The Elastic Properties and Thermal Expansion of Invar-Type Mn–Ge Base Ternary Alloys

Abstract

The magnetization, Young’s modulus and thermal expansion at 150–600 K and crystal structures at room temperature were investigated in the Mn–Ge based ternary alloys containing Ti, V or Cr less than 14%, which had various crystal structures i.e., γ (14% Ge), γ+ε(16–20% Ge) and ε(22% Ge) phases.
Magnetization values of the ternary alloys attainable are 0.4−0.5×10⁻⁶Wb·m·kg⁻¹ in the range of temperatures measured. The temperature dependence of magnetization of the ternary alloys shows a tendency similar to that of the basal binary alloys and distinct variations at T_t, corresponding to the β\ ightleftarrowsγ or the β\ ightleftarrowsε reversible phase transformation and at T_N(ε) and/or T_N(γ) corresponding to the antiferromagnetic Néel points of the ε phase and the γ phase, respectively. At each point of T_t, T_N(ε) and T_N(γ), both Young’s modulus and thermal expansion show remarkable variations. The Elinvar properties are observed in the temperature range between T_t and T_N(ε) or in the range below T_N(γ), and a small thermal expansion coefficient is obtained in a temperature range below T_N(ε) in the alloys having the ε or ε-rich phase. A very small temperature coefficient of Young’s modulus in the neighborhood of room temperature is obtained over a wider composition range in the Mn–Ge–Ti system than in the alloys containing V or Cr. Unlike the case of elasticity, Mn–Ge–Cr alloys show a small thermal expansion coefficient over a wider composition range than in the other alloys. In the case of the Mn–Ge–V alloys, there is a certain composition range where both the thermal expansion coefficient and the temperature coefficient of Young’s modulus become very small at room temperature. Therefore, the temperature coefficient of the ultrasonic delay time shows a relatively small value.