Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation
Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation
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DOI：
10.1016/j.msea.2004.06.055
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发表时间：
2004-11
期刊：
影响因子：
6.4
通讯作者：
Ho-Kyung Kim;W. Kim
中科院分区：
文献类型：
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作者：
Ho-Kyung Kim;W. Kim
采用等通道转角挤压（ECAP）技术，通过两个相交通道的简单大剪切变形，对AZ 31镁合金进行了超细晶组织的制备。控制ECAP温度随道次的增加而降低，以最大限度地提高晶粒细化效率，同时防止开裂。第一次和第二次压制在593 K下进行，而第三次和第四次压制分别在523和473 K下进行。随着道次的增加，晶粒细化程度和晶粒尺寸分布均匀性提高。经过四道次轧制后，获得了由细小等轴晶组成的均匀组织。通过在473和773 K之间的宽温度范围内对四道次材料进行退火来检查ECAP结构在升高的温度下的稳定性。静态晶粒生长的活化能的测量表明，根据所研究的温度范围，存在三个不同的值：Q = 0.78Qgb（激活晶界扩散）在473- 523 K的低温范围内，在523- 673 K的中温区Q = 0.27Qg，在673- 773 K的高温区Q = 0.84QL（晶格扩散激活）或Q = 1.23Qg。在中间温度范围内异常低的Q值可能不代表真正的活化能。随着温度的升高，位错密度逐渐降低，回复率增强，这可能是导致这种结果的原因。经ECAP变形后，AZ 31镁合金的屈服应力降低，延伸率提高。ECAP后应变硬化指数的增大是导致拉伸延伸率提高的主要原因，而织构的改变使ECAP过程中的基面滑移更容易发生则是导致屈服应力降低的主要原因。
Equal channel angular pressing (ECAP) technique, which involves a simple large shear deformation during passage through two intersecting channels, was applied to the AZ31 Mg alloy to make an ultrafine-grained microstructure. ECAP temperature was controlled to decrease with pass number to maximize the grain refinement efficiency with preventing cracking. The first and second pressings were conducted at 593K, while the third and fourth pressings were conducted at 523 and 473K, respectively. The degree of grain refinement and homogeneity of grain-size distribution increased with pass number. After four passes, the reasonably homogeneous microstructure composed of fine and equiaxed grains was obtained. The stability of the ECAPed structure at elevated temperatures was examined by annealing the four-passed materials over a wide range of temperature between 473 and 773K. Measurement of activation energies for static grain growth shows the presence of three different values depending on the temperature range investigated: Q = 0.78Qgb(activation for grain boundary diffusion) in the low temperature range 473–523K, Q = 0.27Qgbin the intermediate temperature range 523–673K and Q = 0.84QL(activation for lattice diffusion) or Q = 1.23Qgbin the high temperature range 673–773K. The abnormally low Q value in the intermediate temperature range may not represent the true activation energy. Progressive decrease in dislocation density by enhanced recovery with increasing temperature may be the cause of the result. After ECAP, the yield stresses (YS) of the ECAPed AZ31 alloys decreased while their elongations increased. Enlarged strain hardening exponent after ECAP is the key factor considered to bring in the tensile-elongation increase, while modification of texture for easier slip on basal planes during ECAP is believed to be responsible for the yield stress decrease.