材料期刊网

研究了铸造铝合金的微观组织参数包括二次枝晶臂间距(SDAS)、Si颗粒的形貌率和体积分数,与拉伸过程中内应力的关系,根据Hollomon和内应力公式建立了应变硬化指数n的定量预测关系式.结果表明:n值反映了材料在较大塑性变形下(在发生塑性松弛以后)的硬化能力.定义了微结构硬化能力参数,并用常见的几种铸造铝合金材料进行了验证.所推导的理论关系式及线性拟合关系式能较好地预测n值,对同一牌号的铸造铝合金材料,n值对颗粒相形貌率和SDAS的依赖性强,而对颗粒相体积分数的依赖性不明显,颗粒相形貌率和SDAS值越大,n值越小.对A319和A356/357铝合金,最佳修正系数分别是0.17和0.11,预测平均误差在10%左右.

Strain hardening exponent (n) of a material is an important parameter reflecting its hardening property whose determination is of great importance. It has a widely application in mate-rial scientific research and engineering fields such as fatigue life prediction, stress-concentration-factor calculation, etc.. The value of strain hardening exponent varies with their microstructures in cast alu-minum alloys, but a few theoretical and experimental investigations have been reported to understand the effects of the microstructural parameters on the strain hardening exponent in these alloys till now. In the present study, the influence of secondary dendrite arm spacing (SADS), aspect ratio and volume fraction of particles on internal stress in aluminum alloys were discussed, and a quantitative prediction of strain hardening exponent was established on the basis of Hollomon and internal stress equations. It shows that the strain hardening exponent represents their hardening ability in relatively large plastic deformation (larger than the upper limit for the no plastic relaxation regime). A new microstructural hardening parameter relatively to strain hardening exponent was defined. Besides, a group of A319 cast aluminum alloys with microstructural heterogeneities were tested. The calculated strain hardening exponents are in agreement with the experimental ones in A319 alloy as well as in some commonlyused cast aluminum alloys. For the same grade of alloys, the microstructural hardening parameter and strain hardening exponent are quite sensitive to SDAS and particle aspect ratio while the influ-ence of volume fraction of particles is relatively little. As the values of SDAS and aspect ratio of particles increase, the value of the strain hardening exponent decreases. A liner relationship between microstructural hardening parameter and strain hardening exponent was proposed. For A319 and A356/357 alloys, the optimum correction coefficients are 0.17 and 0.11, respectively, and the mean prediction error of n is only about 10%.