VおよびNb添加によるCu-Zn-Al形状記憶合金のβ結晶粒微細化と機械的性質の変化

Abstract

Effects of V and Nb addition on the β-grain refining and mechanical properties of Cu-Zn-Al shape memory alloys were investigated by optical microscopy, tensile testing, X-ray analysis and fractography. It was found that the V addition is very effective in reducing β-grain size, and the mean grain size less than 100 μm was obtained by only 0.4 mass%V addition. The Nb addition was also effective in reducing β-grain size and, in this case, more than 1% addition was needed to obtain the mean grain size less than 100 μm. The β-grain size is sensitive to thermomechanical treatments of alloys, and the warm-rolling in the α+β two phase region at about 823 K followed by recrystallization at 973 K was found to be the most effective in both the alloy systems. The minimum β-grain size 60 μm for the V-added alloy and 85 μm for the Nb-added alloy were obtained in the present investigation. Influence of the β-grain size on the mechanical properties was investigated by tensile testing. It was found that the 0.2% offset stress above Ms temperatures of each alloy depends on the β-grain size and shows the Hall-Petch relation with the β-grain size for the specimens having the mean grain size less than 200 μm. Storable energy during the pseudoelastic deformation cycle increases with decreasing β-grain size, while the percentage of energy loss corresponding to the hysteresis loop of the stress-strain curve decreases with decreasing grain size. The latter, however, increased with increasing amount of alloying elements. The SEM observation of the tensile fracture revealed that the grain boundary cracking was remarkably suppressed by the V and Nb addition and the fracture mode changed from the brittle fracture to the ductile fracture with decreasing β-grain size.