Critical Transformation Stress Research Articles

Superelasticity of [0 0 1]-oriented Fe–Mn–Al–Cr–Ni crystals with a negative temperature dependence of transformation stresses

This paper presents, for the first time, the results of studying the superelasticity of the Fe–33Mn–11Al–6Cr–7Ni crystals under compression along [001] orientation after quenching and ageing within a wide temperature range of 203 to 573 K. In quenched crystals, the critical transformation stresses σMs of the BCC–FCC martensitic transformation decrease in temperature ranges of 203–273 K and 320–443 K and, conversely, the σMs increase in temperature ranges of 273–320 K and 473–573 K with increasing temperature on the σMs(T) dependence. In aged crystals, the character of the σMs(T) dependence remains similar to quenched crystals. The maximum reversible strain εrev was 6.2 % and 4.3 % in quenched and aged crystals, respectively.

Toughening mechanism in superstructure Hf6Ta2O17 thin film

Fracture toughness is a key indicator to estimate the anti-peeling performance of thermal barrier coatings (TBCs). A unique superstructure endows Hf6Ta2O17 with an outstanding phase stability and mechanical property, which also poses great challenges for studying its intrinsic toughening mechanism. Here, the superstructure Hf6Ta2O17 single crystal film was epitaxially grown on YSZ(011) substrate by pulsed laser deposition. An uncommon ripple pattern near the Vickers indentation crack tips was first reported. The pop-in signals in the load-displacement curve indicated it may be relevant to ferroelastic domain transformation, which was further confirmed by spontaneous strain distribution in HRTEM. The transformation path, predicted by DFT, was conducted in b→c ferroelastic domain through in-plane rotation of Hf–O/Ta–O polyhedrons. The corresponding critical stress for domain transformation was up to 5.83 GPa. In addition, an intrinsic ductility was confirmed with a high Pugh ratio B/G of 2.49, much larger than the critical value (1.75) for brittle-ductile transition. These two mechanisms make contribution to the superior fracture toughness of Hf6Ta2O17. This work provides a deep insight into the toughening mechanism of ceramics and paves the way to design of next-generation TBCs.

Microstructure and Superelasticity of Cu–Sn Shape-Memory Microwires by Glass-Coated Melt Spinning

Cu–Sn shape-memory microw ires were fabricated by a glass-coated melt spinning method. Effects of Sn content on the microstructure and mechanical properties of microwires were investigated. The phase transforms from martensite to austenite with an increase in Sn from 14.0 atomic percent (at.%) to 16.5 at.%. When the Sn content exceeds 16.5 at.%, a highly ordered intermetallic phase, δ, formed. The fracture stress (σf) and the critical stress for martensitic transformation (σMs) increases with an increase in Sn content. The mechanical properties as well as the superelasticity were greatly improved by a high cooling rate in the glass-coated melt spinning method. A bamboo-grained structure was formed in the Cu–Sn microwire with a Sn content of 16 at.% by annealing at 750 °C for 5 h before quenching in water. The results indicate that two opposite strategies of refining the grain size to the micrometer level, or increasing the grain size to a one dimensional size of specimen, e.g., the diameter of the wire, are both effective in improving the superelasticity of the Cu–Sn alloy.

Effects of porosity and cyclic deformation on phase transformation of porous nanocrystalline NiTi shape memory alloy: An atomistic simulation

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles.

A new model for evaluating rock drillability considering the rock plasticity and chip hold down effect caused by hydrostatic column pressure under high confining pressure

The Yingqiong Basin in the northern part of the South China Sea is a region with large natural gas reserves. However, the high formation pressure and hydrostatic column pressure of the extra-thick mudstone section in this area present significant challenges to drilling operations. The mudstone has high plasticity and is very hard, and its thickness accounts for only 40% of the total footage, which results in pure drilling time accounting for over 75% of the total drilling time. To accelerate the rate of penetration (ROP) in this area, this study investigates the rock mechanics characteristics of mudstone under high confining pressure through experimental methods. The study reveals that the strength of the mudstone increases rapidly with increasing confining pressure, and the critical transformation stress of bottom-hole mudstone is around 45 MPa. Moreover, a cylindrical-cutter micro bit was developed, and the drillability of mudstone was tested under high confining pressure and hydrostatic column pressure, which further confirms that 45 MPa is the critical stress for a sudden increase in mudstone's drillability. Based on multiple regression analysis, the acoustic interval transit time, hydrostatic column pressure, and argillaceous content were identified as strong correlation parameters for the drillability of plastic mudstones. A new model for evaluating rock drillability considering rock plasticity and chip hold down effect caused by hydrostatic column pressure was developed, which provides effective guidance for the optimal design of bits and drilling fluid density in the extra-thick mudstone formations of the Yingqiong Basin. This study provides theoretical and technical support for the economic and efficient development of high-pressure gas fields in this area.

Improvement of mechanical properties and elastocaloric effect in Ag doped Ni-Mn-In magnetic shape memory alloys

Mechanical properties and elastocaloric effect are the most important parameters of Ni-Mn-In shape memory alloys in the refrigeration field. This paper mainly studies the influence of Ag doping and different preparation methods on the mechanical properties and elastocaloric effect of the alloys. The doping of Ag element not only increases the martensitic transformation temperatures and reduces the critical phase transformation stress, but also improves the mechanical properties and elastocaloric effect of the alloys. The adiabatic temperature change (ΔTad) of Ni50Mn34In15.5Ag0.5 prepared by arc melting was − 7 K at 4% strain, and the alloy fractured after 69 cycles with 3% strain. Furthermore, Ni50Mn34In15.5Ag0.5 powders prepared by gas-atomization were used by SPS sintered to obtain 10 µm grains alloys. This preparation method enables the fracture stress and fracture strain to reach 1625.8 MPa and 17.1%, respectively. At the same time it remains stable after 300 cycles with 6.5% strain, indicating the significant improvement of mechanical properties and cyclic stability in Ni-Mn-In metamagnetic shape memory alloy.

The role of texture and restoration mechanisms in defining the tension-compression asymmetry behavior of aged NiTi alloys fabricated by laser powder bed fusion

Tension-compression asymmetry is an inherent property of superelastic NiTi alloys linked to the microstructural evolutions and the stress induced martensite transformation during different deformation modes. Deep understanding of NiTi's asymmetry, a topic that is often overlooked in additive manufacturing, is imperative in designing functional parts that experience multiple deformation modes. Accordingly, in this paper, the asymmetry parameters, namely the critical stress for martensite transformation, transformation strain, and strain recoverability were measured for an aged material with optimum superelasticity, which was fabricated by two different scanning strategies. The acquired results were correlated to the changes in the developed texture components during laser powder bed fusion and subsequent annealing processes, highlighting the role of dynamic and static restoration mechanisms, respectively. The deviation of texture from <001>||building direction, caused by the occurrence of the restoration mechanisms during fabrication and subsequent aging, was found to be dissimilar for samples fabricated by 67° or 90° rotation angles, effectively impacting the behavior of the material in different deformation modes. For a sample with enhanced superelasticity, aged at 550°C for 6 h, 90° scanning strategy resulted in lower asymmetry and superior superelasticity in compression, while 67° rotations led to highest strain recoverability in tension, but an intensified asymmetrical behavior.

Comprehensive shape memory alloys constitutive models for engineering application

The current shape memory alloys (SMA) constitutive models have the problems of complicated calculation, inaccurate results, measuring difficult parameters, and inconvenient application, which brings about challenges for designing, analyzing, and simulating SMA’s macroscopic behavior in engineering. This paper proposed practical SMA constitutive models for accurate engineering calculation, which apply to both the single loading-unloading process and the multiple loading-unloading cycles. The more accurate SMA theoretical tensile curve and temperature-stress phase transformation diagram is introduced to derive the constitutive model. The more accurate calculation method of elastic modulus, phase transformation modulus, and critical phase transformation stress is proposed in this paper. The proposed constitutive model considers the incomplete phase transformation phenomenon and the variation of critical phase transformation stress under multiple loading and unloading conditions. It can accurately evaluate the critical phase transformation stress and stress-strain relationships at various temperatures. Tensile tests under various working conditions were conducted. The results demonstrated that the proposed constitutive model could well simulate the SMA’s macroscopic mechanical behavior at various temperatures, loads, and multiple loading-unloading cycles with accuracy, practicality, and efficiency.

Constitutive model for nanocrystalline NiTi shape memory alloys considering grain size effects and tensile–compressive asymmetry

Mechanical properties of nanocrystalline NiTi shape memory alloys (SMAs) change drastically with grain size. The present contribution develops a constitutive model to reproduce the grain size dependent superelastic behavior and tensile–compressive asymmetry observed in the experiments of nanocrystalline NiTi SMAs. Effects of grain size are incorporated in the developed model by introducing the intrinsic length scale accounting for the transformation hardening as well as the grain-core and grain-boundary phase. In this work, nanocrystalline NiTi SMA is regarded as a two-phase composite material made of inclusions of the grain-core phase dispersed in the grain-boundary phase acting as a matrix. A transformation function allowing for the description of fine-grain strengthening mechanism and tensile–compressive asymmetry is proposed. In the grain-core phase, the evolution law for transformation strain during the forward and reverse transformation is determined. Besides, the constitutive relation of the grain-boundary phase is assumed to be linearly elastic. Based on the equivalent secant bulk and shear modulus of the grain-core and grain-boundary phase, the stress–strain relationship of nanocrystalline NiTi SMAs is derived by using the extended Mori–Tanaka method. Comparisons between experimental and predicted results demonstrate that the proposed model has the ability to reproduce the grain size dependent deformation and asymmetric stress–strain behavior under tension and compression of nanocrystalline NiTi SMAs. In detail, it is found that critical transformation stresses for forward and reverse transformations, dissipation energy density, transformation strain hardening, and maximum transformation strain are sensitive to the grain size and stress states.

Effect of crystallographic anisotropy on phase transformation and tribological properties of Ni-rich NiTi shape memory alloy fabricated by LPBF

This article investigated the effects of crystallographic anisotropy on the microstructure, phase transformation and tribological properties of NiTi Shape memory alloy (SMA) fabricated by laser powder bed fusion (LPBF). The maximum tensile recovery strain of 4.13 ± 0.16% was obtained at room temperature by adjusting the hatch spacing. The contributions of metallurgical factors under different crystal orientations, such as active twinned correspondence variant pairs (CVP), Schmid factor, critical stress for the stress-induced martensitic transformation (SIMT), Yong's modulus and nanohardness to superelastic response and tribological properties was clarified through various building orientations (0°, 45°, 90°). The results indicated that the superelasticity of LPBF-fabricated NiTi alloy was crystallographic orientation-dependent. Sample 0° with a strong (001) texture possessed the highest recoverable superelastic strain of 7.91 ± 0.14%. We describe, in detail, how the sliding friction mechanisms such as delamination, adhesive and formation of iron base oxide layers were attributed to superelasticity and the friction pairs under different loading conditions and building orientations. This new understanding reveals how different LPBF-induced microstructures affect mechanical properties and wear, providing a powerful guide for the tribological design or application and tailoring the functional behavior in LPBF-fabricated Ni-rich NiTi alloy.

Deformation mechanism and load transfer in an in-situ NiTi–Nb composite

In this study, the cyclic deformation behaviours of an in-situ Nb nanowire reinforced NiTi shape memory alloy composite were studied by in-situ synchrotron X-ray diffraction measurements. Localized hardening in Nb nanowire and the corresponding localized relaxation in NiTi matrix were detected in the Lüders band front upon stress-induced martensitic transformation, indicating a phase load transfer from the transformed NiTi to Nb nanowire. Based on the load transfer, a self-propagating transformation process performing under an average phase stress lower than the critical transformation stress was proposed, which should be responsible for the Lüders-like deformation of the composite. The change of deformation mode from Lüders-like localized deformation to homogeneous deformation after the first tensile cycle was attributed to the plastic deformation in Nb nanowire and the formation of internal stress gradient field in NiTi upon pseudoelastic cycling.

On the role of internal stresses on the superelastic behaviour of Ti-24Nb (at.%)

Ti-Nb-based alloys undergo a martensitic transformation that gives rise to superelastic and shape memory behaviours over a wide range of temperatures. The design of such alloys for industrial applications is commonly based around the reported martensite start temperature for a given composition. However, there are significant variations in these values within the literature that remain unexplained using current thermally driven transformation theories. Recently, a marked difference in the transformation behaviour of Ti-24Nb (at.%) in the cold rolled and solution treated conditions has been reported and a stress based mechanism has been postulated to rationalise these results. Here, the veracity of this stress based theory has been investigated through in situ studies of the transformation behaviour of Ti-24Nb (at.%) in the cold rolled condition using synchrotron X-ray diffraction. Upon heating to 350˚C, all αʺ initially present reverted to the β phase by ∼ 300˚C. When cooled from 350˚C, thermally induced martensite was observed below ∼ 270˚C and the intensity of the αʺ peaks increased as the temperature decreased. When loaded at temperatures between -150 and 30˚C, further transformation was observed to occur immediately upon the application of a stress. However, critically, αʺ formed under load was always found to be reversible upon unloading, despite being up to 420˚C below the observed martensite start temperature. This behaviour cannot be accounted for through conventional thermally driven transformation theories for these alloys. Comparison of the stress-strain curves for cold rolled and solution heat treated condition material indicated that their behaviours were similar but offset by a value equivalent to the difference in the critical transformation stress. This offset is readily explained by the total stress-based mechanism proposed and provides further evidence as to its fidelity.

Selective Laser Melting of 60NiTi Alloy with Superior Wear Resistance

In this work, the selective laser melting (SLM) 60NiTi alloy was successfully fabricated. Through designing an orthogonal experiment of parameters optimization including laser power (P) and scanning speed (v), the optimal parameters window with both high forming quality and appropriate composition proportion was established. The SLM 60NiTi can exhibit high relative density (&gt;98%) and low Ni loss (&lt;0.2 at.%) at the parameter window of P = 80–90 W, v = 300–350 mm/s, and energy density of 145–155 J/mm3. The optimally-selected SLM 60NiTi exhibits a high compression strength of 2.2 GPa and large reversible strain of 7% due to the reversible stress-induced martensitic transformation of the NiTi phase and the large elastic strain of the Ni4Ti3 phase. It also exhibits superior wear resistance to conventional casting solution treated 60NiTi because the NiTi phase formed in an SLM repeated thermal cycle possesses a lower solution Ni atom and thus lower critical stress for martensitic transformation, and is more prone to undergo martensitic transformation upon friction and wear.

Transformation yield surface of nanocrystalline NiTi shape memory alloy

Molecular dynamics (MD) simulation is used for investigating mechanical behavior and phase transformation in nanocrystalline NiTi shape memory alloy (SMA) under complex stress states, including uniaxial, biaxial and triaxial stress states. The simulation results indicate that tension-compression asymmetry occurs in terms of transformation strain, critical transformation stress, dissipation energy density, martensite variant and martensite domain. This asymmetry is mainly aroused by the fact that transformation strains under the stress states with tensile loading in the major deformation direction are larger than the counterparts under the stress states with compressive loading in the major deformation direction for most crystallographic orientations. The phase transformation stresses in complex stress states strictly comply with neither the Von Mises criterion without considering tension-compression asymmetry nor the Bouvet and Patoor criteria considering tension-compression asymmetry. Consequently, a new transformation yield criterion on the basis of Bouvet and Patoor criteria is proposed in the present work to precisely describe the transformation stress anisotropy of nanocrystalline NiTi SMA under complex stress states.

Dependence of mechanical behavior on grain size of metastable Ti–Nb–O titanium alloy

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ​K or 773 ​K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ​K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.

Influence of microstructure on the application of Ni-Mn-In Heusler compounds for multicaloric cooling using magnetic field and uniaxial stress

Novel multicaloric cooling utilizing the giant caloric response of Ni-Mn-based metamagnetic shape-memory alloys to different external stimuli such as magnetic field, uniaxial stress and hydrostatic pressure is a promising candidate for energy-efficient and environmentally-friendly refrigeration. However, the role of microstructure when several external fields are applied simultaneously or sequentially has been scarcely discussed in literature. Here, we synthesized ternary Ni-Mn-In alloys by suction casting and arc melting and analyzed the microstructural influence on the response to magnetic fields and uniaxial stress. By combining SEM-EBSD and stress-strain data, a significant effect of texture on the stress-induced martensitic transformation is revealed. It is shown that a <001> texture can strongly reduce the critical transformation stresses. The effect of grain size on the material failure is demonstrated and its influence on the magnetic-field-induced transformation dynamics is investigated. Temperature-stress and temperature-magnetic field phase diagrams are established and single caloric performances are characterized in terms of ΔsT and ΔTad. The cyclic ΔTad values are compared to the ones achieved in the multicaloric exploiting-hysteresis cycle. It turns out that a suction-cast microstructure and the combination of both stimuli enables outstanding caloric effects in moderate external fields which can significantly exceed the single caloric performances. In particular for Ni-Mn-In, the maximum cyclic effect in magnetic fields of 1.9 T is increased by more than 200 % to -4.1 K when a moderate sequential stress of 55 MPa is applied. Our results illustrate the crucial role of microstructure for multicaloric cooling using Ni-Mn-based metamagnetic shape-memory alloys.

Ultrafine shape memory alloys synthesized using a metastable metallic glass precursor with polymorphic crystallization

Metallic glasses have been considered as a precursor for developing nano- or ultrafine-structured alloys based on its crystallization behavior at large undercooled liquid state. However, the multicomponent system, requiring high glass-forming ability, makes it difficult to control the crystallization mechanism, and leads to the formation of brittle intermetallic compounds during crystallization. Here, we present an intermetallic-type metallic glass precursor designed via thermodynamic and physical considerations. The metallic glass precursor polymorphically crystallized into ultrafine-structured single-phase shape memory alloy. The crystallization kinetics and final grain-size strongly depend on isothermal annealing temperatures. The shape memory alloys fabricated using the metallic glass precursor revealed excellent superelastic and shape memory effects, which was affected by the critical stress for martensitic transformation depending on the grain size. Our approach and discovery provide a feasibility of the simple fabrication processing for ultrafine-structured shape memory alloys by one-step heat-treatment technique including thermoplastic forming of metallic glass precursor.

On size-dependent bending behaviors of shape memory alloy microbeams via nonlocal strain gradient theory

This paper focuses on the size-dependent behaviors of functionally graded shape memory alloy (FG-SMA) microbeams based on the Bernoulli-Euler beam theory. It is taken into consideration that material properties, such as austenitic elastic modulus, martensitic elastic modulus and critical transformation stresses vary continuously along the longitudinal direction. According to the simplified linear shape memory alloy (SMA) constitutive equations and nonlocal strain gradient theory, the mechanical model was established via the principle of virtual work. Employing the Galerkin method, the governing differential equations were numerically solved. The functionally graded effect, nonlocal effect and size effect of the mechanical behaviors of the FG-SMA microbeam were numerically simulated and discussed. Results indicate that the mechanical behaviors of FG-SMA microbeams are distinctly size-dependent only when the ratio of material length scale parameter to the microbeam height is small enough. Both the increments of material nonlocal parameter and ratio of material length-scale parameter to the microbeam height all make the FG-SMA microbeam become softer. However, the stiffness increases with the increment of FG parameter. The FG parameter plays an important role in controlling the transverse deformation of the FG-SMA microbeam. This work can provide a theoretical basis for the design and application of FG-SMA microstructures.

Alloy design of metastable α+β titanium alloy with high elastic admissible strain

Owing to the high production cost of a high alloying system, the commercialization of β-Ti alloys with high elastic admissible strain (= yield strength/elastic modulus) for biomaterials is challenging. To reduce the alloying elements and maintain a high elastic admissible strain, a new alloy design of the metastable α+β-Ti alloy (Ti–4Al–2Mo–2Fe–2Cr) is proposed in this study. The microstructure of the alloy consists of an α phase with a high yield strength and a metastable β phase with a low elastic modulus. The existence of the hard α phase increases the critical stress for stress-induced martensitic transformation due to stress and strain partitioning. In addition, heat treatment in the α+β regime is an effective method for grain refinement, resulting in high strength without increasing the elastic modulus. This alloy exhibited an excellent combination of strength and ductility with a high elastic admissible strain. Moreover, electron backscattering diffraction and field-emission transmission electron microscopy were used to understand the mechanical behavior of the α+β Ti alloy.

Transformation-induced fracture toughening in CuAlBe shape memory alloys: A phase-field study

Stress-induced martensitic transformation ahead of a crack tip can relax the stress concentration and produce fracture toughness in shape memory alloys (SMAs). In this manuscript, we utilize a non-isothermal phase-field model (PFM) to study the martensitic transformation induced crack tip toughening in CuAlBe SMA. The force-displacement curve, transformation zone and high stress zone in single crystalline samples show high dependency on the grain orientation with respect to the crack alignment. Comparison between isothermal and non-isothermal simulations reveals that the transformation-induced self-heating decreases the toughening capability by increasing the critical transformation stress. Investigating the high stress zones in CuAlBe and the non-transforming CuAl alloy shows that the toughening is obtained by redistributing the stress concentration far from the crack tip, as the high stress zone follows the moving tip of the transformation zone. Increasing the acuity of the crack tip is found to generate more symmetric martensite wings on both side of the crack axis, and a more localized high stress zone. The polycrystal specimen displays higher toughening due to the internal constraints related to the presence of various grains with difference orientations. Depending on the orientation of the grain inclosing the crack tip, the toughening effect can be lower or higher. Coincidence of the crack tip with a triple junction is found to improve the toughening behavior.