D. P. Dunne

The Effect of Austenite Ordering on the Martensite Transformation in Fe-Pt Alloys Near the Composition Fe3Pt: I. Morphology and Transformation Characteristics

Fe-Pt alloys near the composition Fe3Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing theMs temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography, cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree, the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each cooling cycle at approximately the same temperature.

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.

Sintering and grain growth of 3 mol % yttria zirconia in a microwave field

A comparative study of the sintering and grain growth of 3 mol% yttria zirconia using conventional and microwave heating was performed. Extensive measurements of grain size were performed at various stages of densification, and following isothermal ageing at 1500 °C for 1, 5, 10 and 15 h. Microwave heating was found to enhance densification processes during constant rate heating. The grain size/density relationship for the microwave-sintered samples was shifted in the direction of increased density for density values less than 96% of the theoretical value when compared to conventionally heated samples. This suggests that there may be a difference in the predominant diffusion mechanisms operating during the initial and intermediate stages of sintering. Results of the ageing experiments showed that once densification was near completion, grain growth was accelerated in the microwave field, and exaggerated grain growth occurred.

Influence of heating rate on anisothermal recrystallization in low and ultra-low carbon steels

Abstract High-rate annealing experiments were carried out to study the effect of heating rate on the recrystallization kinetics, grain size and texture of steels with a range of carbon levels (0.003–0.05% C). The steels were cold-rolled to 70% reduction and subsequently annealed at heating rates from 50 to 1000°C s −1 to peak temperatures ( T p ) in the range 600 to 900°C. To minimise masking effects associated with rapid isothermal recrystallization at high temperature, all samples were water-quenched within 0.05 s of reaching T p . For the steels investigated, the rate of anisothermal recrystallization and the final grain size decrease with increasing heating rate. These results do not support previous work in which it was concluded that ultra-rapid softening (which was associated with an observed decrease in recrystallization temperature at high heating rate and increase in grain size) occurs at heating rates in excess of 500°C s −1 . The present results, together with simulation using an anisothermal annealing model, indicate that ‘ultra-rapid softening’ is likely to be an artefact of these earlier experiments.

Cross-sectional transmission electron microscopy characterisation of plasma immersion ion implanted austenitic stainless steel

Abstract Cross-sectional transmission electron microscopy (XTEM), selected area diffraction (SAD) and nano-beam diffraction (NBD) techniques were used to investigate the surface microstructure of 316 stainless steel, implanted with high doses of nitrogen ions at 150, 250, 350, 450 and 520 °C using plasma immersion ion implantation. It has been found that the treatment temperature has a strong influence on the evolution of the microstructure. An amorphous layer of about 1 μm thick with a heavily stressed substrate underneath was observed on the 150 °C implanted sample. Both the 250 and 350 °C implanted samples showed a thin nanocrystalline sublayer at the outermost surface and an amorphous sublayer between the nanocrystalline sublayer and the substrate. A thick amorphous layer up to 3 μ thick was formed on the 450 °C implanted sample whereas at 520 °C, cellular precipitation of CrN and α-ferrite dominated the system. It is suggested that a solid state chemical reaction and the poor mobility of the reactant atoms are the key factors for the solid state amorphisation by nitrogen ion implantation into austenite.

The crystallography of ferrous martensites

The applicability of the phenomenological crystallographic theory to martensitic transformations in ferrous alloys is discussed, and it is concluded that only for the {3, 15, 10}F and {252}F transformations are experimental data sufficiently complete for detailed comparisons with predictions of the theory. Since crystallographic measurements have proved more difficult to obtain in the {252}F case a detailed assessment of electron microscopy observations and of reported measurements of the shape strain and orientation relationship is given for this transformation. The basic theory introduced, in the 1950s accounts satisfactorily for the {3, 15, 10}F transformation but not for the {252}F, case. Recent modifications of the theory based on multiple lattice invariant shears are also reviewed and are examined in the light of the crystallographic measurements. The use of experimental information to infer possible inhomogeneous deformation modes is emphasized.

Plasma nitriding of microalloyed steel

Abstract 3icroalloyed or high strength low alloy (HSLA) steels are carbon-manganese steels containing small amounts of Nb, V or Ti. The excellent mechanical properties of these alloys, particularly high yield strength, usually obviate the need for expensive quench and tempering operations. Furthermore, the presence of a significant amount of nitride-forming elements in some microalloyed steels has generated interest in the applicability of these alloys as a new generation of nitriding steels. In this paper, a study of the plasma nitriding behaviour of a commercially available microalloyed steel MAXIMATM is reported. A comparison is made with a traditional quenched and tempered nitriding steel (En19), plasma nitrided under similar conditions. Optical and scanning electron microscopy in conjunction with microhardness measurements and X-ray diffraction were utilized to characterize the nitrided surfaces. The observed differences in the thickness and structure of the compound layer and the diffusion zone are discussed in terms of chemical composition and microstructure of these steels.

Deformation and annealing of (011)[011̄] oriented Al single crystals

Abstract High purity Al single crystals of the (011)[01 1 ] orientation have been deformed in plane strain compression in a channel die. Deformation was carried out at a strain rate of 0.01 s −1 to true strains of 0.5 and 1.0, and at temperatures of 25, 200 and 300 °C. The as-deformed microstructure has been characterized using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). No recrystallization was detected after deformation, and the deformation texture analysis showed that the stability of the orientation decreased with increasing temperature, contrary to reports for other orientations. Annealing was carried out for various times at 300 °C. Nucleation of recrystallization exhibited periodicity, with distinct bands of recrystallized grains forming parallel to the transverse direction. This recrystallized microstructure has been examined using EBSD. A model is proposed to account for the origin of the periodicity of nucleation and the retention of rods or cylinders of unrecrystallized material after significant annealing times.

Atomic force microscope study of the interface of twinned martensite in copper-aluminium--nickel

Abstract Atomic force microscopic observations of interfacial structures of twinned γ1′ martensite plates in single crystals of Cu–14wt.%Al–3.4wt.%Ni alloy revealed the presence of either side-plate extensions or well-defined twin facets at the martensite/parent phase interface. These results indicate that there are alternative means of accommodating micro-scale mismatch strains at the interface.