In this work, a Ti–29Nb–13Ta–4.6Zr–xO Gum Metal with two significantly different oxygen levels (388 and 3570 ppm) was investigated during deformation. The alloys were compressed during in situ high-energy X-ray diffraction using three different strain rates, 10−4, 10−3, and 10−1 s−1, in order to evaluate their influence on phase stability and mechanical properties. The influence of oxygen on the deformation process was also studied. Deformation takes place by twinning, stress-induced, and reverse martensitic transformation and was observed, for some samples, a spinodal decomposition of the β-phase during elastic deformation. The mechanical properties were similar for the different rates employed when considering the same oxygen level. The alloy with a higher amount of oxygen, however, showed a substantial increase in mechanical strength, with a yield strength of around 680 MPa, which is more than three times higher than for the specimen with 388 ppm of oxygen.

The evolution of fatigue performance and surface mechanical properties of AISI 304 stainless steel induced by the electropulsing-assisted ultrasonic surface rolling process (EP-USRP) was systematically investigated by integrating instrumented indentation, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy. The results indicate that higher hardness, greater strength, finer ultra-refined grains, and higher residual compressive stress are formed within the strengthened layer compared with the original ultrasonic surface rolling process (USRP). EP-USRP with the optimized experimental parameters can produce a higher average rotating bending fatigue strength for AISI 304 stainless steel than USRP. Anomalously and noteworthily, all fatigue specimens treated by EP-USRP showed an incomplete fracture, revealing a higher reservation of safety in practical engineering applications. The further modified structure strengthening and stress strengthening induced by EP-USRP are likely the primary intrinsic reasons for the observed phenomena. Furthermore, the influence mechanism of EP-USRP was discussed scrupulously.

The addition of Co to CuZr-based shape memory bulk metallic glass composites stabilizes the high temperature B2-CuZr and decreases its stacking faulty energy, which promotes an increase in ductility caused by an easier twinning formation. A similar effect is expected for TiCu-based alloys. The present work aims to investigate the effect of Co additions on the phase formation, mechanical properties, and thermal stability of rapidly solidified Ti–Cu-based alloys. Rods of six Ti–Cu-based compositions with different amounts of Co were prepared by Cu-mold suction casting and investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, dilatometry, and compression tests. The results show that the addition of Co decreases the glass-forming ability of Ti–Cu-based alloys and stabilizes B2 Ti(Cu,Ni,Co) at room temperature. The Co-added alloys exhibit an almost identical phase formation and microstructure, but their mechanical behavior is completely different nonetheless, which is mainly connected with the different composition of the B2 phase. The addition of Co makes the stress-induced martensitic transformation of this phase more difficult, which is one of the main reasons for the increase of the yield strength when a higher amount of Co is added.

The present investigation aims to enhance the mechanical properties and shape memory characteristics of Cu–Ni–Al shape memory alloys (SMAs) by alloying additional elements. These additions were found to control the phase morphology and grain size, along with the formation of different volume fractions, sizes, and distributions of precipitates. The features of the precipitates were mainly dependent on the type of alloying element. It was found that a Co (1.14 wt%) alloy gave the best overall improvement in terms of the transformation temperatures, ductility, and shape memory recovery. These improvements were mainly due to the exceptionally high presence of the γ2 phase in the microstructures of the modified alloy. The results of the current investigation were analyzed and compared to those of previous studies related to Cu–Al–Ni SMAs.

In situ laboratory-based and in situ synchrotron X-ray diffraction techniques were employed to study quantitatively the strain-induced austenite-to-martensite (A–M) transformation in thermally cycled (TC) superelastic NiTi alloys. The propagation of the A–M interfaces and the evolution of the microstructure were traced during uniaxial tensile loading. It was shown that the TC material exhibits localized transformation via the propagation of transformation bands. The amount of the martensite phase depends approximately linearly on the applied strain. Analysis of the broadening of the austenite diffraction lines indicates the presence of highly deformed austenite grains within the transformation bands. Analysis of the austenite diffraction-line shifts indicates that the overall lattice strain in the (retained) austenite in the transformation bands differs from that of the austenite in the adjacent untransformed regions.

Martensitic transformation and magnetic entropy change in Co substituted Ni50Mn35−xCoxSn15 (x = 0, 1.0, 1.5, 2.0, and 3.0) Heusler alloys have been investigated by X-ray powder diffraction analysis, differential scanning calorimetry, and magnetic measurements. X-ray diffraction analysis reveals that the Ni50Mn35−xCoxSn15 alloys have L21 Heusler structure at room temperature. The phase decomposition of the sample with x = 3.0, after annealing 48 h at 1173 K, is confirmed by both X-ray powder diffraction analysis and energy-dispersive x-ray spectroscopy in scanning electron microscopy. With the increase of the Co content from 0 to 2.0, the martensitic transformation temperature TM increases from 185 to 245 K, which is in good agreement with the rule of valence electron concentration e/a-dependence of TM. The magnetic entropy change ∆SM is investigated in the vicinity of the martensitic transformation.

Transformation behaviors of the technologically important polycrystalline Ti50.75Ni47.75Fe1.50 shape memory alloy were investigated using differential scanning calorimeter (DSC) and powder diffraction techniques. DSC revealed that there are two-stage (i.e., cubic→trigonal→monoclinic) martensitic phase transformations on cooling and a one-step transformation (monoclinic→cubic) on heating. In situ structural refinement of cubic→trigonal→monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit χ2. Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained.

Crystal structure and phase transformation behaviors in two Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) with compositions of Ni48Mn30Ga22 and Ni53Mn25Ga22 (at. %) as a function of temperature were investigated by in situ neutron diffraction experiments. Neutron diffraction technique proves to be highly efficient in characterizing structural transformation in Ni-Mn-Ga FSMAs, which consist of nearby elements in the periodic table. Our neutron results show that Ni48Mn30Ga22 has a cubic, L21 Heusler structure from 373 to 293 K. Its crystal structure changes into a seven-layered orthorhombic martensitic structure when cooled to 243 K, and no further transformation is observed upon cooling to 19 K. Neutron diffraction results also show that Ni53Mn25Ga22 has a tetragonal I4/mmm martensitic structure from 20 to 403 K. A pre-transformation around room temperature is observed from an abrupt jump in unit-cell volume of Ni53Mn25Ga22, which corresponds with an endothermic peak detected in a heated DSC curve.

Ribbons of the Ni-44.8wt%Ti shape memory alloy are prepared through the melt spinning technique. The study is focused on investigating the effect of the rapid solidification and grain size at characteristic start martensitic (Ms), final martensitic (Mf), start austenite (As) and final austenite (Af) transformation temperatures. Changes on martensitic transformation temperatures in Ti45Ni55 melt spun ribbons are observed as grain size is reduced. Results of optical microscopy and differential scanning calorimetry (DSC) are used to associate grain size with transformation temperatures.

The usual process to produce NiTi shape memory alloys is vacuum induction melting (VIM). Currently a new alternative process to produce NiTi shape memory alloys by rapid solidification structures called Melt Spinning has been studied. In this work, results of ribbons with a chemical composition Ti-55.2 Ni (wt %) alloy prepared by this method are presented. The ribbons are prepared at two different linear velocities: 30 m/s and 50 m/s. After that, samples are heat treated at 350 °C during 1 hour. The alloys are characterized by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction. According to the cycled DSC test, transformation peaks are associated with the B2→R→B19´ transformation during cooling and B19´→R→B2 during heating, showing transformation in multi-peaks. The martensite B19´ start (Ms) is varying from 35 to 39°C and the martensite finish (Mf) from 15 to 21°C, 42-47°C for austenite B2 start (As) and 65-69°C for austenite finish (Af) approximately. All analyzed ribbons show very similar values of transformation hysteresis temperatures at 50% of transformation of around 28°C. In order to change solidification rate, linear velocity is varied during the melt spinning process. Results indicate that linear velocity affects directly the temperature of transformation. When the linear velocity is increasing, crystallographic Ti-rich precipitates are developed, but dendritic growth disappears, changing the microstructure and decreasing these transformation temperatures. Then changes in linear velocity can dramatically affect shape memory properties, and in this case a velocity of 50 m/s produces a more homogenous alloy.

L'objectif de cette étude est de faire dialoguer la simulation numérique et l'expérience pour étudier le comportement en traction d'un multicristal en alliage à mémoire de forme de type CuAlBe. Ce dialogue permet de comparer qualitativement les distributions de transformation martensitique, observée à travers un microscope à grande distance frontale, avec celle obtenue par simulation numérique en utilisant une loi de comportement thermomécanique. Cette loi est basée sur la description de la transformation martensitique à l'échelle du réseau cristallin, elle est implantée dans le code Abaqus®. Les conditions aux limites appliquées lors de la modélisation correspondent aux déplacements mesurés sur les sections frontières par une technique de corrélation d'images. Les propriétés géométriques et cristallographiques des différents grains du multicristal sont quantifiées expérimentalement. L'essai de traction est réalisé à température ambiante où l'alliage a un comportement superélastique. Une bonne corrélation entre les observations expérimentales et les résultats numériques est observée.

Le modèle proposé s'inscrit dans le cadre d'une étude sur le comportement micromécanique métallurgique d'un alliage à mémoire de forme (AMF) Cu-Zn-Al durant la transformation martensitique et l'évolution microstructurale.Le modèle montre que la localisation de la déformation plastique après des sollicitations cycliques entraîne des modifications dans les propriétés mécaniques et thermiques. L'originalité du modèle permet de coupler entre les deux transformations de structures en montrant les mécanismes de déformation nécessaires lors des transformations.

The quest for high particle energies of modern super conducting lepton and hadron storage rings assumes a strong minimisation of the non-magnetic interconnection zones when compared to the total magnetic length of the main quadrupoles and the dipoles. The ratio of non-magnetic to magnetic zones in the LHC is close to 3%. Such a low percentage leads to a very compact design of systems and sub-systems situated in the interconnections, like the mechanical compensation system, composed of the bellows expansion joints, that have to compensate for the thermal contraction of the magnets. Given the complexity of the LHC interconnections, the requirements concerning their reliability were considerably increased since the availability of the collider for physics cannot be compromised. The failure modes taken into account in the present analysis are grouped into 3 categories: material failures, structural stability failures and fatigue failures. Optimisation of stainless steel for cryogenic applications has been carried out. Also, a concept of reliability oriented parametric optimisation of the LHC interconnections is presented.

We propose, analyze, and compare several numerical methods for the computation of the deformation of a pressurized martensitic thin film. Numerical results have been obtained for the hysteresis of the deformation as the film transforms reversibly from austenite to martensite.

We give an analysis of the stability and uniqueness of the simplylaminated microstructure for all three tetragonal to monoclinicmartensitic transformations. The energy density for tetragonal tomonoclinic transformations has four rotationally invariant wells sincethe transformation has four variants. One of these tetragonal tomonoclinic martensitic transformations corresponds to the shearing ofthe rectangular side, one corresponds to the shearing of the squarebase, and one corresponds to the shearing of the plane orthogonal to adiagonal in the square base. We show that the simply laminatedmicrostructure is stable except for a class of special materialparameters. In each case that the microstructure is stable, we deriveerror estimates for the finite element approximation.

We give results for the approximation of a laminate withvarying volume fractions for multi-well energy minimizationproblems modeling martensitic crystals thatcan undergo either an orthorhombicto monoclinic or a cubic to tetragonal transformation. We construct energy minimizing sequences of deformations which satisfythe corresponding boundary condition, and we establish a series of error bounds in terms of the elastic energyfor the approximation of the limiting macroscopicdeformation and the simply laminated microstructure.Finally, we give results for the corresponding finite element approximation of the laminate with varying volume fractions.