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After applying torsion to cylindrical 304 stainless steel samples, a gradient structure along the radial direction was obtained. It was found that the volume fraction of α′-martensite increased gradually from the center to the surface of samples. The possibility of deformation-induced martensitic transformation was analyzed theoretically, whose conclusions were consistent with the experimental results. It was found that torsional deformation could produce abundant deformation twin and deformation-induced martensite, which could affect the tensile property of steel distinctly. The existence of deformation and martensite during torsion could increase the strength but deteriorate the ductility dramatically during the tensile test, which should be attributed to the weakening of work hardening ability. As a result of severe deformation, both strain-induced and stress-induced martensite have been observed. Also, two types of martensitic transformation mechanisms during torsional deformation were discussed.

Thermoelastic martensitic transformation (MT) plays a vital role in shape memory effects and superelasticity of various alloys. To understand the self-accommodation mechanism of the MT of Zr–Co–Pd alloys, which demonstrate typical MT from B2 to B33 structure, the configuration of the alloy’s martensite variants and their crystallographic relationship were investigated by electron backscatter diffraction, transmission electron microscopy, and high-angle annular dark-field scanning transmission electron microscopy. Results suggest that two habit plane variants (HPVs) bounded by {021}B33 compound twinning are formed around each B2 axes of the B2 parent phase. It is determined that the minimum self-accommodation unit, to relax the strain energy accompanying MT, is a pair of trapezoid- or triangle-shaped HPVs. No lattice invariant shear exists in the martensite variants with a trapezoid- or triangle-shape. In the latter stage of MT, each HPV pair may contact and impinge on {110}B2 plane of B2 parent phase, which is shear and shuffling plane on MT. The combination of the four variants forms the roof-type morphology.

The study was carried out by means of transmission electron microscopy on thin foils to investigate the changes in matrix morphology and phase composition occurring in ferritic-pearlitic steel of St2 grade (Russian) under plasma electrolytic surface quenching. In the original state St2 steel is a material which underwent quenching under the temperature of 890 °C (2 – 2.5 h) with cooling into warm water (30 – 60 °C) and further tempering under the temperature of 580 °С (2.5 – 3 h). Surface quenching was conducted in aqueous salt solution during 4 seconds under the temperature of 850 – 900 °C, voltage of 320 V, and current rate of 40 A. In the original state morphological components of the steel matrix were lamellar pearlite and non-fragmented and fragmented ferrite. Surface quenching resulted in the following transformations of morphology and phase composition: 1 – to martensitic transformation (morphological components are lath martensite, lamellar low-temperature and high temperature martensite), 2 – to steel self-tempering (inside all martensite crystals there are thin plate-like precipitations of cementite), 3 – to diffusion transformation γ → α and precipitation of retained austenite (γ-phase) given as thin layers along the boundaries of laths and plates of low-temperature martensite and inside all the crystals of lamellar martensite in the shape of “needles” like in twin type colonies. Surface quenching led to precipitation of special carbides of Мe23С6 phase. It was revealed that carbide precipitation is attributed primarily to decomposition of retained austenite and martensite and also to partial dissipation of cementite and, moreover, it is due to carbon removal from dislocations and the boundaries of α-phase crystals. That means that in all cases carbon from retained austenite, α-solid solution, cementite particles and defects of crystal lattice is used for the formation of special carbides.

H13 die steels with varied yttrium (Y) content were prepared by vacuum induction melting, multiple forging, annealing and quenching treatment with stepwise heating. The effects of Y on the microstructures, internal fraction and martensitic transformation of H13 die steel were investigated using electron backscattering diffraction, transmission electron microscopy and a multifunctional internal friction meter. The results showed that the martensite start temperature first decreased but then increased with the increasing Y content, reaching a minimum in the 0.013Y-H13 steel. The refinement of the prior austenite grain size afforded more nucleation sites in Y-modified H13 die steels. The Snoek–Koster–Ke peak indicated that the solid solution of Y atoms provided additional martensitic transformation dynamics to increase the martensitic transformation rate and promote the formation of V1−V2 (Σ3) variants during the initial stage of transformation. The transformation rate decreased in Y-modified H13 steels during the late stage of transformation (70% completed). Therefore, the addition of Y elements was beneficial for refining the size of the martensite and promoted the formation of twin-type martensite in H13 steel.

The influence of high-temperature quenching and cold treatment on the amount of residual austenite, its stability and ability to deformation martensitic transformation, hardening and wear resistance during the abrasive wear of pearlite grade 150KhNML carbon steel is studied. It is shown that maximum wear resistance during abrasive wear is ensured by a structure consisting of metastable residual austenite and martensite. Such a microstructure is created by high-temperature hardening before testing: during operations, austenite on the working surface, as a result of abrasive particles, turns into dispersed and nanocrystalline martensite with a high level of frictional hardening. This also results in the operability of the secondary steel microstructure due to the micro-TRIP effect. An additional means for increasing abrasion resistance is cold treatment after high-temperature hardening, which provides an increase in carbon martensite cooling before testing for wear resistance and the formation of new portions of carbon martensite deformation during testing.

The present work investigates the influence of constrained groove pressing (CGP) on the microstructural evolution and mechanical properties of 304 austenitic stainless steel. CGP shows a significant improvement in mechanical properties such as tensile strength and micro-hardness. XRD analysis shows the transformation of austenite phase into deformation-induced martensite phase which is also confirmed by vibrating sample magnetometer. Finite element analysis employing Deform 3D software shows the average induced strain of 2.29, after two passes of CGP. Rietveld refinements on XRD patterns affirm the decrease in crystallite size, increase in microstrain and dislocation density. EBSD also shows the formation of substructures due to CGP. The ultimate tensile strength of solution-treated specimens enhances from 729 to 1058 MPa, and the value of micro-hardness increases from 238 to 477 VHN after two passes of CGP. The augmentation in mechanical properties is attributed to the synergic effect of increased dislocation density, martensitic transformation, substructure formation and reduced crystallite size.

The local hydrogen distribution has significant influences on hydrogen embrittlement. In this work, mode-I fractures of (010)[100] pre-cracked α-Fe single crystal containing dissolved and absorbed hydrogen atoms are simulated by molecular dynamics and the time-stamped force-bias Monte Carlo methods. Statistics show that when located near the {112} plane, hydrogen atoms accelerate cleavage fracture and suppress the slip of {112} ; when located on the {110} plane, they promote martensite transformation and increase {110} slip. Most adsorbed hydrogen atoms are concentrated near the inside of the crack surface and suppress fracture early by stress relaxation; therein concentrates stresses inside the matrix, and causes microvoid-coalescence fracture.

The deformation behaviors of W nanowires embedded in a TiNi matrix were investigated by means of in-situ synchrotron high energy X-ray diffraction (HEXRD) and in-situ transmission electron microscopy (TEM) analysis during tensile deformation. The HEXRD measurement indicated that the W nanowires exhibited an average lattice strain of about 1.50 %, whereas the TEM examination revealed a local elastic strain of about 4.59 % in areas adjacent to the TiNi matrix where stress-induced martensitic transformation occurred. This strain corresponds to a stress of ∼15 GPa for the W nanowires. In addition, in areas adjacent to the TiNi matrix where plastic deformation and cracking were generated, the W nanowire showed significant ductile necking with ∼80 % reduction in cross-section area. The ductile necking of W nanowire is attributed to the lack of protection from the stress-induced martensitic transformation of the TiNi matrix.

The article discusses the technology of laser processing of VT6 titanium alloy as a technological operation to improve properties and structure. A promising direction for reducing roughness during laser hardening with reflow can be the development of a process using additives that reduce the surface tension of the melt and increase the wettability of the surface. It was revealed that phase hardening and martensitic transformation β → α are the causes of hardening of titanium during laser processing. This is confirmed by a change in the width of the x-ray lines. It was found that during laser processing with a speed of 3.7 mm/s and higher, a fine-grained structure is achieved. The results show that, at a sufficiently high cooling rate of the substrate, the grain sizes remain practically unchanged. Russian Federation “The influence of magnetic fields and ion implantation on the electrical structure and properties of titanium, aluminum alloys and elementary semiconductors”

International audience; Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e., the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni-Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni-Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni-Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships.

The optimization of the microactuation of shape memory alloy films in bimorph architectures was analysed for various film thickness and substrates material architectures, based on a known model that uses the fraction of phases during the martensitic transformation to evaluate the properties of the shape memory alloy film. It was shown by appropriately selecting the substrate material it is possible to maximize the deflection of the tip of a cantilever-type microactuator. For NiTi films, the increase of the active material thickness, as expected, leads to increased actuation, however the thermoeleastic properties of the substrate contribute to the stress achieved at the onset of the martensitic phase transformation, with higher values corresponding to larger actuation. Also depending on the thermoelastic constants of the substrate, the curvature of the cantilever-type microactuator can be toward the deposited film or toward the substrate. Si, Mo and Ti generated bending toward the shape memory alloy film, while Ni, stainless steel and Upilex S ended up bending toward the substrate. The result are important for the prediction of complex actuation complex architectures with potential to be used in micro-electromechanical systems.

The effect of martensitic transformation on the fatigue damage of AISI 321 stainless steel is investigated. After low-cycle fatigue testing five batches of AISI 321 steel, several specimens were selected, which significantly differed in the amount of α'-martensite measured by the eddy current method. The influence of the variations in chemical composition on the maximum amount of strain-induced α'-martensite is discussed. With an increase in amount of α'-martensite, an increase in shear bands intersections was observed on the optical micrographs. The material density measurements were carried out by hydrostatic weighing. The density reduction resulting from fatigue was quantified. It is shown that a decrease in density linearly depends on the amount of strain-induced α'-martensite formed in the fatigue-damaged material. It is concluded that the effects of damage accumulation and martensitic transformation on the density reduction are inseparable, since both are caused by the same mechanism – the slip of dislocations and the increase in the density of dislocations.

The effect of γ'- and β-phase particles on the γ–α' thermoelastic martensitic transformation (MT) during heating/cooling and under stress in [001]-oriented single crystals of a Fe – 28% Ni – 17% Co – 11.5% Al – 2.5% Ti (at.%) alloy is studied in tension and compression. The influence of the ageing mode on the Ms temperature is shown. During a simultaneous precipitation of γ'- and β-phase particles following two-step ageing for 4+4 h at 873 K, a 100 K increase in the Ms temperature is observed compared to one-step ageing for 8 h. No differences (asymmetry) in the stresses σcr for the start of the stress-induced γ–α' MT nor in the values of α = dσcr/dT in tension and compression are observed in the crystals with γ'- and γ'+β-phase particles. The absence of asymmetry of σcr and α = dσcr/dT is due to close values of the shape memory effect (SME) and superelasticity (SE) in tension and compression. Upon precipitation of β-phase particles the values of SME and SE decrease.

The densification behavior, microstructural development, toughening and strengthening mechanisms of Al2O3 whisker-reinforced 3Y-TZP and 12Ce-TZP composites were systematically and comparatively investigated with varying whisker lengths. Compared with 3Y-TZP/Aw composites, the presence of a Ce-Al-Si-O amorphous phase, caused by the addition of Al2O3 whiskers, promoted the densification and grain growth of 12Ce-TZP/Aw composites. Crack deflection and bridging are proposed as the primary toughening mechanisms for 3Y-TZP/Aw composites, while the t-m martensitic transformation would dominate the toughening and strengthening processes of 12Ce-TZP/Aw composites. Changes in Al2O3 whisker length would vary the distributions of internal stress and amorphous phase within the ceria-stabilized ZrO2 matrix, and hence affect the toughening and strengthening results. It indicates that effective toughening and strengthening of the Al2O3 whisker-reinforced TZP composites can be achieved by taking advantage of collaborative engineering control on the reinforcement morphology and the interface chemistry/structure.

In the dynamic theory of martensitic transformations, temperature Ms of the start of transformation corresponds to conditions that are optimal for generating waves by nonequilibrium d electrons that control the growth of a martensitic crystal. The recognition of relative damping $${{\Gamma}}_{{\text{e}}}^{'}$$ of s electrons plays a substantial role in this case. A general analysis that has made it possible to propose, for the first time, an analytical formula for critical size Dc $$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$$ of austenite grains is used to interpret the results for chromium–nickel steels, in which the transformation is initiated by strong magnetic fields. Under conditions of positive bulk magnetostriction, a magnetic field with intensity H lowers the chemical potential of electrons. As a result, the Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ H) value decreases and austenite with a grain diameter of D, which was stabilized by condition D < Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) in the absence of a field, becomes destabilized, since inequality D > Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ H) is fulfilled. The dynamic theory predicts a sharp increase in the Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) value at $${{\Gamma}}_{{\text{e}}}^{'}$$ → 1. This is evidenced by value Dc $$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$$ ≥ 1 mm for steel 67Kh2N22, which is three orders of magnitude larger than Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) ≈ 1 μm for the Fe–31Ni alloy. Other peculiarities of the effect of magnetic field on the martensitic transformation are also discussed.

As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti45Zr5Pd25Pt20Au5, and Ti45Zr5Pd25Pt20Co5 had the martensitic transformation. The perfect recovery was obtained in Ti45Zr5Pd25Pt20Co5 during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti45Zr5Pd25Pt20Au5 during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti45Zr5Pd25Pt20Co5 is the one possible HT-SMA working between 342 and 450 °C.

Ni-Mn-X (X = group IIIA-VA elements) Heusler alloys have been seen to exhibit multiferroic effects such as magnetic/metamagnetic shape memory (MSM/MMSM), magnetocaloric (MC), direct energy conversi...

Two low-transformation-temperature weld metals, 12Cr6Ni and 8Cr10Ni weld metals, were devised and their martensite start temperatures were 245°C and 225°C, respectively. Despite a small difference ...

Superelastic Shape Memory Alloys (SMAs) provide a high damping capacity due to the hysteretic motion of the inter-phase boundaries during the martensitic transformation. They have demonstrated their ability to control vibrations of SMA-based Civil Engineering and Aerospace structures. In order to improve existing damping devices, characterization of SMA damping capacity is necessary, despite the lack of a standard procedure. Classical characterizations such as tensile or torsion tests on SMA samples are very attractive, the fact that they are common and simple to process. Furthermore, environment and loading conditions are quite easy to control. Different energy-based formulations have been proposed in the literature to explicitly predict SMA damping capacity from the hysteretic mechanical behaviour. The aim of this paper is to classify commonly used formulations from the literature, using a new thermomechanical vibration numerical model of a SMA beam structure. Thus, three energy-based predictions of SMA intrinsic damping ratio measured at the material scale are compared to the damping ratio measured from the free vibration signal at the SMA beam structure scale, taken as the objective reference. The formulation proposed by Piedbœuf and Gauvin provided a better match in three study-cases.

This work aimed to investigate and critically analyze the differences in microstructural features and thermal stability of Cu−11.3Al−3.2Ni−3.0Mn−0.5Zr shape memory alloy processed by selective laser melting (SLM) and conventional powder metallurgy. PM specimens were produced by sintering 106−180 µm pre-alloyed powders under an argon atmosphere at 1060 °C without secondary operations. SLM specimens were consolidated through melting 32−106 µm pre-alloyed powders on a Cu−10Sn substrate. Mechanical properties were measured through Vickers hardness testing. Differential scanning calorimetry was conducted to assess the martensitic transformation temperatures. X-ray diffraction patterns were collected to identify the metallurgical phases. Optical and scanning electron microscopy was used to analyze the microstructural features. β′1 martensite was found, irrespective of the processing route, although coarser martensitic variants were present in PM-specimens. In conventional powder metallurgy samples, intergranular eutectoid constituents and stabilized austenite also formed at room temperature. PM-specimens showed similar average hardness values to the SLM-specimens, albeit with high standard deviation linked to the porosity. The specimens processed by SLM showed reversible martensitic transformation (T0=171 °C). PM-processed specimens did not show shape memory effects.

High-entropy alloys (HEAs) provide a new perspective to design metastable alloys with the stress-induced martensitic transformation (SIMT) for overcoming the strength-ductility trade-off. Here, we report a novel SIMT, orthorhombic to hexagonal close-packed martensite, in a single orthorhombic refractory HEA (Ti16Zr35Hf35Ta14 RHEA), showing a good yield strength-ductility matching. The analysis of the elastic distortion energy (∆Eels) of Ti16Zr35Hf35Ta14 and several other RHEAs reveals that severe lattice distortion is a key factor which causes this SIMT. Combined the “d-electron alloy design” approach with the ∆Eels, the phase configuration and SIMT path in RHEAs can be well predicted. Our work brings new insights between the lattice distortion and SIMT of RHEAs, benefiting the metastable alloy development.

Composition dependence of martensitic transformations as well as the magnetic properties for the Ni2Mn1+xIn1-x (0.25 ≤ x ≤ 0.58) alloys were investigated by using the first-principles calculations. Key results demonstrate that the stability of parent austenite (A) decreases gradually with increasing Mn content whilst it is opposite for the martensitic phase. This causes the total energy difference between the austenite and martensite phases increscent with increasing Mn contents. When x = 0.33, the martensite transformation during cooling is PA→FA→NM. When x ≥ 0.42, an intermartensitic transformation occurs from modulated 6 M martensite to non-modulated (NM) martensite with the martensite transformation sequence of PA→FA→6M→NM. The martensitic transformation from austenite to martensite accompanies the transition from ferromagnetic to ferrimagnetic state. This is a typical magneto-structural coupling transformation. The analysis of the density of states demonstrates that the Ni 3d state plays an important role in the phase stability.

The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated.

An accelerated method for determination of the stability of properties of alloys during their storage and subsequent application by studying the influence of the impact of radiation on the characteristics of shape-memory alloys is considered. The method allows for simulation of loading characteristics effective over the entire operating lifetime.

This study addresses the selectivity of self-accommodation microstructures in the initial stage of martensitic transformation in B19’ TiNi. It is known that specific habit plane variant (HPV) pairs, including { 11 1 ¯ } mtype I twin planes that are called initial stage microstructure (ISM) in this study, are preferentially formed in the initial stage of martensitic transformations. To elucidate on the selectivity of the ISM, the energy barrier at the formation of the twin plane, which is the candidate for the starting point of HPV pair formation, and the geometry of the HPV pair are theoretically analyzed in this paper. Using geometrical non-linear theory analysis, the compatible twin plane which has exact twin orientation relationship can be formed in the specific five groups of HPV pair. Using ab-initio simulations and Eshelby theory analysis results, { 11 1 ¯ } mtype I twin is determined to have a relatively small energy barrier among the twins in TiNi. Therefore, this paper shows that the HPV pair having a twin plane with a small energy barrier of formation and a structure that can effectively reduce the elastic strain is preferentially formed as the ISM.

Nowadays, searching for the materials with multiple magneto-functional properties and good mechanical properties is vital in various fields, such as solid-state refrigeration, magnetic actuators, magnetic sensors and intelligent/smart devices. In this work, the magnetic-field-induced metamagnetic reverse martensitic transformation (MFIRMT) from paramagnetic martensite to ferromagnetic austenite with multiple magneto-responsive effects is realized in Fe-doped Co-V-Ga Heusler alloys by manipulating the magnetic ordering. The martensitic transformation temperature Tm reduces quasi-linearly with increasing Fe-content. In strikingly contrast with the Fe-free alloys, the magnetization difference (ΔM') across martensitic transformation increases by three orders of magnitude for Fe-doped alloys. The increased ΔM' should be ascribed to the reduction of Tm, almost unchanged Curie temperature of austenite and the increased magnetic moment in the samples with higher Fe-content. The large ΔM' provides strong driving force to realize the MFIRMT and accordingly multiple magneto-responsive effects, such as magnetocaloric, magnetoresistance and magnetostriction effects. Meanwhile, giant Vickers hardness of 518 HV and compressive strength of 1423 MPa are achieved. Multiple magneto-responsive effects with exceptional mechanical properties make these alloys great potential candidates for applications in many fields.

The effects of cooling rate on the microstructure and phase transformation mechanism of TC4 alloy during the continuous cooling of β phase region were studied by non-isothermal expansion method combined with microstructure observation. The phase transformation feature points at different cooling rates were obtained by the expansion curve and its first derivative curve. The characteristics of the phase transformation products in the corresponding phase transformation feature points were observed by optical microscopy (OM). The results show that with the increase of cooling rate (excluding water cooling), the nonlinearity of the expansion curve caused by phase transformation changes from one place to two, and the temperature corresponding to the peak value of the phase transformation rate curve also showed different changes. As the cooling rate increases, the microstructure sequentially underwent lamellar α, massive αm, and fine-needle α′ martensite. The phase transformation conditions of massive αm and α′ martensite are significantly different, and showed a competition trend between them. According to the phase transformation feature points and their corresponding microstructure, the continuous cooling diagram for TC4 β solution treatment was plotted.

Deformation-induced martensitic transformation (DIMT) has been investigated for many years mainly under monotonic loading conditions. However, the evolution of the DIMT under non-monotonic loading conditions has rarely been studied. The present work experimentally explored the development of DIMT under tension-compression as well as torsion cyclic loading conditions and showed fluctuating behavior within loading cycles in austenitic stainless steels measured by Feritscope, which was not observed under monotonic loadings. The present study showed that the fluctuation of the martensite measurement was related to the dual-phase features and could be explained by the Villari effect in the martensite phase. The martensite contents are assumed to increase monotonically with the effective plastic strain and are distributed along with principal stresses. A unified kinetics model for DIMT was proposed for both monotonic and cyclic loading conditions and verified experimentally. The anisotropic development of DIMT can be described by the model in combining with cyclic plasticity.

The final carbon content of austenite in equilibrium with tempered martensite can be estimated by the so-called constrained carbon equilibrium in the presence of carbide (CCEθ) model. However, the linear predictions under CCEθ deviate from both the initial and the experimentally measured carbon content. A modified approach to the CCEθ model is proposed, which predicts an increase of the carbon content in austenite with the decrease of temperature below the onset of martensitic transformation.

Boron addition to Ni53.5Mn26.0Ga20.5 alloy is found to modify the microstructure and mechanical properties substantially. Studies on (Ni53.5Mn26.0Ga20.5)Bx alloys reveal that boron addition causes grain refinement which led to an increase in compressive strength in x = 0.5 alloy which also retained multimode twinning. Maximum compression strengths of 124 MPa, 198 MPa, and 111 MPa and corresponding compression strains of 5.2%, 4.3%, and 5.5% respectively have been recorded for B-0, B-0.5, and B-1.0 alloys. Shape memory strain of 1.5% was recorded even after addition of 0.5 at.% boron which reduced to 0.4% at 1 at.% boron and showed a correlation to the extent of multimode twinning. Substantial second-phase segregation rich in Ni was seen at grain boundaries, the extent of which increased with boron content. This led to a compositional shift in the matrix phase which resulted in a reduction in the martensitic transformation temperature and which in turn caused an easy deformation at low stresses and suppression of multimode twinning in x = 1.0 alloy. Microhardness shows an increase with boron content. B-0.5 alloy with larger compression strength, substantial shape memory effect, and higher hardness has a potential for shape memory applications at room temperature.

Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (γ-fcc) to body-centered cubic martensite (α′-bcc) upon plastic deformation. So far, ORs between γ-fcc and α′-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama–Wassermann (N-W) to Kurdjumov–Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of α′-bcc formed directly on γ-fcc was identified as N-W, while those via the hexagonal closed packed e-plate (e-hcp) were the K-S and Pitsch as they were nucleated at the single and two e-plates intersections, respectively. Thus, the SIMT sequence was the direct γ → α′ with the presence of the N-W OR in the high strain depth, while it was the γ → e → α′ as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.

In this work, the CuAlBe shape memory alloy (SMA) with a new composition of 77.05Cu-22.02Al-0.93Be (at.%) detected by the EDS test was fabricated in a vacuum arc melter under a pure argon atmosphere. At the beginning, the high-purity (%99.9) elements of Cu, Al, and Be powders were mixed and the mixture (~ 10 g) was formed as pellets by pressure. By melting the pellets in arc melter, the as-cast ingot alloy was obtained. Then, the ingot alloy was cut into small pieces (~ 30–50 mg) and then all of these samples were all homogenized in the β-phase region (at 900 °C for 1 h) and immediately submerged in traditional iced-brine water to form β1’ martensite phase in the alloy which results from such fast cooling by suppressing the formation of hypoeutectoid precipitations (α and γ2). Then, to uncover and evaluate the existence of martensite structure and the characteristics of shape memory alloy properties of the alloy, the specimens were tested by calorimetric and structural measurements. The results of thermal heating/cooling cycles of the alloy were obtained from differential scanning calorimetry (DSC) measurements that were taken twice at 5 °C/min of heating/cooling rate under constant argon gas flow (100 ml/min) and by using liquid nitrogen cooling support to reach lower temperatures than room temperature. The DSC thermograms of the alloy revealed the characteristic martensitic transformation peaks that occurred endothermic by heating and exothermic by cooling at moderate temperatures ranging between 19 and 66 °C, regarded as evidence for the presence of shape memory effect property in the alloy. Important thermodynamical parameters such as transformation temperatures, hysteresis gap, and entropy and enthalpy change amounts for these back and forward martensitic phase transition peaks were determined directly by using data of DSC peak analyses and by calculation. Differential thermal analysis (DTA) measurement that was taken from room temperature to 900 °C at a heating/cooling rate of 25 °C/min displayed a high-temperature behavior of the alloy as compatible with the common behavior of Cu-Al SMAs. The X-ray test of the alloy conducted at room conditions showed sharp diffraction peaks and their matching crystal planes which indicate the existence of β1′ martensite phase in the alloy and its high single-like crystallinity i.e. its large Debye-Scherrer sub-micrometer crystallite size. Besides, theoretical forecasting about the existence and volumetric dominance of martensite phases was deduced from the calculated value of e/a (average conduction electron concentration per atom) parameter of the alloy. Furthermore, the mechanical Vickers microhardness tests that were performed at room temperature and under 100-gf load applied for 10 s revealed the high ductility and softness features of the alloy. Considering all the results, the highly ductile CuAlBe alloy with new composition owing shape memory alloy properties and intermediate working temperatures can be useful in various kinds of research and applications in which Cu-based SMAs are exploited.

In this experimental research study, quaternary Cu-based shape memory alloy with a chemical composition of 73.23Cu-21.93Al-3.36Fe1.48Mn (at.%) and a valence electron concentration (e/a) value of 1.487 was produced by arc melting method. After production, the identical samples of alloy were all subjected to thermal aging at 150 °C for 1 h and then cooled in different coolant materials. A varied number of thermal and structural tests and analyses were carried out successfully to study the alterations in the alloy samples and to see the effects of the aging and quenching/cooling media on alloy’s characteristic martensitic transformation temperatures, eutectoid points, some other kinetic parameters, and structural features. Among these tests, the DSC analyses showed the variations between the thermodynamical parameters such as the martensitic transformation temperatures of the alloy samples that occurred due to the effects of both aging and various quenching environments. Differential thermal analysis (DTA) measurements showed that some eutectoid point shifts (~ 20-25 °C) occurred on the DTA thermograms of the alloy samples which happened mainly due to the thermal aging. A detailed hypothetical analysis and discussion related to the eutectoid temperature made upon DTA results, since the eutectoid temperature has a key role in some thin-film SMA applications for selecting optimum annealing temperature to make good diffusion of the metal layer on the semiconductor surface without causing a deterioration occurs in SMA properties. The results of structural XRD tests showed that the main martensite peak, the intensities of all peaks, and crystallite size values were changed by the effect of aging. In addition, the intensities of all peaks and crystallite size values were also changed by thermal aging and also by quenching the alloy samples in different environments. All obtained results have reflected that modifying the characteristic parameters of this CuAlFeMn SMA by such ways used here may be helpful in various macroscale SMA-based applications and also in designing miscellaneous micro/nanoscale electromechanical systems (MEMS and NEMS) where different modified SMA properties have been demanding.

An unusual tensile deformation behaviour in the form of a propagating band along the sample gauge was observed in two neutron-irradiated 316 stainless steel samples during room-temperature tests, leading to a combination of high strength and high ductility. These bands were not observed in an unirradiated counterpart. With the help of in situ high-energy synchrotron x-ray diffraction, the phase-specific crystal information was tracked at different deformation levels in each sample. Post-irradiation and post-deformation samples were examined using electron microscopy to characterize various microstructural features. All samples displayed a deformation-induced martensitic phase transformation, which was identified as a second strain-hardening mechanism accompanying the dislocation hardening. The deformation-induced martensitic transformation was rationalized by the effect of applied stress on the effective martensite start temperature. The results showed that the irradiation did not alter the dislocation hardening and the martensitic transformation mechanisms, but the increased yield strength in irradiated materials facilitated the localized phase transformation at the onset of plastic deformation, in contrast to the unirradiated material which required pre-straining. The hardening effect of the martensitic transformation reduced the tendency towards necking and mitigated the loss of ductility in the irradiated material by carrying the deformation in the form of a propagating band. Despite the beneficial effect from the martensitic transformation, this study indicates that this mechanism cannot not be activated at typical operating temperatures of nuclear reactors.

The effect of thermal cycling on the transformation behavior of a Ti–24Nb–1Mo alloy was investigated by means of electrical resistivity measurement, transmission electron microscopy (TEM), X-ray diffraction (XRD), tensile test and Vickers hardness tests. Electrical resistivity changes were not observed in all alloys. It indicates that thermally induced martensitic transformation does not take place in the alloys. After thermal cycling between 298 K and 77 K, clear X-ray diffraction peaks corresponding to ωath phase, which did not exist before thermal cycling, were observed. Volume fraction of ωath phase increased as increasing the number of thermal cycling. ωath phase formed during thermal cycling increased hardness of the alloy. Although thermally induced martensitic transformation did not occur in the alloys, superelastic deformation behavior was observed in the alloys. The superelastic recovery ratio decreased from 81% to 41% by increasing the number of thermal cycling, which came from the increase in the volume fraction of ωath phase.

A new version of the crystallographic theory of martensitic transformation has been developed, where instead of Bain deformation, the deformation of the lattice was performed by a shear along the twinning system of crystals and an additional change in dimensions in three mutually perpendicular directions. In the proposed variant, the angle of relaxation rotation of martensite crystal was about 1.8°; in the standard phenomenological theory, it was 10°. The new version made it possible to establish the mechanism of lattice deformation upon martensitic transformation, to determine the angle of relaxation rotation, and to carry out crystallographic analysis of martensitic transformation in various alloys. The formation of crystals (laths) of dislocation martensite in medium-carbon steel was found to occur with redistribution of carbon.

It is shown that the broadenings of the forward and reverse martensitic transformations are proportional to the second derivatives of the elastic energy by the transformed martensite volume fraction. The shifts of the transitions, beside the effect of chemical contribution to the equilibrium transformation temperature, To, can depend on the change of the elastic and dissipative energies as well. The effect of mechanical stabilization (by making the surfaces roughened) in Ni49Fe18Ga27Co6 single crystal, where the transformation too place by single interface motion, can be well treated in thermoelastic approximation. In Ni50Mn28.5Ga21.5 single crystal (where the usual multivariant martensite structure developed) not only the dissipative energy, but the slope of the second derivative of the elastic energy for cooling has also been altered after mechanical stabilization and there can exist an elastic energy contribution to the width of the hysteresis too. Symmetry conforming short range ordering under uniaxial stress (SIM-aging) in Ni53Mn25Ga22 and Ni51Fe18Ga27Co4 single crystals caused an asymmetric change in the second derivatives of the elastic energy: the sharpening of the corresponding branches of the hysteresis was stronger for heating than cooling. The thermoelastic balance condition did not fulfill for the SIM-aged samples and the elastic energy contributions to the shift of the transformation temperatures could even compensate the increase of To (caused by the stabilization by symmetry conforming short range ordering). It is also illustrated that the dissipated energy per one cycle could decrease even if the area of the hysteresis has been increased.

CuZr-based alloys are being considered as potential shape memory alloys for use in high-temperature applications. We have conducted a study on the effects of several alloying elements on the shape memory properties of these alloys using polycrystalline thin-film samples. Here we report on the explosive formation of martensite in supercooled CuZr, CuZrNi and CuZrCo samples. This explosive transformation behavior is characterized by the following observations: 1) The high-temperature austenitic phase can be supercooled below the martensite finish temperature Mf. At a critical temperature below Mf, austenite transforms to martensite across the entire sample in less than a microsecond. 2) The critical temperature has a narrow distribution and decreases slightly with higher cooling rate. 3) Observation of supercooling and explosive transformation behavior depends on the temperature history above the austenite finish temperature Af. If a sample is quenched immediately after heating above Af, martensite forms gradually on cooling below Ms; if a sample is allowed to dwell a few seconds above Af, the martensite forms explosively. We suggest that the gradual transformation proceeds by martensite growth on defects that accumulate during successive transformation cycles. If the sample is allowed to dwell at a temperature above Af, however, these defects are annihilated and the transformation is nucleation-limited. Nucleation of martensite then requires significant supercooling. The defect annihilation process is highly sensitive to temperature and has an apparent activation energy of 326 kJ/mol, which is too large for a simple diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to the presence of residual martensite.

Here, novel medium-entropy alloys with chemical compositions of Co17.5Cr12.5Fe55Ni10Mo4C1 and Co17.5Cr12.5Fe55Ni10Mo3C2 (at%) exhibiting excellent tensile properties at both room and liquid nitrogen temperatures have been developed. Precipitation of carbides changes the chemical composition and phase stability of the matrix, resulting in the controlled deformation-induced martensitic transformation from face-centered cubic to body-centered cubic of the alloys. The carbide precipitation, lattice distortion, and martensitic transformation led the alloy to have ultra-high yield strength of ~1 GPa and ultimate tensile strength of ~2 GPa with extra work hardening at liquid nitrogen temperature.

Based on the results of electron microscopy examinations, a theoretical study of the distortion tensors of strain localization nanobands with a 90° reorientation of the crystal lattice is performed. The analysis is carried out in the direct-plus-reverse model of FCC→BCC→FCC martensitic transformations in an approximation of the linear theory of small strains. Using the atomic model of martensitic transformations based on a system of cooperative thermal fluctuations of planes in the BCC lattice, the distortion tensors of the contraction and shear deformation modes are estimated and so is their contribution to the total distortion of the reversible martensitic transformation.

Metastable β titanium alloys are widely used in the biomedical, automotive, and aerospace industry, due to their excellent corrosion resistance, fatigue strength, biocompatibility, and easy formability. Besides all these use areas, the suitable microstructure of the alloys for heat treatment increases the efficient usability day by day. In literature research, it has been found that heat treatment types such as cryogenic treatment and precipitation hardening can be applied efficiently to the alloys. Optimum strength/ductility, wear resistance, creep strength, and fatigue strength can be obtained with these heat treatments. For this reason, it has become important to understand the effect of heat treatments on the microstructural and mechanical properties of the alloys and parameters affecting this heat treatment efficiency. Precipitation hardening includes solution and aging treatment steps. The solution treatment can be applied at temperatures below and above the β transition temperature. While the aging treatment can be applied in four different ways, in the review article, the effects of single step and duplex aging treatment, which are applied with high efficiency, are emphasized. Precipitation hardening efficiency affects the chemical composition of the alloys, heat treatment steps, treatment temperature and times, and heating/cooling rate. The cryogenic treatment provides the formation of martensite α phases in metastable β titanium alloys cooled below the martensitic transformation temperature. Higher-strength and hardness have been obtained in the studies where aging treatment was applied after cryogenic treatment. Cryogenic treatment efficiency determines chemical composition of the alloys, treatment temperature and time, the heating/cooling rates, and heat treatments applied after cryogenic treatment.

International audience; In this study, we have employed a repetitive laser scanning strategy to fabricate a three-dimensional functionally graded NiTi alloy by SLM method. The microstructure shows gradient characterized by the increase of volume fraction for B19' phase. An exceptional strain-hardening associated with continuous increase of mechanical recoverable strain is obtained in such material. The gradient of functionality is attributed to the superimposition of multi-deformation mechanisms originating from the microstructural gradient. The deformation mechanisms are dominated by reorientation of B19' variants at early stage of deformation and then by stress-induced martensitic transformation at higher level of stress. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A promising martensitic steel with good hardenability is studied. In the cooling rate range 0.1–30°C/sec and the only transformation recorded by a dilatometer starts at an Ms temperature of 355 ± 10°C. Microstructure and mechanical properties of the steel studied are analyzed after various heat treatment regimes: cooling from the austenitizing temperature at various rates (from 0.02 to 20°C/sec), austempering at 280–380°C, and oil quenching and tempering at 200–600°C. It is shown that a 1000-fold change in the cooling rate has a weak effect on steel strength. Bainitic transformation in the steel in question occurs in the martensitic transformation temperature range, which leads to a reduction in impact strength compared with a martensitic microstructure both after slow cooling and isothermal hardening.

The surface roughening and martensitic phase transformation (MPT) of SUS 304 and SUS 316 were studied through two experiments: a uniaxial tensile stress test, repeated for five cycles, and an Scanning Electron Microscope&ndash, Electron Backscaterr Diffraction (SEM-EBSD) investigation. The MPT and martensitic volume fraction (Mf) were evaluated following the tensile test. The correlation between MPT, Mf, and surface roughening behavior was investigated. The experiment showed that an increase in the strain level from 0.4% to 1% increased the MPT and Mf, which transformed from a metastable austenitic phase in SUS 304. The increased strain level increased the surface roughening for various grain sizes (Dg), from fine grain (Dg &lt, 3 &mu, m) to coarse grain (Dg &ge, m). SUS 304 and SUS 316 are used so that the surface roughening mechanism between SUS 304 and SUS 316, with different phase conditions and at a similar Dg, can be determined. The results showed that the surface roughening increased for both fine and coarse grain at strain levels of 0.4% and 1%, however, a larger increase of surface roughening was obtained for coarse grain. In coarse grain, surface roughening increased significantly not only with a low MPT, but also with a low grain deformation. In coarse grain, the surface roughening increased proportionally to the strain level (&epsilon, p) because of the low MPT and weak grain. In fine grain, the surface roughening did not increase proportionally to the &epsilon, p because of the high MPT, which increased the grain strength in SUS 304. In the fine grain of SUS 304, the increase of surface roughening was nearly the same both at strain levels of 0.4% and 1%, because the MPT and Mf were nearly the same. The surface roughening with the same &epsilon, p and almost the same Dg in SUS 304 and SUS 316 fine grain was nearly the same, because the grain deformation almost produced the same relative inclination between neighboring grains in the initial state direction to the surface. The inter-grain movement changed the grain orientation. Based on kernel average misorientation (KAM) mapping, the local grain misorientation in SUS 304 was higher than that in SUS 316. This indicated that the fine-grain SUS 304 is harder than the fine-grain SUS 316. There is no MPT in SUS 316 because of the higher austenitic phase, which is affected by the austenitic former, such as Ni.

The investigation addresses the influence of tensile deformation on the magnetic properties of 304SS samples in their as-received state as well as those with martensite content of 12 and 17%. Non-destructive electromagnetic techniques like magnetic Barkhausen emission (MBE) and magnetic hysteresis loop have been used to measure variation in MBE voltage and coercivity, respectively, during plastic deformation through tensile loading. As both the techniques use surface probe, the present investigation will be useful for in situ evaluation of structural components. With progressive plastic deformation, those measurements revealed different stages of deformation indicated by change in MBE signal and magnetic coercivity. The stages of magnetoelastic response, strain-induced martensitic transformation, dislocation pile-ups and formation of voids reflected different patterns of magnetic Barkhausen emission and magnetic coercivity variation with progressive straining of austenitic stainless steel samples. The changes in true strain with variation in martensite content along gauge length of fractured samples have been analyzed with respect to martensite transformation kinetics. Mathematical fitting methodology has been adopted to distinguish as-received and pre-strained martensitic transformation characteristics. In situ magnetic NDE and associated martensitic transformation parameters may be useful for structural health monitoring of in-service components.