Effect of betatizing time on the grain growth and transformation characteristics in rapid-solidified Cu-12.7Al-3.9Ni based SMA ribbons with different solidification structures

Abstract
The effect of betatizing time on the microstructure, grain growth rate and transformation characteristics of Cu-AI-Ni based shape memory alloy was investigated in rapid-solidified ribbons with equiaxed, columnar and fibrous structures. The initial grain sizes of the as-spun ribbons were 6, 2.5 and 0.5 m in the order of wheel speed of 10, 35 and 50 m/sec. However, the grain size after betatizing at 973 K for 10 ⁴ sec was almost same value at approximately 10 m in all ribbons due to the rapid grain growth rate of columnar and fibrous structures. Comparing the peaks with betatizing time at each wheel speed, the sharp peaks of the ribbons with wheel speeds of 10 and 35 m/sec became broader due to a decrease in the grain boundary area acting as a nucleation site. However, the broad peak of the ribbon with the wheel speed of 50 m/sec became normal by betatizing for 10 ² sec because of the disappearance of the parent phase retained in the as-spun state. The higher wheel speed was, the more the dM ₃/dt was increasing due to the rapid grain growth of columnar and fibrous structures by betatizing. The M ₅ temperatures of all the melt-spun ribbons after betatizing for 10 ⁴ sec were around 420 K, which came close to that of the mother alloy because of the high surface area/volume ratio of the melt-spun ribbon.