Effect of grain size on the pseudoelastic properties of Cu-20.8Zn-6.1Al shape-memory alloy

Abstract

The fl-phase in copper alloys is prone to rapid grain growth. Coarse grain sizes should be avoided: they decreases the uniformity and reproducibility of properties, they can cause pseudoelastic brittleness, due to the high compatibility stresses between neighbouring grains leading to intergranular fracture, and they decrease the stress-corrosion resistance [1]. The grain size produces changes in the transformation temperatures and in the stressinduced martensite. The applied stress necessary to stress-induce martensite also depends on the orientation of the parent with respect to the tensile axis. Thus, different orientations of single crystals will give different values of stress [2]. In this work the martensitic transformation temperatures and stress-induced martensite were studied in relation to grain growth for different heattreatment temperatures and times. Linear decreases in the Ms and As temperatures and a linear increase in the critical stresses with respect to the grain growth were observed. Grain size parameters were obtained by an image analysis technique [3] in /3-phase alloy at room temperature with the chemical composition 20.8 wt % Zn and 6.1 wt % A1. Thirty slices were cut from the same bar, measuring 5 mm in diameter and 4 mm in height. Two of these were used as reference samples, whereas the rest were subjected to different heat treatments at 750, 800, 850 and 900 °C and for 3, 5, 10, 15, 20, 30 and 60 rain at each temperature. For each sample after heat treatments the Ms and As transformation temperatures were determined. The calorimetric system used was described in previous papers [4-6]. The flow calorimeter measures differential signals by means of Melcor thermopiles. The temperature was measured by means of a standard Pt-100 probe.