Advances in martensitic transformations in Cu-based shape memory alloys achieved by in situ neutron and synchrotron X-ray diffraction methods

This article deals with the application of several X-ray and neutron diffraction methods to investigate the mechanics of a stress induced martensitic transformation in Cu-based shape memory alloy polycrystals. It puts experimental results obtained by two different research groups on different length scales into context with the mechanics of stress induced martensitic transformation in polycrystalline environment

Relationship between the volume of the unit cell of hexagonal-close-packed Ti, hardness and oxygen content after α-case formation in Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy

In this study, the influence of oxygen diffusion on the physical properties of Ti–6Al–2Sn–4Zr–2Mo–0.1Si was examined. Measurements were carried out directly on sample cross sections which were preoxidized at high temperature. The lattice parameter evolution was measured using synchrotron X-ray diffraction and was coupled with microhardness and electron probe microanalyzer results with the aim of highlighting their relationships. The results show that the hardness and oxygen gradients along the oxygen diffusion zone in the alloy are similar to the evolution of the [alpha]α-phase unit-cell volume quantified by X-ray diffraction. Linear relationships were found between these three parameters

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

The heterogeneous precipitate microstructure of a AA2050 Al–Li–Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction and size of both T1 platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure

Measurement of lattice rotations and internal stresses in over one hundred individual grains during a stress-induced martensitic transformation

To better understand the properties of polycrystals at a microscopic scale during cyclic mechanical loading we have measured the relationship between grain orientations, their positions inside the sample and their internal stresses. In this work, in-situ 3DXRD technique was performed on over hundred grains during the stress-induced martensitic transformation in a Cu-Al-Be shape memory alloy. Information about the position, orientation, and stress field was obtained for each austenitic grain. These results have been used to develop a procedure that allows automatic processing for a large number of grains, matching them during loading and leads to a quantitative stress field. A strong heterogeneity of stress state between the grains at the surface and in the volume is evident

In situ synchrotron analysis of lattice rotations in individual grains during stress-induced martensitic transformations in a polycrystalline CuAlBe shape memory alloy

Two synchrotron diffraction techniques, three-dimensional X-ray diffraction and Laue microdiffraction, are applied to studying the deformation behaviour of individual grains embedded in a Cu74Al23Be3 superelastic shape memory alloy. The average lattice rotation and the intragranular heterogeneity of orientations are measured during in situ tensile tests at room temperature for four grains of mean size 1 mm. During mechanical loading, all four grains rotate and the mean rotation angle increases with austenite deformation. As the martensitic transformation occurs, the rotation becomes more pronounced, and the grain orientation splits into several sub-domains: the austenite orientation varies on both sides of the martensite variant. The mean disorientation is 1 . Upon unloading, the sub-domains collapse and reverse rotation is observed

Observation using synchrotron X-ray diffraction of the crystallographic evolution of α-titanium after oxygen diffusion

In this study, the effect of oxygen diffusion on the crystallographic evolution of α-titanium has been studied using synchrotron X-ray diffraction. Measurements were carried out directly on sample cross-sections that were directly pre-oxidized at high temperature. Changes in hardness, oxygen content and lattice parameters after oxidation were determined by coupling microhardness measurements and electron probe microanalyzer results

Microstructure distribution in an AA2050 T34 friction stir weld and its evolution during post-welding heat treatment

This paper presents a systematic study where the distribution of precipitate microstructures is mapped in the cross-section of a friction stir weld made with an AA2050 Al–Cu–Li alloy in the naturally aged temper, as well as the evolution of this microstructure during subsequent post-welding heat treatment (PWHT). This study is carried out using spatially resolved small-angle X-ray scattering, supported by transmission electron microscopy, differential scanning calorimetry and microhardness mapping. The as-welded microstructure is dominated by solute clusters, while very little precipitation has taken place during the welding operation. During PWHT, the precipitation kinetics in the different zones of the weld is mainly controlled by the local dislocation density inherited from welding, and by the amount of solute available for precipitation, which depends on the volume fraction of welding-induced intermetallics. Pre-deforming the weld before the PWHT results in a very effective strength recovery and a nearly homogeneous distribution of hardness

Solid-state phase transformation in a lithium disilicate-based glass-ceramic

The solid-state phase transformation in a lithium disilicate-based glass-ceramic (IPS e.max® CAD) was revisited on the basis of quantitative data. IPS e.max® CAD is widely used as material for the dental restoration in the dental industry. In-situ X-ray diffraction and differential scanning calorimetry accompanied by scanning electron microscopy observations were applied to understand phase transformation during heat treatment in a dental ceramic. In-situ X-ray diffraction evidences the concomitant formation of cristobalite and lithium orthophosphate at 770 °C. Then, the formation of lithium disilicate occurred at the expense of a complete dissolution of cristobalite and lithium metasilicate. No phase transformation occurred during cooling. The quantitative results of microstructural features (amount of each phase, morphology, and number density of lithium disilicate and lithium metasilicate) indicate that lithium disilicate is probably formed by diffusional process at the lithium metasilicate/cristobalite interface, which acts as favorable nucleation sites. The energy barrier is probably too high for lithium disilicate nucleation in the amorphous matrix. The quantitative results will provide the background for further modeling of phase transformation kinetics, which may have potential industrial benefits

Characterization of the thermo-mechanical properties of p-type (MnSi1.77) and n-type (Mg2Si0.6Sn0.4) thermoelectric materials

The thermoelectric generators reliability is highly sensitive to the materials they are made off. This work aims to characterize the structure, microstructure, emissivity and thermo-mechanical properties of p-type (MnSi1.77) and n-type (Mg2Si0.6Sn0.4) highly promising thermoelectric materials. In particular, the temperature dependence of the Young's modulus, evaluated by impulse excitation technique, shows for both materials values higher than those reported in literature. Fracture toughness measured by both single edge notched beam and indentation methods exhibits similar values to those reported in the literature. Micro-indentation measurements reveal that the hardness values of the MnSi1.77 are significantly higher than the Mg2Si0.6Sn0.4 one

Elastic properties of the α' martensitic phase in the Ti-6Al-4V alloy obtained by additive manufacturing

Elastic behaviour of Ti-6Al-4V alloy elaborated through additive manufacturing process is studied both experimentally and trough atomistic simulation. By studying rough samples after fabrication and after heat treatment, the elastic properties of the α' martensitic phase is compared to that of α+β microstructure. Atomistic calculations were also performed on various super-cells varying their chemical composition in order to simulate α and α' phases and their full elastic stiffness tensors were determined. Both experimental and simulation results show that Youngmodulus of the α' phase is lower than that ofα phase,while it presents a more anisotropic behaviour