Hot-deformation behaviors of AZ31 alloys with different initial states

WANG Bing-shu(汪炳叔)¹, XIN Ren-long(辛仁龙)¹, HUANG Guang-jie(黄光杰)^{1, 2},

CHEN Xing-pin(陈兴品)¹, LIU Qing(刘 庆)^{1, 2}

1. College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China;

2. National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400050, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The hot-deformation behaviors of three types of AZ31 samples, extruded sheet, hot rolled sheet and cast rod were studied. These samples had different initial grain size and texture. Compression deformation of these samples was carried out using a Gleeble 1500D under a series of thermal deformation conditions. Microstructure and texture of the initial and deformed samples were examined using electron backscatter diffraction (EBSD) techniques. The flow curves for all these three types of samples shifted upward with strain rate increasing. Significant grain refinement was noticed in the hot rolled sheet sample. The grain size was reduced to 3.7 μm after 50% (ε=0.69) compression. The DRX grains in both the extruded rod and hot rolled sheet samples presented the same basal plane texture, irrespective of the difference in the initial texture of the samples.

Key words: magnesium alloy; hot deformation; texture; dynamic recrystallization

Magnesium alloys are promising materials because of their low density, high specific strength and stiffness, superior damping capacity, high thermal conductivity, and good electromagnetic shielding characteristics[1-3]. However, magnesium alloys exhibit very poor ductility at room temperature, which limits their structural applications[4-5]. To improve forming ability, wrought magnesium alloys are normally processed at warm temperatures. Therefore, the study of thermal deformation behavior is one of the focuses of magnesium alloys research.

In previous studies, it was found that wrought magnesium alloys showed distinct strain softening during high-temperature deformation. The strain softening was also considered a typical characteristic in the present elevated flow stress models of magnesium alloys[6]. It is commonly believed that the presence of strain softening is attributed to dynamic recovery(DRV) and dynamic recrystallization(DRX). Furthermore, the initial microstructure and texture of magnesium alloys are closely related to the kinetic of DRX, which thus determines the hot deformation behaviors[7]. For magnesium alloy, the response to plastic deformation is strongly dependent on the crystal orientation. And the texture evolution during deformation of magnesium alloys has therefore been the subject of studies[8-14]. However, there is still a need for a more detailed characterization of the response to plastic deformation of such metals.

In this work, AZ31 cast rod, extruded rod and hot rolled sheet samples were prepared, which had different initial microstructure and texture. Microstructure and texture evolution of the samples were observed before and after the compression tests. Preliminary results showed that the initial grain size and texture of the samples had effects on hot deformation behaviors.

2 Experimental

The starting materials were AZ31 (Mg-3%Al-1%Zn) cast rod, extruded rod, and hot rolled sheet in different initial states. The cylinders with the diameter of 8 mm and the length of 12 mm were cut from the materials for compression testing. The extruded rod samples were cut with the cylinder (compression) axis perpendicular to the extrusion direction (ED) of the rod. The hot rolled sheet samples were cut with the cylinder (compression) axis parallel to the normal direction (ND) of the sheet. The cast samples were cut randomly. After the homogenization annealing at 673 K for 1 h, compression tests were carried out using a Gleeble 1500D in a vacuum of 1×10^-3 Pa. The isothermal compression tests were conducted at the temperature of 573 K and the strain rate of 0.001, 0.01, 0.1, 1 and 5 s^-1. The deformed samples were water-quenched immediately after the test and were cut along the compression axis for metallographic examination. Automated EBSD orientation maps were obtained in beam-control mode using FEI Nova 400 FEG-SEM. Representative regions were mapped using a step size of 1 μm, in each case using a grid of 200×200 points.

3 Results and discussion

3.1 Flow curves of three types of AZ31 samples

Fig.1 shows the true stress—true strain curves for compression of the magnesium alloy samples at the temperature of 573 K and the strain rate of 0.01 s^-1. With temperature fixed and strain rate increased, the flow curves in all the cases shifted upward. Strain hardening and strain softening were observed in all these three types of samples. The strain softening (appearing after the maximum stress) was attributed to the competition of work hardening and DRX softening. Since working hardening and DRX softening are related to grain size and strain rate, the rates of strain softening differ among the samples with different grain sizes or the same sample at different strain rates. The flow stress of extruded rod and hot rolled sheet samples tends to be steady when work hardening and DRX softening reaches equilibrium at a low strain rate (Figs.1(b) and 1(c)).

3.2 Evolution of microstructure during compression

The orientation maps of the extruded rod and hot rolled sheet samples at different strains are shown in Fig.2. The samples were compressed at the temperature of 573 K and the strain rate of 0.01 s^-1. A few recrystallized grains and elongated deformed grains are presented in the original extruded rod sample (Fig.2(a)); whereas the hot rolled sheet sample exhibits a homogeneous and equiaxed microstructure (Fig.2(d)). The average grain sizes of the original extruded rod and the hot rolled sheet samples are approximately 6.9 μm and 18 μm, respectively.

Significant grain refinement was noticed in the hot rolled sheet but not in the extruded rod sample. With 50% (ε=0.69) compression, the average grain size of the hot rolled sample was reduced to about 3.7 μm, whereas that of the extruded rod sample was slightly reduced to

Fig.1 True stress—true strain curves of three types of AZ31 samples: (a) Cast rod; (b) Extruded rod; (c) Hot rolled sheet

about 4.8 μm. The DRX does not completely occur at 573 K. The new DRX fine grains formed at original grain boundaries. As shown in Figs.2(b) and 2(e), the DRX grains can be clearly distinguished from the original comparatively coarse grains in the samples with 20% (ε=0.22) compression. The grain sizes of the extruded rod and hot rolled sheet samples were close after 50% compression. Thus the two types of samples showed comparable strength afterward.

Fig.2 EBSD maps of AZ31 sample at different strains: (a-c) Extruded rod; (d-f) Hot rolled sheet; (a, d) ε=0; (b, e) ε=0.22; (c, f) ε= 0.69

3.3 Evolution of texture during compression test

The textures of the extruded rod and hot rolled sheet samples at different strains are shown in Fig.3. The initial extruded rod sample presents a strong fiber texture, with the basal plane aligned almost parallel to the extruded orientation (Fig.3(a)). The initial hot rolled sheet sample presents a strong basal plane texture centered close to normal direction (ND) (Fig.3(d)). The textures of the deformed samples are illustrated in pole-figures drawn with the compression axis in the centre of the pole-figures in Figs.3(c) and 3(f). It can be seen that the basal plane texture of the hot rolled sheet sample was retained and enhanced during compression. However, the initial fibre texture of the extruded rod sample was transformed to a basal texture with 50% compression at 573 K, but minor texture components

Fig.3 Pole figures of AZ31 samples at different strains (contour densities for {0001} pole figures are (2, 4, 6, 8, 10, 12)×random):  (a-c) Extruded rod; (d-f) Hot rolled sheet; (a, d) ε=0; (b, e) ε=0.22; (c, f) ε=0.69

corresponding to regions with basal planes inclined to Y direction.

The texture evolution of AZ31 samples is related to the microstructure evolution of the samples induced by compression. Under 50% compression, DRX largely occurred in both the extruded rod and hot rolled sheet samples (Figs.3(c) and 3(f)). Surprisingly, the DRX grains in both samples presented the same basal plane texture though the initial texture of the samples was different. This result was contradicted to the observations by BACKX et al[15], in which they found that the DRX grains formed with random orientations upon compression. The consistent texture of the DRX grains observed in different type of samples agreed with the DRX mechanism proposed by HUMPHREYS et al[16]. DRX occurred mainly due to progressive subgrains rotation and the new grains gradually rotating to basal orientation.

1) The true stress—true strain curves revealed obvious strain softening in all the magnesium alloys: AZ31 cast rod, extruded rod and hot rolled sheet samples. The flow curves in all these cases shifted upward with strain rate increasing.

2) Significant grain refinement was noticed in the hot rolled sheet sample which initially had a large grain size (18 μm) and a strong basal plane texture. After 50% compression and grain refinement, the average grain size of the hot rolled sample was reduced to about 3.7 μm.

3) The DRX grains in both the extruded rod and hot rolled sheet samples presented the same basal plane texture, irrespective of the difference in the initial texture of the samples.

[1] Mwembela A, Konopleva E B, McQueen H J. Microstructural development in Mg alloy AZ31 during hot working [J]. Scripta Mater, 1997, 37: 1789-1795.

[2] YU Kun, LI Wen-Xian, ZHAO Jun. Plastic deformation behaviors of a Mg-Ce-Zn-Zr alloy [J]. Scripta Mater, 2003, 48: 1319-1323.

[3] Galiyev A, Kaibyshev R, Gottstein G. Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60 [J]. Acta Mater, 2001, 49: 1199-1207.

[4] WANG Y N, HUANG J C. Texture analysis in hexagonal materials [J]. Mater Chem Phys, 2003, 81: 11-26.

[5] YANG P, YU Y, CHEN L. Experimental determination and theoretical prediction of twin orientations in magnesium alloy AZ31 [J]. Scripta Mater, 2004, 50: 1163-1168.

[6] Barnett M R. Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31 [J]. Journal of Light Metals, 2001, 1: 167-177.

[7] Yi S B, Zaefferer S, Brokmeier H G. Mechanical behaviour and microstructural evolution of magnesium alloy AZ31 in tension at different temperatures [J]. Mater Sci Eng A, 2006, 424: 275-281.

[8] Kim W J, Hong S I, Kim Y S. Texture development and its effect on mechanical properties of an AZ61 Mg alloy fabricated by equal channel angular pressing [J]. Acta Mater, 2003, 51: 3293-3307.

[9] Agnew S R, Yoo M H, Tome C N. Application of texture simulation to understanding mechanical behavior of Mg and solid solution alloys containing Li or Y [J]. Acta Mater, 2001, 49: 4277-4289.

[10] Philippe M J, Serghat M, Houtte P V. Modelling of texture evolution for materials of hexagonal symmetry (II): Application to zirconium and titanium α or near α alloys [J]. Acta Metall, 1995, 43: 1619-1630.

[11] Philippe M J, Wagner F, Mellab F E. Modelling of texture evolution for materials of hexagonal symmetry (I): Application to zinc alloys [J]. Acta Metall, 1994, 42: 239-250.

[12] Styczynski A, Hartig Ch, Bohlen J. Cold rolling textures in AZ31 wrought magnesium alloy [J]. Scripta Mater, 2004, 50: 943-947.

[13] Barnett M R, Nave M D, Bettles C J. Deformation microstructures and textures of some cold rolled Mg alloys [J]. Mater Sci Eng A, 2004,386: 205-211.

[14] YOO M H, AGNEW S R, MORRIS J R, HO K M. Non-basal slip systems in HCP metals and alloys: Source mechanisms [J]. Mater Sci Eng A, 2001, 319: 87-92.

[15] Backx P, Kestens L. Dynamic recrystallization during compression of Mg-3%Al-1%Zn [J]. Materials Science Forum, 2005, 495/497: 633.

[16] Humphreys F J, Hatherly M. Recrystallizatio and related annealing phenomena [M]. 2nd ed. Elsevier, Oxford, UK, 2004.

(Edited by YANG Bing)

Foundation item: Project(2007CB613703) supported by the National Basic Research Program of China; Project(2006BAE04B09-1) supported by the National Key Project of Scientific and Technical Supporting Programs of Ministry of Science and Technology, China

Corresponding author: HUANG Guang-jie; Tel: +86-23-65112334; E-mail: gjhuang@cqu.edu.cn