X-phase precipitation in aging of Cu–Al–Ni–xTi shape memory alloys and its influence on phase transition behavior

Abstract

The present work aims to investigate the aging treatment effects on the microstructure, transition characteristics, and mechanical properties of Cu–Al–Ni–xTi shape memory alloys. The aging treatment of the homogenized and modified alloys of Cu–Al–Ni–xTi results in variations of the martensite morphology, volume fraction, and precipitate size. These variations in the features of precipitates have an obvious effect on transition behavior and mechanical properties. The transition temperatures are increased slightly with an increase in the aging time and temperatures; such increase is mainly attributed to changes in the γ′1 and β′1 morphology and distribution, along with the precipitates. The thermodynamic parameters (ΔH and ΔS) tend to decrease with an increase in the aging time and temperatures. Moreover, the brittleness, low strength, and poor processability of Cu–Al–Ni alloys greatly limit their application. We attempted to improve the mechanical properties without losing the thermoelastic properties through the additions of Ti and appropriate aging treatments. The highest tensile strength and strain of 640 MPa and 4.3 % were obtained in alloy of Cu–Al–Ni-0.7 mass% Ti aged at 250 °C for 24 h and 100 °C for 48 h, respectively. The shape memory behavior of the alloys was studied using a specially designed machine. The strain recovery by shape memory effect of the alloys increases with an increase in the aging tomes and temperature, where the highest recovery (100 %) is obtained with the alloy of Cu–Al–Ni-0.7 mass% Ti after being aged at 250 °C for 24 h.