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

This review encompasses the story of the Verwey transition in magnetite over a period of about 90 years, from its discovery up to the present. Despite this long period of thorough investigation, the intricate multi-particle system Fe3O4 with its various magneto-electronic interactions is not completely understood, as yet - although considerable progress has been achieved, especially during the last two decades. It therefore appeared appropriate to subdivide this retrospect into three eras: (I) from the detection of the effect to the Verwey model (1913-1947), being followed by a period of: (II) checking, questioning and modification of Verwey's original concepts (1947-1979). Owing to prevailing under-estimation of the role of crystal preparation and qualitiy control, this period is also characterized by a series of uncertainties and erroneous statements concerning the reaction order (one or two) and type of the transition (multi-stage or single stage). These latter problems, beyond others, could definitely be solved within era (III) (1979 to the present) - in favour of a first-order, single-stage transition near 125 K - on the basis of experimental and theoretical standards established in the course of a most inspiring conference organized in 1979 by Sir Nevill Mott in Cambridge and solely devoted to the present topic. Regarding the experimental field of further research, the remarkable efficiency of magnetic after-effect (MAE) spectroscopy as a sensitive probe for quality control and investigation of low-temperature (4 K Tv) into a Wigner crystal (T<Tv), describes most adequately the various low-temperature mechanisms in Fe3O4 in terms of tunnelling and variable range hopping of small polarons. On the other hand, the well-elaborated Ihle-Lorenz model, assuming a superposition of polaron-band and -hopping conductivity, is in better agreement with the high-temperature data (Tv<T<600 K).

https://iopscience.iop.org/article/10.1088/0953-8984/14/12/203/pdf

The Verwey transition - a topical review

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

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

We describe an electron microscopic investigation of an as-quenched 63 at. % Ni-Al alloy. This material exhibits strong tweed contrast and associated diffraction effects, commonly called ‘premartensitic effects’. Anomalous diffraction effects in martensite-forming β3 Hume-Rothery phases have been studied extensively, but the mottled microstructures have received little attention. This paper describes an attempt to characterize the microstructure systematically. In addition to some well-known characteristics, we have discovered that in many ways tweed contrast resembles the fringe contrast observed on stacking faults. This includes the variation of fringe spacing with foil orientation, with the extinction distance of the diffraction vector and with the deviation from the Bragg condition. Such behaviour provides a basis for understanding the origin of premartensitic effects in Hume-Rothery phases, and tweed microstructures in general.

Tweed microstructures I. Characterization in β-NiAl

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.

In this second in a series of papers on tweed microstructures in phases of the Ni-Al system, we describe the electron microstructure of the 3R and 7R martensites, the γ phase and the newly discovered Ni5Al3 phase in an alloy of average composition 63 at.% Ni-Al. The alloy was subjected to ageing treatments at temperatures ranging from 300 to 850°C for periods of up to 15 days, which resulted in the precipitation of the γ and Ni5Al3 phases from the supersaturated β phase. The metastable Ni2Al phase has also been found to occur, and its effect on the tweed microstructure in the β phase is examined. We conclude that Ni2Al precipitation is not the cause of tweed in β-Ni-Al. The cubic γ phase exhibits a tweed microstructure almost identical to that of the β phase. The tweed microstructures of the tetragonal 3R martensite and orthorbombic Ni5AI3, differ from those of the cubic phases in ways which reflect their different symmetries. These differences provide further clues to the origin of tweed microst...

C. M. Wayman

Papers

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

Tweed microstructures I. Characterization in β-NiAl

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

Premartensitic phases of Ti50Ni47Fe3.

Tweed microstructures II. In several phases of the Ni-Al system