Stress-induced Transformation Research Articles

Stress-induced transformation on the cubic perovskite RbTaO3 for high-temperature applications: a DFT approach

Stress-induced transformation on the cubic perovskite RbTaO3 for high-temperature applications: a DFT approach

Effect of burst-type martensitic transformation on superelastic and elastocaloric properties in a 〈116〉 oriented Cu70.5Al17.5Mn12 single crystal

We report the occurrence of stress-induced burst-type martensitic transformation in a 〈116〉A oriented Cu70.5Al17.5Mn12 single crystal. The burst-type transformation is observed in the reverse transformation of unloading from 250 K to 370 K in tension, showing strong coupling with plastic deformation in the temperature range between 310 K and 370 K. Such coupling greatly exacerbates the stress hysteresis and energy dissipation in association with the stress-induced transformation, resulting in a significant enhancement in the mechanical damping and degradation in the elastocaloric response. In particular, the merit index for damping exhibits a remarkably high value of 0.903 at 370 K, demonstrating the huge potential of coupled burst-type transformation and plastic deformation in manipulating damping properties. Moreover, it is evidenced that the microstructural evolution of burst-type transformation greatly deviates from that of the thermoelastic transformation.

Influence of Strain Rate on Metallurgical and Mechanical Properties of Friction Stir Spot Welded Aluminium Joints

Nowadays, the substitution of copper with aluminium is widely pursued in order to save weight and material costs, for battery components and wire connectors. Additionally, cost reductions can be further enhanced with effective reduction of energy consumption through efficient manufacturing. Therefore, friction stir spot welding as a solid-state welding technique is a potential choice with low energy demands and high joining performances. However, the joining of aluminium and its alloys with solid-state welding techniques is still a challenging task due to a persistent and chemically stable aluminium oxide layer formed at the sheets prior to the welding, due to the reaction between aluminium and atmospheric oxygen. In this paper, the influence of strain rate induced during friction stir spot welding process on the metallurgical, mechanical and electrical properties of friction stir spot welding of AA 5754-H111 was studied. The strain rate was calculated according to the rotational speed of the tool and the effective (average) radius and depth of the stir zone. It was observed that the specimens welded with a lower strain rate endured a 15 % higher average strain failure load compared to the specimens welded at a higher share rate. The microhardness profiles of the specimens obtained at low strain rates imply strain hardening mechanisms in the weld zone, while the microhardness of specimens welded at high strain rates expressed thermal softening. It was also found that the friction welded sheets, regardless of the strain rate, show increased electrical resistance compared to the base material, however, it decreases with an increase in strain rate. Microstructural analysis reveals a stress-induced metallurgical transformation in the narrow zone around the weld-faying interface.

The many faces of thrombosis in post-cardiac surgery newborn

We present a clinical case showing the development of two episodes of postoperative thrombosis of varying mechanisms in one patient. The first episode of thrombosis in the systemic-pulmonary anastomosis was due to shear stress-induced secretion and transformation of the von Willebrand factor (vWF) in the performed anastomosis. The altered vWF/ADAMTS-13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, member 13) ratio in favor of the former was accompanied by platelet adhesion and activation in the anastomotic area that resulted in its thrombosis. The second episode of thrombosis within the superior and inferior vena cava was developed in parallel with escalated infectious process and characterized not only by thrombocytopenia, but also by decreased fibrinogen concentration. In this clinical situation, resolution of the infectious process as well as the use of proper antithrombotic therapy led to patient recovery.

Toughening oxide glasses through paracrystallization.

Glasses, unlike crystals, are intrinsically brittle due to the absence of microstructure-controlled toughening, creating fundamental constraints for their technological applications. Consequently, strategies for toughening glasses without compromising their other advantageous properties have been long sought after but elusive. Here we report exceptional toughening in oxide glasses via paracrystallization, using aluminosilicate glass as an example. By combining experiments and computational modelling, we demonstrate the uniform formation of crystal-like medium-range order clusters pervading the glass structure as a result of paracrystallization under high-pressure and high-temperature conditions. The paracrystalline oxide glasses display superior toughness, reaching up to 1.99 ± 0.06 MPa m1/2, surpassing any other reported bulk oxide glasses, to the best of our knowledge. We attribute this exceptional toughening to the excitation of multiple shear bands caused by a stress-induced inverse transformation from the paracrystalline to amorphous states, revealing plastic deformation characteristics. This discovery presents a potent strategy for designing highly damage-tolerant glass materials and emphasizes the substantial influence of atomic-level structural variation on the properties of oxide glasses.

Stress-Induced Transformations of Polarization Switching in CuInP2S6 Nanoparticles

Using the Landau-Ginzburg-Devonshire approach, we study stress-induced transformations of polarization switching in ferrielectric CuInP2S6 nanoparticles for three different shapes: a disk, a sphere, and a needle. Semiconducting properties of a nanoparticle are modeled by a surface charge layer, whose effective screening length can be rather small due to the field-effect. We reveal a very strong and unusual influence of hydrostatic pressure on the appearance of polarization switching in CuInP2S6 nanoparticles, hysteresis loops shape, magnitude of the remanent polarization, and coercive fields, and explain the effects by the anomalous temperature dependence and "inverted" signs of CuInP2S6 linear and nonlinear electrostriction coupling coefficients. In particular, by varying the sign of the applied pressure (from tension to compression) and its magnitude (from zero to several hundreds of MPa), quasi-static hysteresis-less paraelectric curves can transform into double, triple, pinched, or single hysteresis loops. Due to the sufficiently wide temperature and pressure ranges of double, triple, and pinched hysteresis loop stability (at least in comparison with many other ferroelectrics), CuInP2S6 nanodisks can be of particular interest for applications in energy storage (in the region of double loops), CuInP2S6 nanospheres maybe suitable for dynamic random access multibit memory, and CuInP2S6 nanoneedles are promising for non-volatile multibit memory cells (in the regions of triple and pinched loops). The stress control of the polarization switching scenario allows the creation of advanced piezo-sensors based on CuInP2S6 nanocomposites.

Microstructure and mechanical properties of WC-TiC0.4 composites doped with ZrO2 or 3Y-ZrO2

The ZrO2 or 3Y-ZrO2 incorporated WC-TiC0.4 composites were consolidated employing spark plasma sintering (SPS) technology at different temperatures. The impacts of ZrO2 and 3Y-ZrO2 as toughening agents on the densification, phase composition, microstructure and mechanical properties of the WC-based composites were assessed. The results revealed that the WC-5 wt% TiC0.4–9 wt% ZrO2 (W5Ti9Zr) composite required a lower sintering temperature of 1700 °C for nearly full densification than WC-5 wt% TiC0.4–9 wt% 3Y-ZrO2 (W5Ti9Zr-3Y) composite. The diffraction peaks of monoclinic ZrO2 (m-ZrO2) existed only in the W5Ti9Zr composite. The evenly dispersed ZrO2 and 3Y-ZrO2 achieved the relatively homogeneous and fine microstructure in W5Ti9Zr and W5Ti9Zr-3Y composites. Moreover, the semi-coherent interfaces of (W1-x, Tix)Cn layer with WC, (W1-x, Tix)Cn layer with m-ZrO2, and (W1-x, Tix)Cn with m-ZrO2 brought about a rise in the fracture toughness of W5Ti9Zr composite. The W5Ti9Zr composite yielded the better comprehensive mechanical properties with a combination of hardness (20.8 GPa) and fracture toughness (9.9 MPa·m1/2) as compared to those of W5Ti9Zr-3Y composite. Furthermore, the main toughening mechanisms of W5Ti9Zr and W5Ti9Zr-3Y composites included grain refinement of WC matrix, a mixed transgranular and intergranular fracture, stress-induced transformation of t-ZrO2 to m-ZrO2, and crack deflection.

Superlubricity of Titanium Alloy Enabled by MoS2 Flakes and a-C:H Film

Titanium alloys are often used in engineering fields including aerospace, cryogenic technologies, and weaponry due to their remarkable qualities. However, several issues including a high coefficient of friction, weak wear resistance, and low hardness hinder their widespread usage. Despite several efforts to enhance their tribology, achieving ultra-low friction on titanium alloy surfaces remains a challenging problem in materials science. Here, we report on the superlubricity of a MoS2 + a-C:H (Mo-a films) composite film, prepared by magnetron sputtering and spraying to lubricate titanium alloy surfaces. Robust superlubricity was achieved by the Mo-a composite films with a coefficient of friction (COF) below 0.007 in a helium environment. Compared to the reference titanium alloy substrates, the introduction of Mo-a composite film reduced the friction coefficient to roughly 1%, and the a-C:H film reduced wear by three orders of magnitude. High-resolution characterizations indicate that this enhanced tribology can be attributed to the formation of transfer film, which is enriched with nanostructured graphene sheets and MoS2 nanoscrolls, and is formed due to shear stress-induced structural transformation of a-C:H films and MoS2 nanosheets. This transfer film transitioned the initial high-resistance steel-to-a-C:H contact to super low-resistance steel-to-transfer film contact, thus achieving superlubricity and a remarkable wear reduction. This work outlines a pathway to solving the poor wear resistance and high friction coefficient problem of titanium alloy surfaces, which can be an important guideline for applications of titanium alloys in mechanical engineering.

Atypical phase transformation behavior of Fe-33%Ni alloys induced by shot peening

The phase transformation behaviors of two Fe-33 mass%Ni alloy specimens subjected to shot peening (SP) were investigated. The first sample consisted solely of an austenite (γ) phase while the second also contained a high proportion of a martensite (α′) phase. When the specimen containing only the γ phase was subjected to SP, a stress-induced martensitic transformation occurred. In contrast, the second sample underwent the reverse transformation from the α′ to γ phase. Moreover, the volume fraction of the γ phase in the vicinity of the peened surface of each specimen was found to plateau as the SP treatment was prolonged. The final volume fraction of the γ phase was the same in both specimens regardless of their initial microstructure prior to the SP process. In addition, the volume fraction of the γ phase was found to increase with increasing peening pressure, meaning that the reverse transformation was promoted by SP at higher pressures. Calculations of the deformation heat generated during SP indicated that this heat was not primarily responsible for the reverse transformation induced by SP. Estimation of the change in the phase transformation temperature resulting from the residual compressive stress imparted by SP showed that this treatment changed the value of T0 (the arithmetic mean of the martensitic transformation start temperature and the reverse transformation start temperature) for the Fe-33 mass%Ni alloy from 328.5 K to 287.8 K. Because T0 after SP was lower than the temperature to which the specimens were raised during SP, this process could have caused stress-induced reverse transformation. Therefore, the decrease in the phase transformation temperature stemming from residual compressive stress is believed to explain the reverse phase transformation observed during SP of this alloy.

Microstructural, crystallographic, and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

• Magnetic field-assisted directional solidification (MFADS) can control structure. • Macro-/micro-segregation and orientation can be adjusted by MFADS. • Microsegregation-induced microstructure influences mechanical properties. • Stress-induced crystallographic evolution is traced by in-situ EBSD. Abstract In this study, the effect of transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field, causing the emergence of martensite clusters in the austenite matrix. Moreover, the magnetic field alleviates the microsegregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of the above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation. The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the microsegregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor. The stress-induced superelasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Predicting reaction barriers in solid-state systems under stress via second-order energy expansion

Solid-state transformations are important in many areas of science and technology. Herein, a model for predicting the relative energies of stationary points along the reaction pathways for stress-induced solid-state transformations is assessed and applied. The model is based on a second-order expansion of the energy of the system with respect to changes in the unit cell, and requires a small number of parameters that can be obtained through quantum chemical calculations. Comparison of the model with the results of quantum chemical calculations indicates that the model accurately reproduces changes in energy occurring during stress-induced transformations over a reasonable range of stresses. A procedure for applying the model to identify stress tensors that are most likely to promote a desired reaction is illustrated. The results also indicate that this procedure provides insight into the connection between the form of a stress tensor and the changes in energy occurring during a stress-induced solid-state transformation.

Strain rate response of mechanical behaviors for Fe-8Mn-6Al-0.4C duplex low-density steels with different austenite stabilities

Duplex low-density steels exhibit excellent mechanical properties under quasi-static tensile tests with the help of the transformation-induced plastic (TRIP) effect produced by the deformation-induced martensitic transformation (DIMT), which is sensitive to the strain rate. Therefore, adjusting the microstructure to accommodate the deformation at different strain rates is crucial for the optimal design of high-performance duplex low-density steels. In this study, Fe-8Mn-6Al-0.4C low-density steel was subjected to intercritical annealing (IA) treatment to obtain duplex microstructures with an inhomogeneous distribution of austenite grain sizes and tensile tests were performed at two strain rates. The response of the mechanical behavior to the strain rate was investigated, and the mechanical stability of austenite was evaluated using a method based on the grain size. The results show that the diverse mechanical behaviors of experimental steel depend mainly on the mechanical stability of austenite rather than the stacking fault energy (SFE). The mechanical stability of austenite increased with decreasing annealing temperature and was further enhanced by adiabatic heating at a higher strain rate. Accordingly, the DIMT mode changed from a stress-induced transformation to a strain-induced transformation until the DIMT was inhibited by adiabatic heating. The improvement in the mechanical properties of the experimental steel is attributed to the sustainable DIMT with a reasonable mechanical stability of austenite.

A mechanical criterion for Lüders-type deformation of polycrystalline NiTi

NiTi shape memory alloys are known to exhibit a Lüders-type deformation behavior associated with its stress-induced martensitic transformation. Such phase-transformation-caused Lüders deformation presents a different metallurgical mechanism to the dislocation-related Lüders deformation of mild steels. To explain this phenomenon, many hypotheses have been proposed in the literature. However, all these explanations seem to contradict some experimental evidence in one way or another. In this work, a new explanation for the occurrence of the Lüders-type deformation in NiTi is proposed and a model is derived based on the criterion of conservation of sample length changes produced by the stress-induced transformation, the crosshead movement of the testing machine and the elastic contraction. The model is verified by tensile experiment and digital image correlation measurement. The new model is able to describe the upper-lower yielding phenomenon and the plateau stress of the Lüders-type deformation behavior.

Enhanced thermo-mechanical processing through stress-induced martensitic transformation and its effect on grain refinement in metastable β titanium alloys

Stress-induced transformations in retained metastable phase are known deformation mechanisms that provide additional plasticity in several alloy systems. In the present study on metastable β Ti-10V-2Fe-3Al (Ti-1023) and Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloys, stress-induced martensitic transformation (SIMT) has been effectively demonstrated to result in achieving a noticeable cold reduction of ∼43% and ∼45%, respectively. A significant fraction of stress-induced α″ martensites provide preferential nucleation sites for new strain-free grains formation upon recrystallization annealing. The thermomechanical treatment resulted in grain refinement by ∼93% and ∼80%, respectively, for Ti-1023 and Ti-5553 alloys. Electron Probe Micro Analyzer (EPMA) confirms the role of alloying element (such as Fe, Mo, V, Cr and Al) diffusivities resulting in altering the stability of the β phase as a function of recrystallization annealing time. Further, the effect of grain size on the SIMT ability based on microhardness variations has been deduced.

Lüders band evolution in geometrically graded NiTi plates

This study investigated the process of the Lüders band formation and propagation within geometrically graded plate samples of a pseudoelastic NiTi during the stress-induced martensitic transformation by means of digital image correlation full-field strain measurement. The geometrically graded plate samples had parabolic width profiles in concave and convex forms along the length. Such sample geometries created a situation of progressively increasing applied load and a continuous variation of the sample width during the process of the stress-induced transformation. The size (along the sample length direction) of the Lüders band front transition zone was found to remain unchanged with variation of the sample width during the process, but the local strain rate across the transition zone was varied. The transition zone of the martensite Lüders band during the forward transformation was wider than that of the austenite Lüders band on the reverse transformation, and the local strain rate was lower than that of the latter.

Concentration of transformation-induced plasticity in pseudoelastic NiTi shape memory alloys: Insight from austenite–martensite interface instability

Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined with in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid–solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite–martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves “smoothly” with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite–martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved.

Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): A first-order structural phase transition

Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-$s$-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot spots which control detonation initiation. Using classical molecular dynamics, we predict the formation of similar nanoscale shear bands in the \ensuremath{\alpha}-RDX crystal subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core softening of the effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.

Microstructure and mechanical properties of zirconia toughened nacre-like alumina ceramics

In this study, the nacre-like alumina ceramics, with introduction of zirconia, are fabricated by freeze casting followed by hot pressing. The experimental results show that zirconia particles are uniformly distributed between the layered alumina ‘brick’ in the form of ‘mortar’. Due to the incorporation of zirconia with proper particle size, both flexural strength and fracture toughness of the nacre-like alumina are greatly improved, much stronger and tougher than pure alumina. In regard to the sample containing 13.3 wt% zirconia, the flexural strength reaches 522 MPa, ∼47% increase in comparison with the monolith. Particularly, when zirconia content is 10 wt%, the fracture toughness is increased by ∼66%, from 6.1 MPa m1/2 to 10.1 MPa m1/2. Microstructure observation and fracture behavior analysis reveal that the coupling effects of bioinspired nacre-like structure and stress-induced transformation toughening are responsible for the enhanced mechanical properties. The results show that it is an effective method to increase the mechanical properties of layered alumina ceramics by introducing proper interlayer phase.

Damage of the martensite interfaces as the mechanism of the martensite stabilization effect in the NiTi shape memory alloys

The aim of the paper was a comprehensive experimental study of the martensite stabilization effect (MSE) in NiTi alloys under various conditions to find the mechanism behind this phenomenon. The MSE was studied in the quenched Ni50Ti50 and Ni51Ti49 alloys subjected to pre-deformation to various strains in three ways: deformation in the martensite state, cooling under stress through a temperature range of the forward transformation and deformation in the austenite state due to the stress-induced martensite formation. It was found that after pre-deformation by the martensite reorientation, the MSE was observed even after a small pre-strain. After pre-deformation by cooling under stress or by stress-induced transformation, the MSE was small if the pre-strain was less than 5%; otherwise, the value of this effect was significant. A new mechanism of the martensite stabilization effect was considered. It was assumed that the damage to the martensite boundaries during the pre-deformation (without plastic strain) decreased their mobility and a larger thermodynamic force needed to be provided to move the interfaces during the reverse transformation.

On the Lüders band formation and propagation in NiTi shape memory alloys

• Both the B19′ martensite and B2 austenite can form Lüders band. • Lüders band initiation strain rate is two orders of magnitude of the applied strain rate. • Lüders band forms with a finite minimum width. • Lüders band propagates with a transition zone half of the band minimum width. • Transition zone local strain rate is more than one order of magnitude of the applied strain rate. Near-equiatomic NiTi shape memory alloys are known to exhibit Lüders-type deformation associated with a stress-induced transformation. Many studies have been conducted in the past mainly focusing on the macroscopic characteristics of the phenomenon and some theories have been proposed in the literature to explain its mechanisms, but some aspects of this phenomenon are still unclear, particularly at the microscopic scale. This study investigated the local strain evolution during the initiation and propagation of Lüders band in a pseudoelastic NiTi alloy during tensile deformation by means of digital image correlation (DIC) analysis. Based on the evidence collected, distinct stages of Lüders band formation and propagation are defined and the corresponding local strain rates are obtained. These local strain rates are much higher than the global strain rate of the testing, giving insight to the mechanism of this phenomenon.