Influence of alloying elements and aging treatment on the phase transformation and shape memory behavior of Cu-Al-Ni shape memory alloys

Abstract

Nickel-Titanium (Ni-Ti) shape memory alloys (SMAs) have been used in many engineering and medical applications. However, their use is limited, due to their low transformation temperatures, difficulties in processing and high cost of the raw materials. As an alternative material to Ni-Ti alloys, copper-based alloys are successfully being used. Among copper-based SMAs, Cu-Al-Ni alloys are used in a wide range of applications, particularly if high temperatures are required. However, Cu-Al-Ni SMAs also have limitations such as very low ductility and low shape recovery strain. Therefore, this research aims to enhance the ductility and shape memory effect of Cu-Ni-Al by alloying additions and aging heat treatment. The base metal, Cu-Al-Ni, was cast without and with different amounts of the fourth alloying elements, namely, titanium (Ti), manganese (Mn) and cobalt (Co). The modified and unmodified alloys were homogenized and aged at 373 K, 423 K and 523 K for 24 and 48 hours. Phase transformation and microstructural changes were characterized using techniques such as optical microscopy, field emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS), differential scanning calorimetry (DSC), x-ray diffractometry (XRD) and transmission electron microscopy (TEM). The tensile properties and hardness were determined using a universal Instron tensile machine and Vicker‟s hardness test machine, respectively. The shape memory test was performed using a specially designed tensile machine equipped with a heating tape. The results revealed that the alloying elements and aging treatment were found to control the phase morphology, orientations and grain size along with the formation of precipitates, thereby improving the shape memory characteristics, ductility and hardness. The volume fraction, size and distribution of the precipitates are mainly dependent on the type and amount of alloying element as well as the condition of aging treatment. The Cu-Al-Ni with the addition of 0.76 wt. % Ti and age treated showed complete recovery after the shape memory test. This may be attributed to the high volume of X-phase precipitates and grain refinement that led to the restricted mobility of martensite variant interfaces and dislocations. It was found that the alloy with 1.14 wt. % of Co gave the best overall improvement in terms of the transformation temperatures, ductility and shape memory recovery. These improvements were mainly due to the exceptionally high content of the gamma-2 (γ2) phase in the microstructures of the modified alloy. Furthermore, the ductility of the Cu-Al-Ni SMAs increased from 1.65 to 7.0 % when 1.14 wt. % Co was added and the alloy undergone aging treatment where the fracture surfaces showed more ductile features and less brittle cleavages. It was also found that Cu-Al-Ni SMAs with 1.14 wt. % Co obtained full shape recovery after being aged at 523 K for 48 hours.