Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10⁻²/s to 10⁻¹/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.