Stress-induced Martensite Formation Research Articles

Self-Heating and Fatigue Assessment of Laser Powder Bed Fusion NiTi Alloy with High Cycle Fatigue Mechanisms Identification

Rapid methods for assessing the fatigue properties of materials have been developed, among which the self-heating method stands out as particularly promising. This approach analyzes the thermal signal of the specimen when subjected to cyclic loading. In this research, the self-heating method was utilized for the first time with laser powder bed fusion (LPBF) of NiTi alloys, examining two specific loading conditions: loading ratios of 0.1 and 10. A thorough examination of the material self-heating behavior was conducted. For comparative purposes, conventional fatigue tests were also conducted, alongside interrupted fatigue tests designed to highlight the underlying mechanisms involved in high cycle fatigue and potentially self-heating behavior. The investigation revealed several key mechanisms at play, including intra-grain misorientation, the emergence and growth of persistent slip bands, and the formation of stress-induced martensite. These findings not only deepen our understanding of the fatigue behavior of LPBF NiTi alloys but also highlight the self-heating method potential as a tool for studying material fatigue.

Influence of isothermal omega precipitation aging on deformation mechanisms and mechanical properties of a β-type Ti-Nb alloy

In this study, the influence of ωiso precipitates on the active deformation mechanisms and the mechanical properties of the biomedical β-type Ti-40Nb alloy are revealed. Low temperature heat treatments (aging) at 573 K for durations up to 108.0 ks were carried out for a cold-rolled and recrystallized sample state. After an aging time of 3.6 ks the ωiso phase was determined by means of synchrotron XRD and the fraction and the crystallite size of ωiso increased progressively with increasing aging time. Due to the high intrinsic Young’s modulus of the ωiso phase, the Young’s modulus increased gradually with the aging time from 63 GPa, for the recrystallized reference condition, to values of 70 GPa (3.6 ks), 73 GPa (14.4 ks), 81 GPa (28.8 ks) and 96 GPa (108.0 ks). Depending on the aging time, also a change of the active deformation mechanisms occurred, resulting in significantly altered mechanical properties. For the single β-phase reference microstructure, stress-induced martensite (SIM) formation, {332}113̅ twinning and dislocation slip were observed under tensile loading, resulting in a low 0.2% proof stress of around 315 MPa but a high elongation at fracture of 26.2%. With increasing aging time, SIM formation and mechanical twinning are progressively hindered under tensile loading. SIM formation could not be detected for samples aged longer than 3.6 ks. The amount and thickness of deformation twins is clearly reduced with increasing aging time and for samples aged longer than 14.4 ks deformation twinning is completely suppressed. As a result of the changed deformation mechanisms and the increase of the critical stress for slip caused by ωiso, the 0.2% proof stress of the aged samples increased gradually from 410 MPa (3.6 ks) to around 910 MPa (108.0 ks). With regard to application as new bone implant material, a balanced ratio of a low Young’s modulus of E = 73 GPa and higher 0.2% proof stress of 640 MPa was achieved after an aging time of 14.4 ks.

Integrated multi-scale modeling of variant selection during stress-induced martensite formation in metastable β Ti-alloys

Stress-induced martensite formation is the predominant mechanism during the deformation of various functional and structural Ti- alloys. To predict the performance of these alloys, stress-induced martensite formation was modeled as a function of crystal orientation and stress state. We present an integrated micromechanical modeling approach using finite element (FE) analysis and an elastic spectral solver based on Fast Fourier Transforms (EFFT), which allows direct correlation of the available work from martensite formation under complex stress-states in an in-situ characterized microstructure. The model is applied as a virtual analogue of an experimental 3-point bending test of a metastable β Ti–10V–2Fe–3Al alloy containing 5% α. The FE model incorporates the experimental β microstructure from electron backscattering diffraction (EBSD) with anisotropic elastic behavior. The EFFT solver uses strain fields in two local regions from the FE model to predict local stresses in experimental microstructures containing β and α phases. The stress and orientation data are used to predict the available work from stress-induced α″ martensite formation of the six different α″ variants. It was found that stress and available work concentrated around the tips of lamellar α, making these regions preferred nucleation sites for α″ formation, which is in excellent agreement with the in-situ experimental observations. This suggests that the local stress conditions in the β phase play a more important role in triggering α″ formation than the compositional inhomogeneity in the β phase at α-β interfaces. Using the integrated model, the first variants forming at low levels of deformation could be confidently predicted. However, the transformation behavior at elevated levels of deformation could not be properly captured. This indicates the need of further model improvement, including Dirichlet boundary conditions for the EFFT solver and accounting for the role of plasticity induced by phase transformation.

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 impact of nanometric γ’ precipitates on the tensile deformation of superelastic Co49Ni21Ga30

Results are presented reporting on the martensite domain variant selection and stress-induced martensite morphology in [001]-oriented superelastic Co49Ni21Ga30 shape memory alloy (SMA) single crystals under tensile load. In situ neutron diffraction, as well as in situ optical- and confocal laser scanning microscopy were conducted focusing on three differently treated samples, i.e. in the as-grown, solution-annealed and aged condition. An aging treatment performed at 350 °C promotes the precipitation of nanoprecipitates. These second phase precipitates contribute to an increase of the number of habit plane interfaces, while reducing lamellar martensite plate thickness compared to the as-grown and solution-annealed (precipitate free) samples. During tensile loading, all samples show a stress-induced formation of martensite, characterized by one single domain variant (“detwinned”) and one set of parallel habit planes in a shear band. The results clearly show that γ’ nanoprecipitates do not necessarily promote multi-variant interaction during tensile loading. Thus, reduced recoverability in Co-Ni-Ga SMAs upon aging cannot be solely attributed to this kind of interaction as has been proposed in literature so far.

Functional behaviour of TiNi shape memory alloy after high strain rate deformation

This paper presents an investigation of the influence of strain rate on the functional behaviour of TiNi alloy. Specimens were deformed in compression and tension modes using the Kolsky method for a split Hopkinson pressure bar, and its modification in martensitic, austenitic, and pre-martensitic states at 500, 1000, and 1500 s−1 strain rates. One-way shape memory and two-way shape memory (TWSM) effects were measured and compared with ones after quasi-static deformation. With an increase in the strain rate, the one-way shape memory effect decreased, but the TWSM effect increased. Functional behaviour indirectly demonstrated the sensitivity of the martensite reorientation and the formation of stress-induced martensite to strain rate. Strain rate sensitivity led to differences in the shape recovery curves during thermocycling.

Microstructural evolution and properties of a Ti-Nb-Ta-Zr-O prepared by high-pressure torsion

In this work, the Ti-29Nb-13Ta-4.6Zr-0.07O (TNTZO) alloy was processed by high-pressure torsion (HPT), and its microstructure, phase stability, elastic behavior, mechanical properties and deformation mechanisms were determined for samples prepared under different conditions. An increase in β-phase stability relative to martensite and a significant hardening was observed after severe plastic deformation. The increase in β-phase stability is attributed to the sharp increase in the system’s Gibbs free energy that occurs when the α"-laths reach a critical nanometer-size during the HPT processing. This system’s Gibbs free energy increase triggers the occurrence of the α"→ β reverse martensitic transformation. The greater hardness can be related to three factors: microstructural refinement; high dislocation density and stress-induced formation of the ω-phase. Plastic deformation is based on conventional slip, detwinning, stress-induced martensite formation, as well as the reverse martensitic transformation. The reverse martensitic transformation observed during HPT corresponds to a recently discovered mechanism that has not been well addressed in the literature until now. In this paper, this new mechanism is clearly detected and detailed discussed based on a systematic Rietveld refinement, in-depth microscopy, and calorimetry.

Effect of Yttrium on the microstructure, phase transformation and superelasticity of a Ti–Ni–Cu shape memory alloy

The purpose of this work is to systematically investigate the effect of substituting Yttrium for Ti on the microstructure, phase transformation and superelasticity of Ti50Ni44Cu6 alloys. The results show that a single-step martensitic transformation occurs in all the selected Ti–Ni–Cu–Y alloys. With increasing Yttrium content, the transformation temperatures first increase approximately by 10–15 °C and then decreases slowly. In the cyclic compression experiments of pre-strain from 2% to 10%, the main reason for the increase of the residual strain is the plastic deformation of the alloys caused by the increasing pre-strain. In the cyclic compression experiments of 5% fixed pre-strain, the recovery rate and the total recoverable strain can be effectively improved to 92.4–96.6% and 4.6–4.83%, respectively. With increasing cycles, as the formation of stress-induced martensite is precluded by high density dislocations, the superelasticity-dominated recovery transforms to elasticity-dominated recovery, which may lead to the gradual decrease of residual strain. In the two kinds of cyclic compression experiments, the results show that the alloys with higher Y content normally have better superelasticity.

Cyclic deformation characteristics of the metastable β-type Ti–40Nb alloy

The present study investigates the cyclic deformation behavior of a new metastable β-type Ti–40Nb alloy (wt%) in two different microstructural conditions. Severe cold rolling followed by recrystallization annealing were performed on cast and homogenized Ti–40Nb rods to obtain a single β-phase microstructure at room temperature. Additional aging at 573 K was carried out to generate isothermal ω-phase precipitates in the β-matrix. Fatigue tests at cycles up to 2 × 106 and a stress ratio of R = −1 were realized with samples with electrochemically polished surfaces. EBSD analysis was carried out after the fatigue tests. Furthermore, detailed fractographic investigations as well as TEM analysis were executed. Results revealed significant differences in the cyclic deformation behavior and a higher fatigue strength for the aged condition. Reasons for superior fatigue properties of the aged condition are a pronounced precipitation hardening effect of the ω-phase as well as a complete suppression of stress-induced martensite formation and deformation twinning due to a barrier function of the ω-precipitates. In the vicinity of the fatigue crack tip, where the localized plastic zone dominates the damage evolution, precipitation-depleted channels can be observed. Within these ω-depleted channels dislocations have an increased mobility, allowing highly localized plastic deformation. Signs of other deformation features with increasing distance to the fatigue crack are not observed. EBSD analysis of the recrystallized samples with initial single β-phase showed on the contrary changes in the microstructure caused by the cyclic loading and the high instability of the β-phase. A dominant development of stress-induced α’’-martensite towards a full martensitic microstructure dependent on the applied stress amplitudes and cycles as well as additional isolated {332} <113> twinning were detected.

Deformation Induced Hierarchical Twinning Coupled with Omega Transformation in a Metastable \u03b2-Ti Alloy

Hierarchical twinning, at multiple length scales, was noted in a metastable body-centered cubic (bcc) β-titanium alloy on tensile deformation. Site-specific characterization within the deformation bands, carried out using EBSD and TEM, revealed {332} <113> type primary bcc twins, containing different variants of secondary and tertiary twins, as well as the formation of stress-induced martensite (α”). Within the primary {332} <113> type twin, “destruction” of the prior quenched-in athermal ω phase was observed, while a stress-induced ω phase reforms within the tertiary twins, revealing the intricate nature of coupling between deformation twinning and displacive ω transformation.

Microstructural characteristics and tensile behavior of medium manganese steels with different manganese additions

Manganese is normally considered as the most important alloying element in medium Mn steels. However, the influence of Mn additions on the microstructural characteristics and the ensuring mechanical properties has not been much studied. This was addressed in the present study by investigating two variants of medium Mn steels with compositions of 0.2C-7/10Mn-3Al (in wt%), subjected to intercritical annealing (IA) and tensile testing. Results showed that a higher Mn addition can effectively increase the fraction of retained austenite (RA) at ambient temperature. However, the mechanical stability of RA was reduced, due to the lower C concentration partitioned into austenite during IA treatments. The higher fraction and lower mechanical stability of RA in the 10Mn steel gave rise to an enhanced transformation-induced plasticity (TRIP) effect, which was the main contributor to its higher strain hardening rate and improved mechanical properties, confirmed by a dislocation density-based strain hardening model. The yielding behavior and the Portevin-Le Chatelier (PLC) effect of the investigated steels were also found to be altered with different austenite mechanical stability as a consequence of different Mn additions and IA temperatures. Higher deformation-induced martensite transformation (DIMT) kinetics, i.e. lower mechanical stability of austenite, resulted in a lower yield point elongation, and eventually changed the yielding to a continuous manner due to the formation of stress-induced martensite. The PLC effect only occurred in the intermediate austenite stability range, and the critical strain for the onset of jerky flow showed a first decrease and then an increasing trend with higher DIMT kinetics. The observed beneficial effect of Mn additions on mechanical property improvement in medium Mn steels offers a significant aspect for further alloy design of such steels.

On the competition between the stress-induced formation of martensite and dislocation plasticity during crack propagation in pseudoelastic NiTi shape memory alloys

Abstract

Ternary Ti-Zr-Nb and quaternary Ti-Zr-Nb-Ta shape memory alloys for biomedical applications: Structural features and cyclic mechanical properties

Ti-18Zr-14Nb, Ti-18Zr-13Nb-1Ta, Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta (at%) shape memory alloys were obtained by vacuum arc remelting and subjected to different thermomechanical treatment (TMT) schedules. The transmission electron microscopy analysis revealed that a TMT, which consisted of cold rolling with a true logarithmic strain of e = 0.3 and post-deformation annealing at 600°C (30min), led to a subgrained structure formation with a size of structural elements of 200–300nm and to a room temperature superelastic behavior. The X-Ray diffraction (XRD) analysis showed that a gradual replacement of Nb content by its Ta equivalent decreases the impact of alloying elements on the β-phase stabilization phenomenon and suppresses the ω-phase formation: at room temperature, Ti-18Zr-14Nb and Ti-18Zr-13Nb-1Ta alloys contain mainly parent β-phase, while Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta alloys contain considerable amount of α”-martensite. The crystallographic resource of recovery strain for all the alloys exceeds 5% and reaches 6%, for the Ti-18Zr-11Nb-3Ta alloy. The fatigue life of the Ti-18Zr-13Nb-1Ta alloy is almost twice as high as that of the Ti-18Zr-14Nb alloy. During multi-cycle mechanical testing, the following regularity appeared for all the alloys tested: during 10–15 first loading-unloading cycles, the mechanisms of plastic deformation and stress-induced martensite formation and stabilization prevail, while after that, the β⇄α” martensitic transformation mechanisms become dominant, and the alloys’ behavior turns from elasto-plastic into superelastic.

Local behavior of an AISI 304 stainless steel submitted to in situ biaxial loading in SEM

The microstructural response of a coarse grained AISI 304 stainless steel submitted to biaxial tensile loading was investigated using SEM and X-ray diffraction. The specimen geometry was designed to allow for biaxial stress state and incipient crack in the center of the active part under biaxial tensile loading. This complex loading was performed step by step by a micromachine fitting into a SEM chamber. At each loading step FSD pictures and EBSD measurements were carried out to study the microstructural evolution of the alloy, namely grain rotations and misorientations, stress-induced martensite formation and crack propagation. According to their initial orientation, grains are found to behave differently under loading. Approximately 60% of grains are shown to reorient to the [110] Z orientation under biaxial tensile loading, whereas the 40% left undergo high plastic deformation. EBSD and XRD measurements respectively performed under loading and on the post mortem specimen highlighted the formation of about 4% of martensite.

Structure and properties of self-expanding intravascular NiTi stents doped with Si ions

Here we present research results on the structure, functional properties, and mechanical characteristics of self-expanding nickel-titanium (NiTi) stents the surface of which is doped with silicon (Si) by plasma immersion ion implantation. The stents are of two dimension types used to recover the lumen of peripheral vessels (carotid artery and abdominal aorta). When implanted, Si ions penetrate into the stents, forming a Si-doped layer of thickness up to 80 nm at their inner and outer surfaces. After deformation at room and lower temperature, the surface-modified stents fully recover their shape under unloading at body temperature. The surface modification by plasma immersion ion implantation decreases the stress for the formation of stress-induced martensite under loading and increases the hysteresis under unloading. The research results allow the conclusion that Si-doped stents are promising for treatment of peripheral vascular disease. However, related studies on laboratory animals are required.

Stress-ageing effects on the functional properties of high-strength [001]-oriented Ni51.0Ti37.3Hf12.5 single crystals

The effects of stress-free and stress-assisted ageing at 873 K for 3 h on the crystal microstructure, the B2-B19' martensitic transformation (MT), the critical stress level for the stress-induced martensite formation, and the superelasticity (SE) in Ni-rich Ni51.0Ti37.3Hf12.5 (at.%) single crystals, oriented along the [001]B2 direction, were investigated. For the first time, it is experimentally demonstrated that ageing, under a compressive stress of 150 MPa along the [001]B2 direction, leads to the oriented growth of H-phase particles. The temperature range of SE and the critical stress level for martensite formation are determined by variation in the size (40-150 nm) and the number of crystallographic variants of the H-phase particles. Reversible stress-induced B2-B19' MTs at high levels of applied compressive stress up to 1750-1970 MPa, with the reversible strain of 1.1%, are observed in all investigated Ni51.0Ti37.3Hf12.5 single crystals.

Shape memory response and mechanical properties of warm deformed NiTi intermetallic alloy

The warm deformation method as an effective thermomechanical processing (TMP) route was applied on the equiatomic NiTi alloy to enhance its shape memory and mechanical properties. To this end, warm compression tests at different temperatures in the range of 100–500°C under different strain rates of 0.003, 0.03 and 0.3s−1 were performed. The material flow behaviors at 100°C revealed the occurrence of stress induced martensite formation; this caused the specimens to fail by fracture. However, the material flow behavior at 300°C and 500°C demonstrated a dynamic restoration phenomenon. The higher stability of austenite phase in the processed specimens at 300°C was indicated by microstructural and XRD analysis. In addition, the bimodal martensite packets were distinguished in the materials processed at 500°C. The latter might well designate the occurrence of dynamic recrystallization phenomena. Furthermore these existence of austenite phase in 300°C processed samples lead to an unfavorable shape memory response, while the fully martensitic structure of 500°C processed samples materials exhibited a better shape memory recovery. The final mechanical properties of the alloy were improved through TMP, and applying the TMP at 500°C increased the shape recovery strain of the alloy from approximately 1.5% (in as-hot rolled material) to 3.6%.

Twinning-Induced Elasticity in NiTi Shape Memory Alloys

Pseudoelasticity (PE) in shape memory alloys relies on the formation of stress-induced martensite during loading and on the reverse transformation during unloading. PE yields reversible strains of up to 8 % and is applied in applications such as medical implants, flexible eye glass frames, damping elements, and others. Unfortunately, PE shows a strong temperature dependence and thus can only be exploited within a relatively narrow temperature window. The present work focuses on a related process, which we refer to as twinning-induced elasticity (TIE). It involves the growth and shrinkage of martensite variants which are stabilized by dislocations, which are introduced by appropriate cold work. TIE yields reversible strains of the order of 3 %. The TIE effect does not suffer from the strong temperature dependence of PE. The weak temperature dependence of mechanical TIE properties makes TIE attractive for applications where temperature fluctuations are large. In the present work, we study the TIE effect focusing on Ni50Ti50 shape memory alloy wires. The degree of plastic pre-deformation of the initial material represents a key parameter of the ingot metallurgy processing route. It governs the exploitable recoverable strain, the apparent Young’s modulus, and the widths of the mechanical hysteresis. Dynamic mechanical analysis is used to study the effects of pre-deformation on elementary microstructural processes which govern TIE.

Synchrotron analysis of toughness anomalies in nanostructured bainite

High-resolution synchrotron X-ray diffraction has been used to characterise the notch root regions of Charpy impact test specimens of a superbainitic steel, both before and after loading. The changes in the volume fraction of austenite induced by the application of a three-point-bending load were quantified. Analysis of diffraction peak shifts revealed the extent of residual tensile and compressive strains present due to both machining and an applied load. The results lend support to the hypothesis that the comparatively low energies absorbed during Charpy impact testing of superbainitic steels, <7 J, are due to the formation of stress-induced martensite at the notch root, prior to crack initiation.

Superelasticity in Polycrystalline Ni-Mn-Ga-Fe Microwires Fabricated by Melt-extraction

Ni48Mn26.4Ga19.7Fe5.9 microwires with grain size of 1-3 micron were successfully fabricated by melt-extraction. The superelastic effects in the microwires under various temperatures and loads were systematically demonstrated. The as-extracted microwires displayed partial superelasticity when attended at relatively high temperature. The critical stress for stress-induced martensite formation increases linearly with temperature and follows the Clausius-Clapeyron relationship. The temperature dependence of the as-extracted polycrystalline Ni48Mn26.4Ga19.7Fe5.9 microwires is 16.4 MPa/K, which is higher compared with Ni-Mn-Ga single crystals. In addition, the as-extracted microwires display excellent shape memory behavior with the recovery strain and recovery ratio of 1.26% and 86%, respectively, when the total strain reaches 1.47% at 310 K.