Microstructure and mechanical properties of AZ91 magnesium alloy subject to deep cryogenic treatments

Abstract

AZ91 magnesium alloy was subjected to a deep cryogenic treatment. X-ray diffraction (XRD), scanning electronic microscopy (SEM), and transmission electronic microscopy (TEM) methods were utilized to characterize the composition and microstructure of the treated samples. The results show that after two cryogenic treatments, the quantity of the precipitate hardening β phase increases, and the sizes of the precipitates are refined from 8–10 μm to 2–4 μm. This is expected to be due to the decreased solubility of aluminum in the matrix at low temperature and the significant plastic deformation owing to internal differences in thermal contraction between phases and grains. The polycrystalline matrix is also noticeably refined, with the sizes of the subsequent nanocrystalline grains in the range of 50–100 nm. High density dislocations are observed to pile up at the grain boundaries, inducing the dynamic recrystallization of the microstructure, leading to the generation of a nanocrystalline grain structure. After two deep cryogenic treatments, the tensile strength and elongation are found to be substantially increased, rising from 243 MPa and 4.4% of as-cast state to 299 MPa and 5.1%.

Keywords

magnesium alloys / cryogenic treatment / microstructure / mechanical properties

References

[1] Li GR, Zhao YT, Dai QX, Zhang HJ, Wang HM. In-situ fabrication of particulate reinforced aluminum matrix composites under high-frequency pulsed electromagnetic field. J. Univ. Sci.Technol. Beijing, 2007, 14(5): 460.

[2] Li GR, Zhao YT, Dai QX, Cheng XN, Wang HM, Chen G. Fabrication and properties of in situ synthesized particles reinforced aluminum matrix composites of Al-Zr-O-B system. J.Mater. Sci., 2007, 42(14): 5442.

[3] Chu YJ, Chen J, Li XQ, Wu SQ, Yang ZH. Effects of thermomechanical treatments on the microstructures and mechanical properties of GTA-welded AZ31B magnesium alloy. Int. J. Miner. Metall. Mater., 2010, 19(10): 945.

[4] Wang B, Yang YS, Sun ML. Microstructure refinement of AZ31 alloy solidified with pulsed magnetic field. Trans. Nonferrous Met. Soc. China, 2010, 20(9): 1685.

[5] Asai S. Recent development and prospect of electromagnetic processing of materials. Sci. Technol. Adv. Mater., 2000, 1(4): 191.

[6] Zhao ZD, Mao WM, Li SL, Zhong RM. Preparation of semisolid AlSi7Mg alloy slurry through weak traveling-wave electromagnetic stirring. Int. J. Miner. Metall. Mater., 2009, 16(5): 554.

[7] Akbaripanah F, Fereshteh-Saniee F, Mahmudi R, Kim HK. Microstructural homogeneity, texture, tensile and shear behavior of AM60 magnesium alloy produced by extrusion and equal channel angular pressing. Mater. Des., 2013, 43, 31.

[8] Bayani H, Saebnoori E. Effect of rare earth elements addition on thermal fatigue behaviors of AZ91 magnesium alloy. J. Rare Earths, 2009, 27(2): 255.

[9] Gill SS, Singh H, Singh R, Singh J. Cryoprocessing of cutting tool materials-a review. Int. J. Adv. Manuf. Technol., 2010, 48(1–4): 175.

[10] Asl KM, Tari A, Khomamizadeh F. Effect of deep cryogenic treatment on microstructure, creep and wear behaviors of AZ91 magnesium alloy. Mater. Sci. Eng. A, 2009, 523(1–2): 27

[11] Liu Y, Shao S, Xu CS, Yang XJ, Lu DP. Enhancing wear resistance of Mg-Zn-Gd alloy by cryogenic treatment. Mater. Let., 2012, 76, 201.

[12] Jiang Y, Chen D, Chen ZH, Liu JW. Effect of cryogenic treatment on the microstructure and mechanical properties of AZ31 magnesium alloy. Mater. Manuf. Processes, 2010, 25(8): 837.

[13] Deng KK, Wu K, Wu YW, Nie KB, Zheng MY. Effect of submicron size SiC particulates on microstructure and mechanical properties of AZ91 magnesium matrix composites. J. Alloys Compd., 2010, 504(2): 542.

[14] Zhang D, Wu MS, Feng R. Micromechanical investigation of heterogeneous microplasticity in ceramics deformed under high confining stresses. Mech. Mater., 2005, 37(1): 95.

[15] Alam ME, Han S, Nguyen QB, Hamouda AMS, Gupta M. Development of new magnesium based alloys and their nanocomposites. J. Alloys Compd., 2011, 509(32): 8522.

[16] Hunphreys FJ, Hatherly M. Recrystallization and Related Annealing Phenomena, 2004 2nd Ed. Oxford, The Elsevier Ltd. Press, 121

[17] Wolf D, Yamakov V, Phillpot SR, Mukherjee A, Gleiter H. Deformation of nanocrystalline materials by molecular-dynamics simulation: relationship to experiments. Acta Mater., 2005, 53(1): 1.