Effects of Ge Addition on the Microstructure and Thermal Stability of Co-V-Si High-Temperature Shape Memory Alloys

A study of the Ge substitution for Si in the Co₆₄V₁₆Si₂₀ shape memory alloy was performed in this work. The microstructure, martensitic transformation, thermal cycle stability, and microstructural evolution during thermal cycling of the Co₆₄V₁₆Si_20-xGe_x (x=2, 4, 6, 8, at%) alloys were studied. When x=2, the alloy was single D0₂₂ martensite. With increasing Ge content to 4 at% and 6 at%, two-phase microstructure of (αCo)+D0₂₂ martensite was observed. When Ge content was up to 8 at%, three-phase microstructure of (αCo)+D0₂₂ martensite+R phase was observed. The results show that the reversible martensitic transformation temperatures increase by nearly 50℃ through adding Ge compared with that of the Co₆₄V₁₆Si₂₀ matrix alloy. Although the (αCo) or R phases precipitate during thermal cycling process, the reversible martensitic transformation peaks are observed during ten times thermal cycles. It reveals that the Ge addition can improve thermal cycling stability of Co-V-Si high-temperature shape memory alloy.