The effect of DO3 ordering on the parent ⇄ martensitic transformation in the CuZnAl shape-memory alloy

The character of D03 ordering resulting from different quenching rates and its effect on the characteristic martensitic transformation temperatures and shape-memory effect in the Cu-21.5 at % Zn-12.5 at % Al alloy has been analysed. ln room temperature water-quenched samples with a cooling rate 1000° C sec^–1 showing B2 long-range order and DO₃ Short-range order, e significant stabilization of martensite in the reverse transformation was observed. This stabilization was eliminated in the air-cooled samples with a cooling rate 20° C sec^–1 showing DO₃ long-range order. Mechanical tests revealed a more complete shape recovery in the air-cooled samples, when compared to room temperature waterquenched ones.     

The effect of quenching on the martensitic transformation in ß1 phase of Au-Cd alloys

The effect of quenching on the martensitic transformation mechanism in ₁ Au-Cd alloys has been investigated by measurements of the electrical resistivity and X-ray diffraction. In the case of the Au-47.5 at%Cd alloy, the ₂-martensite is the characteristic product under quenching conditions, but it always exists with the equilibrium ₂-martensite phase. Consequently, the _{1 2} and ₂transformations occur simultaneously during the heating and cooling cycles. The corresponding resistivity behaviour is very complicated and extremely sensitive to thermal treatments such as quenching temperature and thermal cycling. On the other hand, in the case of the Au-49.0 at%Cd alloy, only the _{1 2} transformation occurs even when quenched, and the transformation is unaffected structurally by quenching. A distinct resistivity anomaly, which is considered to be due to the disappearance of quenched-in vacancies, is observed in quenched alloys. Some important characteristics of this anomaly are determined. In particular, the quenching effect disappears when the specimen is heated above the temperature at which the resisitivity anomaly begins. This result suggests that the quenched-in vacancies play an essential role in the martensitic transformation process under quenching conditions.     

Coherency strains influence on martensitic transformation of γ-Fe particles in compressed Cu-Fe alloy single crystals

Single crystals of a Cu-Fe alloy, which contained spherical -Fe particles, were compressed up to shear strain of 0.17 in liquid nitrogen bath. In the process of straining the structural (optical and TEM) observations in as-deformed and post-deformation annealed samples were provided. The substructure of deformed samples was characterized by slightly developed cell structure and lack of distinct layer-like arrangements of dislocations. Three kinds of particles were found: coherent and semi-coherent f.c.c. -Fe and martensiticly transformed b.c.c. -Fe. The critical diameter for coherency loss was found to be 58 nm at the initial stage of deformation decreasing with strain to 50 nm. These values coincided with the theoretical estimations. It was suggested that relaxation of coherency strains around -Fe particles by the generation of interface dislocations might occur prior to the martensitic transformation. This assumption might explain the particle size dependency of martensitic transformation.     

Microcrack nucleation and its effect on the plastic deformation of FL γ-TiAl alloy

Microcracks and their effect on the plastic deformation of -TiAl alloy with fully lamellar microstructure (FL) at relatively low strain rate (1 × 10^–5/s) has been investigated. It is found that a large number of microcracks nucleate within the grains. The microcrack density increases with the increase of grain size. Most of the microcracks nucleate at the /₂ interfaces and gather at grains with soft-orientations. Based on the observation and analysis, a model of microcrack nucleation in FL -TiAl alloy is built up. The plastic elongation of -based TiAl alloys with FL microstructure changes non-monotonously with the increase of grain size, which results from cooperation of micro-deformations and microcracks.     

Investigation on the martensitic transformation in γ-Fe(n) nanoparticles

γ-Fe(5.90–10.9 at.% N) nanoparticles ranging in diameters from 30 to 100 nm were prepared by laser-induced pyrolysis of mixtures of Fe(CO)₅ and NH₃. During the quenching from high temperature to room temperature and even down to 4.2 K, no martensitic transformation occurred in these particles. Based on the effects of surface tension and refinement of grain size on the yield strength of the nanoparticles, the dependencies of nucleation driving force and the martensite-start temperature on the particle size was revealed, from a viewpoint of thermodynamics of martensitic transformation in the γ-Fe(N) nanoparticles. It is suggested that the yield strength increment due to the surface tension and to the very fine crystallite grains is probably the predominant aspect responsible for inhibiting the nucleation of martensite and drastic decrease of the martensite-start temperature of the nanoparticles. High pressure of 0.5–4.0 GPa were adopted to induce the martensitic transition of the γ-Fe nanoparticles. It is found that the extent of martensitic transformation increased with the increasing pressures, most individual γ-Fe particles were partially transformed to martensite after pressurization.     

The effect of δ alumina fibre arrays on the age-hardening characteristics of an Al-Mg-Si alloy

The effect of alumina fibre arrays on the age-hardening characteristics of an Al-Mg-Si alloy (6061) has been investigated by means of hardness and electrical resistivity measurements, and optical, scanning and transmission electron microscopy. It is shown that the fibre array can have a considerable effect on the age-hardening response of the matrix alloy in metal matrix composites, causing suppression of GP zone formation which inhibits natural ageing and considerably reduces the peak hardening produced during artificial ageing. The reduced hardening potential of the composites during artificial ageing is shown to result from a competition between GP zone formation and heterogeneous nucleation of the intermediate precipitate on lattice defects. The most probable cause of both phenomena is shown to be lack of quenched-in vacancies following solution treatment, due to the availability of a large number of vacancy sinks at the fibre-matrix interfaces.     

Grain size effect on deformation twinning in Mg–Al–Zn alloy

Abstract

Specimens of Mg–3Al–1Zn alloy with a wide range of grain size distribution were compressed along different directions. The compressed microstructure was examined to clarify the grain size effect on deformation twinning in magnesium alloys. Small strains were used to reveal the twinning behaviour. The results show that the grain size affects the formation of deformation twins in an Mg–Al–Zn alloy. The reason for a different result being previously reported is given. This study also reports the different deformation microstructures in specimens compressed along different directions.     

Study of the martensitic transformation in NiTi–epoxy smart composite and its effect on the overall behavior

In this work, the effect of wire phase transformation on the overall thermo-mechanical behavior of NiTi–epoxy composites has been investigated. The shape memory wire received in as drawn condition was subjected to three heat treatments which results to different transformation characteristics. Composite specimens were manufactured by casting followed by curing and post curing process. The mechanical behavior of samples has been determined using standard tensile test. The effect of wire volume fraction and test temperature was investigated as well.It is found that the martensitic transformation occurring in the wire affects the mechanical behavior of the composite specimens. In this way, using the wire with higher transformation stress improves the composite tensile strength. This is achieved either by increasing the test temperature or by using the wires heat treated at lower temperatures. From the experimental results, the martensitic transformation can change the debonding mode. It seems that on the constraint of matrix, the transformation occurs simultaneously at several points in wires that result in regular debonded/undebonded pattern.     

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the + phase field at 900 C. The microstructures obtained at room temperature after cooling from 900 C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the -phase, molybdenum and silicon to the -phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the -phase and the resultant microstructure. In quenched material containing transformed , substantial age hardening can be obtained in the range 350 to 600 C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.     

The effect of γ-irradiation on the optical properties of polyoxymethylene compacts

The effects of -irradiation on the optical properties of polyoxymethylene (POM) compacts are studied at room temperature. A UV-visible spectrometer is used to study the optical absorbency in the wavelength range 200–1100 nm. The optical energy of both direct and indirect transitions as well as the energy band tail E is determined as a function of exposure dose. The results obtained showed that the POM compacts became brittle as the -radiation dose reaches 5 Mrad. In the meanwhile, the wavelength of the optical band tail g increases to a maximum at 2 Mrad and the energy band tail E reaches 5 eV at the same dose. Both direct and indirect energies of transition decrease with the irradiation dose. The results can be explained on the basis of -irradiation-induced changes due to cross-linking in the POM.     

The stress-induced martensitic transformation and superelasticity in a βCu–Zn alloy containing α1 plates

The stress-induced martensitic transformation and superelasticity effect in a βCu–Zn alloy containing α₁ plates have been investigated. The results showed that the α₁ plates have no obvious effect on the deformation behavior in the aged specimens. The deformation behavior was dominated by the formation of stress-induced martensite (SIM). The cyclic loading–unloading behaviors and characteristics of the aged specimens were dependent on the test temperatures, which are similar to that of single-phase specimens. In addition, the mechanism of superelasticity in the alloy containing α₁ plates was not affected by the presence of the α₁ plates. The α₁ plates remain unchanged during stress cycling, which was independent of the test temperatures. The interface of SIM moved smoothly and was not hindered by the α₁ plates during the reversible martensitic transformation on loading–unloading cycle. Upon loading, the SIM cannot penetrate the α₁ plates ahead of them during their nucleation and growth, but may form the same variant on both sides of the α₁ plate at the same time or in sequence.     

Mössbauer study of martensitic transformations in an Fe-29.6% Ni alloy

The application of an external stress may form band shaped strain-induced martensites in the austenite structure of Fe alloys. Mössbauer spectroscopy and transmission electron microscopy techniques were used to clarify certain properties of strain-induced martensite in an Fe-29.6% Ni alloy. The reverse transformation mechanism between thermal plate martensite and the matrix austenite was also studied. Mössbauer spectroscopy made it possible to examine the same area of the austenitic thin foils during the thermal cycles, and the volume fraction changes were determined. The habit plane and orientation relationship of strain-induced martensite were measured from the electron diffraction patterns and the latter parameter was found to be K-S type as with thermal plate martensites of the Fe-Ni alloys. The isomery shifts caused by the deformation and cycling procedures were also calculated for both austenite and martensite structures and the hyperfine magnetic field parameter of Fe-29.6% Ni strain-induced martensite was found to be equal to that of Fe-Ni-C alloys reported earlier.     

The effect of cooling rate on α-phase ordering in Cu-12.4wt% Al alloy

The formation of a superlattice in the-phase of Cu-12.4wt% Al alloy was studied during cooling. Specimens cooled at different rates were examined using electron microscopy and differential thermal analysis. The superlattice structure formed was described by means of theD-parameter which determines the position of superspots in the reciprocal lattice. Variation of theD-parameter with cooling rate has a linear form, hence it may be concluded that the superlattice in the-phase is formed as a transitional structure. The relationship between theD-parameter and enthalpy suggests that the latter can be taken as a measure of superstructure development.     

The effect of strontium on the mechanical properties of aluminum–silicon alloy

Technical Physics Letters - We have studied the influence of strontium additives on the microstructure and mechanical properties of an aluminum alloy with 15 wt % silicon prepared by directional...