Development of High Strain Rate Superplastic Aluminum-SiC Particulate Composits

采用机械合金化工艺制备了体积分数分别为5%、10%和15% （MA7075+5SiC、+10SiC、+15SiC复合材料）的SiC颗粒增强7075铝合金。在793 K.1的初始应变速率范围为10^<-5> ^<-1>到2 × 10 s^<-1>时，研究了这些复合材料的超塑性性能。预应变后，SiC颗粒在基体内均匀分布，形成极细的晶粒结构，平均晶粒尺寸小于1 μ m。在三种复合材料的对数流动应力-对数应变率曲线中，识别出两个应变率区域，分别为应变率敏感指数m的低应变率和高应变率区域，即区域1和区域2，这与超塑性中通常观察到的情况相同。在5 × 10^<-1>到2 × 10^<-1>（区域2）的应变速率下，3种复合材料的m值均大于0.36，超塑伸长率均大于200%。相比之下，在较低应变率下（区域1），m值小于0.13，出现低伸长（< 50%）。随着SiC颗粒含量的增加，2区向应变速率较高的区域移动。区2晶界滑动比区1晶界滑动更明显。采用外推法估算的阈值应力sigma_<th>随SiC含量的增加而增大。对于MA 7075+SiC复合材料，应变率与有效应力的双对数图sigma_e （=sigma-sigma_<th>）可以用斜率约为0.5的直线近似表示。复合材料的高应变速率超塑性可以用位错滑移调节的晶界滑动模型来解释。晶界上的SiC颗粒能够抵抗晶界滑动，从而产生阈值应力sigma_< >。

7075 aluminum alloys reinforced with SiC particulates, of which volume fractions are 5,10, and 15 % (MA7075+5SiC,+10SiC,+15SiC composite, respectively), have been developed by using mechanical alloying process. Superplastic properties of these composites have been investigated at the initial strain rates ranging from 10^<-5> s^<-1> to 2x10 s^<-1> at a temperature of 793 K.1. After the pre-straining, SiC particulates are distributed uniformly within matrix, and a extremely fine grain structure, with average grain size less than 1 mu m is developed.2. In log flow stress-log strain rate curves for the three composites, two strain rate regions showing low and high strain rate sensitivity exponent, m, (region 1 and 2, respectively) were recognized, as in usually observed in superplasticity. At very high strain rates from 5x10^<-1> to 2x10 s^<-1> (region 2), values of m were larger than 0.36, and superplastic elongations were larger than 200% in the three composites. In contrast, m values were less than 0.13 at lower strain rates (region 1), where low elongations (< 50%)appeared.3. With increasing of the SiC particulate content, region 2 moved to the range of higher strain rate. Grain boundary sliding took place more remarkably in region 2 rather than in region 1.The threshold stress, sigma_<th>, estimated by using an extrapolation method increased with increasing of the SiC content.4. A double logarithmic plot of the strain rate and the effective stress, sigma_e (=sigma-sigma_<th>), can be approxi- mated by straight lines with a slope of about 0.5 for MA 7075+SiC composites.5. The high strain rate superplasticity of the composites can be explained by the grain boundary sliding model accommodated by dislocation slip. The SiC particulates on grain boundaries can resist the boundary sliding, thus resulting the threshold stress, sigma_<th>.