SEM ANALYSIS OF FRACTURE SURFACE OF THE CuAlNi SHAPE MEMORY ALLOY AFTER HEAT TREATMENT

Ivana Ivanić, Stjepan Kožuh, Andrej Vračan, Borut Kosec, Mirko Gojić
unpublished
CuAlNi shape memory alloys (SMA) belongs to a group of alloys that exhibit shape memory effect which is related to the thermoelastic martensitic transformation. In this paper the influence of heat treatment (quenching and tempering) on fracture surface morphology of the CuAlNi SMA after tensile testing was examined by scanning electron microscope. It is observed a small difference on alloys mechanical properties and surface morphology between casted and quenched (850°C/60'/WQ) condition. But
more » ... ) condition. But after tempering at 300 °C/60'/WQ the mechanical properties and surface morphology is drastically changed. In both states (casted and quenched) can be noticed a transgranular type of fracture showing that the small amount of plastic deformation occurred, and along long oriented grains it is observed an intergranular type of fracture. After tempering the CuAlNi sample shows mostly intergranular type of fracture after tensile testing. Keywords: shape memory alloys, CuAlNi, heat treatment, fracture analysis, microstructure 1. INTRODUCTION Shape memory alloys (SMAs) are a special class of multi-functional materials. These materials have the ability to undergo shape change and then recover to its original shape under the influence of external stimuli, which could be mechanical, thermal, electrical, or magnetic [1]. Among many alloy systems which exhibit shape memory effect, the most familiar are Ni-Ti based and Cu-based (Cu-Al-Ni and Cu-Zn-Al) shape memory alloys which have been studied over the years extensively [2]. Although Ni-Ti is the most widely used shape memory alloys for technological applications, Cu-based alloys have been used as an excellent alternative because they offer a wide range of transformation temperatures, a large superelastic effect, small thermal hysteresis and high damping coefficient. Cu-Al-Ni SMAs have been used in a wide range of applications, especially when the high transformation temperatures are required, due to their high thermal stability and high transformation temperatures. On the other hand, these alloys have some disadvantages like low reversible deformation and high brittleness and the reason for that behavior lies on the intergranular breakdown at low stress rate [3]. The main problems in the actual utilization of Cu-based SMAs are mainly due to their low thermal stability and unsatisfactory mechanical strength. They often suffer from martensite stabilization and finally lose the thermoelastic properties. Moreover, intergranular cracking usually occurs in Cu-based SMA during the manufacturing process and in service, hence improving the thermal stability and the mechanical properties are important issues for the prospect of Cu-based shape memory alloys [4]. The Cu-Al-Ni alloy is prone to intergranular fracture because this alloys have high elastic anisotropy and large grain size (often in order of 1 mm), and also because of the existence of brittle γ2 (Cu9Al4) phase. The problem with large grain size can be resolved by using elements which are grain size refinements or by producing an alloy with rapid solidification techniques. Mechanical properties improvement can be provided by adding alloying elements and by heat treatment [5]. The heat treatment procedure has an influence on shape memory properties. Even the small changes in shape memory effect (SME) might worsen the applicability of the alloy. The martensitic transformation and the associated mechanical shape reversibility in Cu-based SMAs are strongly influenced by quenching and aging treatments [6]. This paper studies the influence of heat treatment (quenching and tempering) on mechanical properties and fracture surface morphology of the alloy. The results obtained after heat treatment will be compared with the results obtained on the sample in as-cast state. 2. EXPERIMENTAL Cu-12.8%Al-4.1%Ni (wt.%) shape memory alloy was produced by melting pure elements (99.9 %) in vacuum induction furnace and casted by vertical continuous casting procedure, Figure 1. A bar of 8 mm in diameter
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