Transformation behaviour of a Ti₅₀Ni₄₇Fe₃ alloy II. Subsequent premartensitic behaviour and the commensurate phase

Abstract

A second ‘premartenaitic’ effect in a Ti₅₀Ni₄₇Fe₃ alloy was found to occur approximately 12°C below the onset of the initial ‘normal-to-incommensurate’ transition described in the previous paper. This subsequent phase change involves a structural transition from a ‘distorted’ CsCl parent to a rhombohedral product, during which the ‘1/3’ superlattice reflections are shifted to precise 1/3 positions. The rhombohedral distortion is effected by a homogeneous expansion along 〈111〉 directions, allowing the lattice and CDWs to lock in and become commensurate. Microstructural changes associated with this structural transition involve the formation of four crystallographic variants of ‘needle’-like domains (three variants and the ‘matrix’). Each such domain has its unique rhombohedral axis, reflecting the rhombohedral distortion along one of the four 〈111〉 body diagonals of the cubic unit cell. The ‘needle’ domains and the matrix are twin-related with respect to a {110} twinning plane. Small antiphase-like microdomains are found within both the ‘needles’ and the ‘matrix’. Their size is independent of temperature. Electron diffraction patterns show that within a single needle domain 1/3(111) superlattice reflections lie along only one 〈111〉∗ direction, and the 1/3(110) super-lattice reflections lie only along those 〈110〉∗ directions which are normal to 〈111〉∗. The disappearance of the needle domains on heating shows some thermal hysteresis, indicative of a first-order phase change. The above phenomena are interpreted in terms of a first-order incommensurate-to-commensurate structural change, with the formation of twins leading to a reduction in long-range strain distortion.