FCC/HCP Martensitic Transformation and High-Temperature Shape Memory Properties in Co-Si Alloys

The microstructure, martensitic transformation temperatures and shape memory properties of Co-Si binary alloys were investigated by means of optical and transmission electron microscopy, differential scanning calorimetry and a shape memory test. The ε martensite phase with a hexagonal close-packed (hcp) structure was observed to coexist with the face-centered cubic (fcc) γ parent phase at room temperature. The γ⁄ε martensitic transformation temperatures linearly increased with Si content up to 4 mol%, but the trend flattened beyond 4 mol%Si while the transformation hysteresis and intervals monotonously increased. The Co-Si alloys exhibited shape recovery at high temperatures up to 900°C and a relatively high thermal stability up to 600°C. These results suggest that Co-Si alloy system has a possibility for high-temperature shape memory alloys.