and the extrusion direction was set to be 0 degrees. Prior to testing, each specimen was held at the testing temperature for 30 min to establish equilibrium. Microstructure of the specimens deformed to failure was investigated byan optical microscope. Also, the fractured surface and side surface were investigated bya scanning electron microscope. 3. Results The nominal stress-nominal strain curves at 473 K are shown in Fig. 2, where the strain rate is 1: s 1 and 1: s 1, respectively. At 1: s 1, the specimen with the coarse grain size of 450 mm exhibited clear work-hardening up to failure, while the others did not show work-hardening. At 1: s 1, significant work-hardening to failure was not observed for anyspecimens. It should be noted that the specimen with the small grain size of 4 mm showed large elongation at 1: s 1, compared to the others. The nominal stress-nominal strain curves at 673 K are shown in Fig. 3, where the strain rate is 1: s 1 and 1: s 1, respectively. At 1: s 1, there

Influence of Grain Size on Elongation at

(the grain size = 12 mm), (c) the annealed

mm) and (d) the remelted specimen (the grain

2 The nominal stress-nominal strain curves at

2 Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy491 20µ m 20 µ m (c) (d) 50 µ m Fig. 1 Microstructures of the AZ31 specimens, the extruded specimen (the grain size = 4.3 mm), the annealed specimen at 623 K for 28 h (the grain size = 12 mm), (c) the annealed specimen at 693 K for 72 h (the grain size = 60 mm) and (d) the remelted specimen (the grain size = 450 mm). Fig. 2 The nominal stress-nominal strain curves at 473 K, 1: s 1 and 1: s 1. Fig. 3 The nominal stress-nominal strain curves at 673 K, 1: s 1 and 1: s 1.

3 492 M. Mabuchi, Y. Chino and H. Iwasaki was no distinct grain size dependence on elongation. On the other hand, at 1: s 1, the specimens with the small grain sizes of 4 and 12 mm showed large elongation of 385 and 308%, respectively. This is attributed to superplastic behavior, as shown later. It should be noted that the specimen with the coarse grain size of 450 mm showed much lower elongation and higher stress than the others. The variation in stress and elongation at 473 K as a function of strain rate is shown in Fig. 4, where the stress corresponds to the maximum true stress. The specimen with the grain size of 450 mm showed the higher stress than the specimen with the grain size of 60 mm at 1: s 1.This is attributed to the work-hardening for the specimen with the grain size of 450 mm, as shown in Fig. 2. The elongation at 1: s 1 decreased with increasing grain size. In particular, the specimen with the coarse grain size of 450 mm showed much lower elongation of 17%. Similarly, the elongation at 1: s 1 decreased with increasing grain size. The specimen with the small grain size of 4 mm exhibited large elongation more than 100%. This results from the high strain rate sensitivityof about 0.2 for the specimen with the small grain size of 4 mm, though the other specimens showed the low strain rate sensitivityof about 0.1. The variation in stress and elongation at 573 K as a function of strain rate is shown in Fig. 5, where the stress corresponds to the maximum true stress. The elongation increased with decreasing grain size in the stain rate range investigated. This trend at 573 K is the same as that at 473 K. The specimen with the small grain size of 4 mm exhibited large elongation of 530% at 1: s 1. In addition, this specimen showed the high strain rate sensitivityof 0.4 at 1: : s 1. Thus, superplastic behavior was Fig. 5 The variation in stress and elongation at 573 K as a function of strain rate, where the stress is the maximum true stress. Fig. 6 The variation in stress and elongation at 673 K as a function of strain rate, where the stress is the maximum true stress. Fig. 4 The variation in stress and elongation at 473 K as a function of strain rate, where the stress is the maximum true stress. attained at 573 K with 1: : s 1 for the specimen with the grain size of 4 mm. The variation in stress and elongation at 673 K as a function of strain rate is shown in Fig. 6, where the stress corresponds to the maximum true stress. The specimens with

Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy493 the small grain size of 4 and 12 mm exhibited large elongation above 300% at 1:7 10 5 s 1 and the high strain rate

It should be noted that at 1:7 10 5 s 1, the elongation of the specimen with the grain size of 60 mm (¼ 234%) was much larger than that of the specimen with the grain size of 450 mm (¼ 35%), though

4 Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy493 the small grain size of 4 and 12 mm exhibited large elongation above 300% at 1: s 1 and the high strain rate sensitivityabove 0.4 at 1: : s 1. Thus, superplastic behavior was obtained at 673 K for the specimen with the grain size of 12 mm as well as the specimen with the grain size of 4 mm. It should be noted that at 1: s 1, the elongation of the specimen with the grain size of 60 mm (¼ 234%) was much larger than that of the specimen with the grain size of 450 mm (¼ 35%), though the values of the strain rate sensitivityfor the specimens with the grain size of 60 and 450 mm were almost the same (= about 0.2). 4. Discussion The present investigation revealed that the grain size has unique influence on the elongation in the non-superplastic region. For example, at 473 K with 1: s 1, the elongation of the specimen with the grain size of 450 mm was particularlylow (¼ 17%), though the strain rate sensitivityof each specimen, except the specimen with the grain size of 4 mm, was almost the same. The plastic stabilitydepends on not onlythe strain rate sensitivity, but also the workhardening rate. According to Hart, 15) the stabilitycriterion is given by ln _A ln A ¼ þ m 1 ð1þ m where A is the cross section area of a specimen, is the workhardening rate and m is the strain rate sensitivity. When ln _A= ln A > 0, the deformation is stable. Equation (1) indicates that the plastic stabilityincreases with increasing work-hardening rate. In the present investigation, however, the specimen with the grain size of 450 mm showed clear work-hardening behavior to failure, but, verylow elongation at 473 K with 1: s 1. Obviously, the grain size dependence of elongation cannot be explained onlyfrom the viewpoint of the plastic stability. Fractured surfaces of the specimens deformed at 473 K with 1: s 1 are shown in Fig. 7 for the specimen with the grain size of 60 mm and Fig. 7 for the specimen with the grain size of 450 mm, respectively. Ductile fracture was observed for the specimen with the grain size of 60 mm, while cleavage fracture was observed for the specimen with the grain size of 450 mm, indicating that the difference in elongation between the specimens with the grain sizes of 60 and 450 mm is related to the difference in fracture mode. Microstructures near the fractured surface of the specimens deformed at 473 K with 1: s 1 are shown in Fig. 8 for the specimen with the grain size of 60 mm and Fig. 8 for the specimen with the grain size of 450 mm, respectively. Many twins were observed in the specimen with the grain size of 450 mm. This trend is in agreement with the result bylahaie et al. 11) that twin formation is enhanced in Mg with a large grain size. A twin serves as an obstacle for dislocation movement. 16) Hence, when twins are formed during deformation, the number of dislocations piled up at the twins increases with straining, which leads to workhardening behavior. In such a case, the stress concentration is caused due to pile-up of dislocations bythe twins, resulting in premature fracture. Therefore, significant twin formation is 100 µ m Fig. 7 Fractured surfaces of the specimens deformed at 473 K with 1: s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm. likelyto be responsible for the low elongation at 473 K with 1: s 1 for the specimen with the grain size of 450 mm. One of characteristic results in the present investigation is that the specimen with the grain size of 60 mm exhibited large elongation at 673 K, in spite of the low strain rate sensitivity of 0.2. For example, large elongation of 234% was attained at 673 K with 1: s 1 for the specimen with the grain size of 60 mm, while the specimen with the grain size of 450 mm showed low elongation as 35%, though the strain rate sensitivityat 673 K with 1: s 1 was the same (¼ 0:2) for both specimens. This cannot be also explained from the viewpoint of the plastic stability. Fractured surfaces of the specimens deformed at 673 K with 1: s 1 are shown in Fig. 9 for the specimen with the grain size of 60 mm and Fig. 9 for the specimen with the grain size of 450 mm, respectively. Ductile fracture was observed for both specimens and there was no difference in fracture mechanism between the specimens with the grain sizes of 60 and 450 mm. At 673 K with 1: s 1, as shown in Fig. 6, the stress for the specimen with the grain size of 60 mm was lower than that for the specimen with the grain size of 450 mm. Recently, it has been reported that the strength is decreased bygrain boundarysliding for the Mg alloy. 8) Therefore, the

$8 Microstructures near the fractured surface of the specimens deformed at 473 K with 1:7 10 1 s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm.$ 200 µ m Fig. 9 Fractured surfaces of the specimens deformed at 673 K with 1:7 10 5 s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm.

200 µ m Fig. 9 Fractured surfaces of the specimens deformed at 673 K with 1:7 10 5 s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm.

Side surfaces of the specimens deformed at 673 K with 1:7 10 5 s 1 are shown in Fig.

photograph with low magnification for the specimen with the grain size of 450 mm, respectively.

5 494 M. Mabuchi, Y. Chino and H. Iwasaki 50 µ m 100 µ m 50 µ m Fig. 8 Microstructures near the fractured surface of the specimens deformed at 473 K with 1: s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm. 200 µ m Fig. 9 Fractured surfaces of the specimens deformed at 673 K with 1: s 1, the specimen with the grain size of 60 mm and the specimen with the grain size of 450 mm. lower stress for the specimen with the grain size of 60 mm maybe related to grain boundarysliding. Side surfaces of the deformed specimens were observed to investigate grain boundarysliding. Side surfaces of the specimens deformed at 673 K with 1: s 1 are shown in Fig. 10; is a photograph with low magnification for the specimen with the grain size of 60 mm and is a photograph with high magnification for the specimen with the grain size of 60 mm and (c) is a photograph with low magnification for the specimen with the grain size of 450 mm, respectively. For the specimen with the grain size of 60 mm, irregularities, whose spacing was roughlyin agreement with the grain size, were observed on the side surface of the deformed specimen. The irregularities are attributed to grain boundarysliding. 8) For the specimen with the grain size of 450 mm, however, such irregularities were not observed and there was no evidence for grain boundarysliding. Mabuchi et al. 8) investigated the contribution of grain boundarysliding to total strain at 573 K with 1: : s 1 for the Mg alloywith the grain size of 60 mm and theyrevealed that the contribution of grain boundarysliding to total strain is 5 30%, which is much lower than those for superplasticity(¼ 50{80% 17,18) ). Clearly, grain boundary sliding is not the dominant deformation process for the specimen with the grain size of 60 mm. The fact that the specimen with the grain size of 60 mm showed the lower stress indicates that grain boundarysliding serves to relax the stress concentration caused at grain boundaries. Therefore, it is suggested that the large elongation at 673 K for the specimen with the grain size of 60 mm is attributed to relaxation of the stress concentration bygrain boundarysliding. The present investigation revealed that the unique effects of grain size on elongation result from twining and grain boundarysliding. Twining occurs due to the lack of slip systems. Also, active grain boundary sliding is attributed to the large grain boundarydiffusion coefficient for Mg. 12,13) The enhancement of twining and grain boundarysliding is the specific feature of Mg. These features give rise to the unique grain size dependence of elongation that cannot be explained from the plastic stability. 5. Conclusions Mechanical properties of an AZ31 Mg alloyin the grain size range of mm were investigated bytensile tests at 473, 573 and 673 K with 1: : s 1. The results are concluded as follows. (1) The specimens with the grain size of 4 and 12 mm exhibited superplastic behavior at 673 K with 1: : s 1. However, superplasticity was not attained in anyconditions for the specimens with the grain sizes of 60 and 450 mm.

Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy495 (2) At 473 K with 1:7 10 1 s 1, the elongation of the specimen with the grain size of 450 mm was verylow (¼ 17%),

Also, the specimen with the grain size of 60 mm exhibited large elongation of 234% at 673 K with 1:7 10 5 s 1, in spite of the low strain rate sensitivityof 0.2. These unique behaviors could not be explained from the viewpoint of the plastic stability.

1 for the specimen with the grain size of 60 mm.

6 Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy495 (2) At 473 K with 1: s 1, the elongation of the specimen with the grain size of 450 mm was verylow (¼ 17%), though the strain rate sensitivityof each specimen was almost the same, except the specimen with the grain size of 4 mm. Also, the specimen with the grain size of 60 mm exhibited large elongation of 234% at 673 K with 1: s 1, in spite of the low strain rate sensitivityof 0.2. These unique behaviors could not be explained from the viewpoint of the plastic stability. (3) Microstructural observation revealed significant twin formation at 473 K with 1: s 1 for the specimen with the grain size of 450 mm and active grain boundarysliding at 673 K with 1: s 1 for the specimen with the grain size of 60 mm. The enhancement of twining and grain boundarysliding gave rise to the unique influence of the grain size on elongation which could not be explained from the plastic stability. REFERENCES (c) 100 µ m Fig. 10 Side surfaces of the specimens deformed at 673 K with 1: s 1, photograph with low magnification for the specimen with the grain size of 60 mm, photograph with high magnification for the specimen with the grain size of 60 mm and (c) photograph with low magnification for the specimen with the grain size of 450 mm. 1) H. Yoshinaga and R. Horiuchi: Trans. JIM 4 (1963) ) H. Yoshinaga and R. Horiuchi: Trans. JIM 5 (1963) ) M. H. Yoo: Metall. Trans. A 12A (1981) ) K. Kubota, M. Mabuchi and K. Higashi: J. Mater. Sci. 34 (1999) ) T. Mohri, M. Mabuchi, M. Nakamura, T. Asahina, H. Iwasaki, T. Aizawa and K. Higashi: Mater. Sci. Eng. A290 (2000) ) M. Mabuchi, T. Asahina, H. Iwasaki and K. Higashi: Mater. Sci. Tech. 13 (1997) ) M. Mabuchi and K. Higashi: Acta Mater. 44 (1996) ) M. Mabuchi, Y. Chino and H. Iwasaki: Mater. Trans. 43 (2002) ) F. E. Hauser, C. D. Starr, L. Tietz and J. E. Dorn: Trans. ASM 47 (1955) ) J. W. Christian and S. Mahajan: Prog. Mater. Sci. 39 (1995) ) D. Lahaie, J. D. Embury, M. M. Chadwick and G. T. Gray: Scripta Metall. Mater. 27 (1992) ) M. Mabuchi and K. Higashi: Philos. Mag. A 74 (1996) ) H. Watanabe, T. Mukai, K. Ishikawa, M. Mabuchi and K. Higashi: Mater. Sci. Eng. A307 (2001) ) J. A. Chapman and D. V. Wilson: J. Inst. Metals 91 ( ) ) E. W. Hart: Acta Metall. 15 (1967) ) A. Serra, D. J. Bacon and R. C. Pond: Metall. Mater. Trans. A 33A (2002) ) Z.-R. Lin, A. H. Chokshi and T. G. Langdon: J. Mater. Sci. 23 (1988) ) K.-T. Park, S. Yan and F. M. Mohamed: Philos. Mag. A 72 (1995)

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