N.F. Kennon

Aging Effects in Copper-Based Shape Memory Alloys

Aging of three copper-based shape memory alloys was studied by measuring the time dependence of hardness, martensitic transformation temperatures, lattice parameters, and shape memory capability at temperatures in the range 200 to 450°C. The ultimate loss of the shape memory effect in each alloy was preceded by changes in the other properties which resulted from thermally activated processes having activation energies in the range 60 to 80 kJ mol-1. At temperatures above about 300°C the aging process involved the eventual formation of α and γ2 phases. Although the activation energy appears to be insensitive to temperature and alloy composition, at lower temperatures other thermally activated processes, such as change in the type or degree of order, may, at least in the initial stages, be significant aging phenomena.

Factors influencing shape memory effect and phase transformation behaviour of Fe–Mn–Si based shape memory alloys

Abstract The objective of this research work was to investigate the factors influencing the shape memory effect and phase transformation behaviour of three Fe–Mn–Si based shape memory alloys: Fe-28Mn-6Si, Fe-13Mn–5Si-10Cr-6Ni and Fe-20Mn-6Si-7Cr-1Cu. The research results show that the shape memory capacity of Fe–Mn–Si based shape memory alloys varies with annealing temperature, and this effect can be explained in terms of the effect of annealing on γ ↔ e transformation. The nature and concentration of defects in austenite are strongly affected by annealing conditions. A high annealing temperature results in a low density of stacking faults, leading to a low nucleation rate during stress induced γ → e transformation. The growth of e martensite plates is favoured rather than the formation of new e martensite plates. Coarse martensite plates produce high local transformation strains which can be accommodated by local slip deformation, leading to a reduction in the reversibility of the martensitic transformation and to a degradation of the shape memory effect. Annealing at low temperatures (≤673 K) for reasonable times does not eliminate complex defects (dislocation jogs, kinks and vacancy clusters) created by hot and cold working strains. These defects can retard the movement and rearrangement of Shockley partial dislocations, i.e. suppress γ → e transformation, also leading to a degradation of shape memory effect. Annealing at about 873 K was found to be optimal to form the dislocation structures which are favourable for stress induced martensitic transformation, thus resulting in the best shape memory behaviour. Transmission electron microscopy observations supported the concept that the regular overlapping of stacking faults can result in the formation of bulk e martensite plates. Stacking faults were also found to exist in e martensite plates, and it is inferred that these faults can act as embryos for e → γ reverse transformation.

A study of the abrasive wear of carbon steels

Abstract The abrasive wear behaviour of 0.10%–1.4% carbon steels heat treated to various micro-structures and hardnesses was studied using a pin-on-drum machine. For constant hardness and carbon content less than 1.0%, the results show that bainite had the highest wear resistance, followed by tempered martensite and annealed structures. For 1.2%C steel, the annealed structure had wear resistance superior to the quenched and tempered structure and spheroidized structure. Additionally, the relationship between relative wear resistance and hardness was linear for annealed steels, but the slope for hypoeutectoid steels was lower than for hypereutectoid steels. A non-linear relationship between wear resistance and hardness of tempered martensite was confirmed for both 0.38%C and 0.75%C steels. This behaviour indicates that abrasive wear resistance is not simply related to the hardness of materials, but is determined also by the microstructure and fracture properties. Microscopical studies showed the dominant wear mechanism to be microcutting with significant microploughing for very low carbon hypoeutectoid steel, and substantial cracking and spalling in higher carbon steels and in quenched and low temperature tempered medium carbon steels.

The Morphology and Mechanical Properties of Bainite Formed from Deformed Austenite

Deformation of austenite containing 0.85 pct C is shown to significantly increase the ten-sile strength of bainite formed during subsequent transformation. Quantitative metallo-graphic measurements indicate that strengthening is due primarily to an increased dislo-cation density in the ferrite and reduced carbide size, with consequent finer distribution, compared with the bainite formed from undeformed austenite. It is also shown that de-formed austenite transforms to upper bainite at temperatures at least as low as 200°C due to enhanced nucleation and/or growth at slip band heterogeneities generated by the defor-mation process while the only effect on the formation of lower bainite is a retardation of the transformation and reduction of ferrite plate size.

Schematic transformation diagrams for steel

This paper examines the diffusional transformations of austenite and concludes that separateC-curves are required for pearlite, upper bainite, lower bainite and isothermal martensite. A schematic isothermal transformation diagram incorporating the four curves is presented for a plain carbon eutectoid steel and used to develop a schematic continuous cooling transformation diagram. These diagrams are shown to be more compatible with the available experimental information than are the usual diagrams based on a single transformation curve.

Isothermal transformation of austenite to pearlite and upper bainite in eutectoid steel

Austenite containing 0.80 pct C, 0.77 pct Mn is shown to transform isothermally to both pearlite and upper bainite at temperatures between 400 and 600 °C according to independent overlapping C-curves in the isothermal transformation diagram.

Thermal characteristics of a nitinol heat engine

Temperature profiles in the nitinol working elements of a simple rocking shape-memory engine have been measured and attributed to the heat transfer characteristics of the system. The temperature extremes evidently lie between the reverse transformation temperaturesAs andAf for the constrained wires, indicating that the device functions under the driving force generated by stress-induced formation (and reversion during heating) of a small volume fraction of martensite. Metallographic observations of relief effects associated with changes in the microstructure of wires simulating the working elements are consistent with this hypothesis.