Physical metallurgy of Ti–Ni-based shape memory alloys

The uniaxial behavior of a nearly equiatomic NiTi alloy is studied experimentally. Experiments are conducted in a temperature and deformation regime in which the alloy exhibits the shape memory effect and pseudoelasticity. These characteristics are due to the displacive nature of transformation between the two major phases of the material, austenite and martensite, and to the fact that in this alloy martensite accommodates deformation by twinning. A series of uniaxial experiments is conducted on NiTi wire at temperatures in the range of approximately −20 to 100 °C where the fundamental material response changes drastically. In addition, the loading rate and the choice of ambient medium were found to have a significant influence on the recorded stress-strain responses due to a complex interaction between the inherent mechanical properties of the material and the prevailing heat transfer conditions of the experiment. It is demonstrated that local measurements of strain and temperature can help clarify events that take place at different stages of a typical loading-unloading history. These local measurements are used to track the movement of the interfaces between phases during stress-induced phase transformations at different loading rates.

Thermomechanical aspects of NiTi

Recent developments in the research of shape memory alloys

Deformation behavior associated with theR-phase (rhombohedral phase) transition and the subsequent martensitic transformation was studied systematically in Ti-Ni alloys by tensile testing over a wide temperature range covering belowM
 f to aboveT′
 R (>Af). Since the deformation and transition characteristics showed a strong dependence on thermo-mechanical treatment and Ni-content, internal structures were examined by electron microscopy in specimens with various Ni-content and thermo-mechanical treatment. As a result precipitates and/or dislocations were revealed in the specimens in which theR-phase transition occurs. Based on the above results, the effects of thermo-mechanical treatment and Ni-content on the deformation and transition characteristics were clarified for both theR-phase transition and the martensitic transformation.

Deformation and transition behavior associated with theR-phase in Ti-Ni alloys

Effect of thermal cycling on the transformation temperatures of TiNi alloys

The transformation behaviour of TiNiFe alloys of nominal composition Ti50Ni47Fe3 has been studied by transmission electron microscopy and diffraction, and electrical resistance measurements between room temperature and −196°C. Based on the present results and complementary neutron and X-ray diffraction data, charge density wave (CDW) phenomena and associated phase transitions were found to be involved in the ‘premartensitic’ behaviour of the Ti50Ni47Fe3 alloys. The crystal structure of the high-temperature parent phase was confirmed to he the CsCl (B2) structure with a0, = 3·02 A. On cooling from room temperature, 1/3(110) and 1/3(111) superlattice reflections appeared, coincident with the onset of the electrical resistance anomaly (increase). The ‘1/3’ superlattice reflections were observed to intensify with decreasing temperature, and close inspection showed that such reflections deviated slightly from the exact 1/3 positions relative to the parent CsCl structure, suggesting the formation of an...

Transformation behaviour of a Ti50Ni47Fe3 alloy I. Premartensitic phenomena and the incommensurate phase

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-lik...

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

Premartensitic phases of Ti50Ni47Fe3.

The premartensitic phases of TiNi  (Fe) have been studied by means of neutron diffraction, specific heat, and electron microscopy. Evidence for an incommensurate charge density wave has been found, with a wavevector near, but not at, (h/3, k/3, 0) positions. A lock‐in transition occurs at lower temperatures, accompanied by the formation of needle‐shaped domains oriented along [110] directions.

A netron scattering study of premartensitic phases of TiNi (Fe)

The transformation behavior of TiNiFe alloys of nominal composition Ti50Ni47Fe3 has been studied by transmission electron microscopy and diffraction, and electrical resistance measurements, between room temperature and -196°C. Based on the present results and complementary neutron and x-ray diffraction data, charge density wave (CDW) phenomena and associated phase transitions were found to be involved in the "premartensitic" behavior of the Ti50Ni47Fe3 alloys. The crystal structure of the high temperature parent phase was confirmed to be the CsCl(B2) structure. Upon cooling below room temperature, 1/3(110) and 1/3(111) superlattice reflections appeared, coincident with the onset of an electrical resistance anomaly (increase). The "1/3" superlattice reflections were observed to intensify with decreasing temperature, and close inspection showed that such reflections deviate slightly from the exact 1/3 positions relative to the parent CsCl structure, suggesting the formation of an incommensurate superlattice over a 12°C temperature range. The appearance of the "1/3" superlattice reflections is interpreted to be associated with the formation of CDW's. A second "premartensitic" effect was found to occur approximately 12°C below the onset of the initial "normal-to-incommendurate" transition. 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 directions, allowing the lattice and CDW's to lock-in and become commensurate.

https://hal.archives-ouvertes.fr/jpa-00222144/document

M. Meichle

Papers

Transformation behaviour of a Ti50Ni47Fe3 alloy I. Premartensitic phenomena and the incommensurate phase

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

Premartensitic phases of Ti50Ni47Fe3.

A netron scattering study of premartensitic phases of TiNi (Fe)

TRANSFORMATION BEHAVIOR OF Ti50Ni47Fe3 ALLOY :I. INCOMMENSURATE AND COMMENSURATE PHASES