Stress-induced Martensitic Transformation Research Articles

Mn vaporization control and stress-induced martensite transformation in high-manganese steel welds under different laser keyhole conditions

Mn vaporization control and stress-induced martensite transformation in high-manganese steel welds under different laser keyhole conditions

Stress-induced martensite transformation and mechanical properties of fine-grained Ti-10V-2Fe-3Al alloy fabricated by friction stir processing

Ti-10 V-2Fe-3Al, as the metastable β-type titanium alloy, has several applications in the aerospace industry due to its good mechanical properties. However, the study of stress-induced martensitic transformation behavior and corresponding mechanical properties in its small-sized grains (<20 μm) needs to be further clarified. This study investigates the relationship between grain size, stress-induced martensitic transformation, and mechanical properties of Ti-10 V-2Fe-3Al after friction stir processing. When the spindle speed increased from 200 rpm-100 mm/min to 350 rpm-100 mm/min, the grain size grew from 7.53 μm to 13.06 μm, and the martensite triggering stress decreased from 287 MPa to 111 MPa. The phase boundaries formed by α" and β, as well as the grain boundaries formed by α" and α", enhanced the plasticity of Ti-10 V-2Fe-3Al. The Crussard-Jaoul analysis method was used to investigate the effect of stress-induced martensitic transformation on Ti-10 V-2Fe-3Al, it was found that the martensitic transformation leads to dynamic softening. The formation of α" martensite results in an increased work hardening rate.

Assessment of the mechanical and functional properties of nitinol alloys fabricated by laser powder bed fusion: Effect of strain rates

Laser Powder Bed Fusion-fabricated (LPBFed) nitinol shape memory alloys (SMAs), which undergo solid-solid phase transitions between austenite and martensite, are increasingly utilized in various technical fields and are subjected to a wide range of strain rates during service. This study pioneers the investigation of a comprehensive range of strain rates (3.16 × 10−6-10°/s), encompassing the strain rates used in quasi-static tensile tests reported in the existing literature, to understand their effects on the deformation mechanisms of LPBFed nitinol SMAs. Utilizing multi-scale characterization, including macroscopic mechanical evaluation, mesoscale analysis of deformation surfaces, and microscale examination of microstructure, we reveal distinct deformation behaviors at different strain rates. At lower strain rates, the mechanical behavior exhibits little sensitivity to changes in the strain rates, with stress-induced martensite transformation or martensite detwinning and variant reorientation being the primary deformation mechanisms. However, at higher strain rates, we observe significant variability in mechanical response, dominated by dislocation-induced slip and reduced occurrence of stress-induced martensitic transformation. This study provides the first comprehensive database for the engineering applications of LPBFed Nitinol SMAs and offers critical insights into optimizing mechanical and functional assessments for these advanced materials.

Dependence of fretting wear resistance on the α morphology and stress-induced martensite transformation in a metastable β titanium alloy

Systematic experimental investigations concerning the influence of α morphology on the fretting wear behaviors of metastable β titanium alloys are carried out. A Ti-10V-2Fe-3Al titanium alloy was subjected to different heat treatment routes to create dual phase microstructures consisted of β phase plus α phase with three different morphologies. The effect of α morphology on the fretting wear resistance, the resulting failure mechanisms, and stress induced martensite transformation (SIMT) were unraveled. Results show that the α morphology has a significant influence on fretting wear behaviors depending on the fretting run regimes. In the partial slip regime (PSR) and mixed fretting regime (MFR), the microstructure with lath α morphology has a lowest fretting wear volume accompanied by relatively strong SIMT effect. While in gloss slip regime (GSR), the globular α microstructure has a lowest fretting wear volume along with strongest SIMT response, yet a highest fretting wear volume for the acicular α morphology microstructure due to its brittleness nature. The subsurface observations demonstrates that a compacted and thick plastic deformation layer, especially for the mechanical mixture layer, can well protect the material surface against the fretting wear damage.

A high-performance TRIP Mg-Sc-Zn alloy enhanced by fine grain strengthening and nano-precipitate strengthening

The Mg–Sc alloys have attracted considerable attention from researchers due to their martensitic transformation behavior. However, the relatively low yield strength limits their practical application. Thus, Zn alloying combined with annealing treatment was used to improve the mechanical properties by modulating its microstructure. Zn alloying can effectively delay the recrystallization rate and refine the grain size of the alloy. The β/α intensity ratio and grain size of β phase increase with the increasing annealing temperature. When the annealing temperature is 600 °C, the major phase is β-Mg-Sc with minor α-Mg-Sc, ScZn and Mg3Sc phases. Further increasing the temperature to 650 °C would lead to the dissolution of ScZn and Mg3Sc phases into the matrix, resulting in smaller precipitations. The Mg-Sc-Zn alloy annealed at 600 °C for 60 min owns the best comprehensive mechanical properties, with yield strength, ultimate tensile strength, and elongation of 307.2 MPa, 330.5 MPa, and 21.1%, respectively. Further TEM analysis reveals that fine grain reinforcement, precipitation reinforcement, and stress-induced martensitic transformation during tensile deformation are the main factors for the enhancement of strength and ductility in this alloy.

Influence of β Grain Orientation on the Stress‐Induced Martensite Formation in Cold‐Rolled Metastable β Ti–5Mo–5V–5Al–3Cr Alloy

Solution‐treated metastable β Ti–5Mo–5V–5Al–3Cr (Ti 5553) alloy processed through cold rolling at strain levels of 5–45% results in stress‐induced martensitic transformations (SIMT). Further, the cold‐deformed sample is annealed at 860 °C for 5 min to achieve recrystallization and relieve internal stresses. The influence of recrystallized β grain orientations on Ti‐5553 deformation behavior is characterized using nanoindentation tests. As β grain orientation changes from near &lt;111&gt; to &lt;001&gt; direction, the alloy exhibits an increase in hardness (from ≈6.3 to ≈6.6 GPa) and a decrease in elastic modulus (from ≈127 to ≈120 GPa). It can be inferred that a threshold stress is necessary to activate SIMT in each orientation due to the stress gradient from the top surface to the indent tip at a certain penetration depth in the sample. Additionally, the influence of alloying elements (e.g., Mo, V, Al, and Cr) on β phase stability as a function of grain orientations is confirmed using the electron probe microanalyzer. Observations indicate that &lt;111&gt; grain orientation is most preferred, followed by &lt;101&gt; for promoting stress‐induced martensite, while &lt;001&gt; is least preferred.

Quasi-linear superelasticity and associated elastocaloric effect in boron-doped polycrystalline Ni-Mn-Ti alloys

Ni-Mn-Ti shape memory alloys show great potential in solid-state elastocaloric cooling owing to very prominent elastocaloric effect along with first-order stress-induced martensitic transformation. However, large stress hysteresis inherent to martensitic transformation greatly restricts the energy efficiency and cyclic stability of elastocaloric response. Here, we demonstrate the effective manipulation of stress hysteresis as well as the resulting elastocaloric effect through doping boron to Ni-Mn-Ti alloys. With the incremental boron content in (Ni50Mn31Ti19)100–xBx (x = 0, 0.2, 0.5, 1, 1.5) alloys, a plateau-type superelastic behavior with large stress hysteresis gradually evolves into a quasi-linear one with slim hysteresis, giving rise to significant improvement in the energy conversion efficiency of elastocaloric response. In a (Ni50Mn31Ti19)99B1 alloy, the coefficient of performance of material (COPmat) can be as high as 24 ∼ 33. Moreover, under a compressive strain of 4%, large cooling |ΔTad| values higher than 6.5 K in the (Ni50Mn31Ti19)99B1 alloy are maintained for over 8000 superelastic cycles, showing an enhancement of one order of magnitude in the cyclability with respect to that of boron-free Ni50Mn31Ti19 alloy. We attribute the enhanced elasocaloric response to the significantly improved mechanical properties but reduced stress hysteresis endowed by relatively high content of boron doping.

A strategy for simultaneously enhancing the strength and ductility of metastable β titanium alloys: Coupling heterogeneous structure and TRIP effects

The applicability of metastable β titanium alloys is frequently constrained by their relatively low yield strength. This study has successfully designed a metastable β titanium alloy with heterogeneous microstructures (HS) comprising both deformed and recrystallized grains, achieved through simple cold rolling followed by short-term annealing. Compared to traditionally coarse-grained (CG) specimen that is entirely crystallized and equiaxed, the HS specimen exhibit not only a postponed initiation of double yielding but also superior yield strength (enhanced by 27.8 %) and ductility (enhanced by 7 %). The delayed double yielding and enhanced strength in the HS specimen are ascribed to the restriction of dislocation motion by α" martensite and the heterogeneous deformation induced (HDI) strengthening mechanisms. The ductility of the HS specimen exceeds that of the CG specimen, owing to the combined effect of HDI hardening and the transformation-induced plasticity (TRIP) effects. The more pronounced TRIP effect in the HS specimens results from the synergistic effect of the elevated HDI stress and the applied external stress, which promotes the occurrence of stress-induced martensitic phase transformation. This study offers a novel strategy for the development of high-performance metastable β titanium alloys.

Comparative analysis of microstructure and mechanical properties in FeMnAlNi alloys fabricated via casting and additive manufacturing

This study provides a comprehensive analysis of FeMnAlNi alloys fabricated via casting, laser-based directed energy deposition (DED-LB), and powder bed fusion-laser beam (PBF-LB) techniques, highlighting the profound impact of manufacturing processes on their microstructural characteristics and mechanical properties. The investigation reveals distinct two-phase distributions, with PBF-LB samples retaining the high-temperature α-phase due to rapid cooling, contrasting with the γ-phase predominance in as-cast and DED-LB samples. Notably, PBF-LB samples also exhibit fine γ′ martensites, likely a result of stress-induced martensitic transformation. Mechanical testing shows significant variations across the samples: as-cast and DED-LB samples demonstrate similar microhardness values (211 ± 22 and 209 ± 7 HV0.2, respectively), while PBF-LB samples exhibit higher microhardness (321 ± 9 HV0.2) due to the dominance of α-phase. Specifically, the DED-LB samples demonstrate elongation exceeding 50 %, with a notable improvement in ductility compared to the as-cast samples. The Kurdjumov–Sachs orientation relationship between α-phase and γ/γ′-phases is found. These insights reveal that considerable work remains to be done, including exploring post-processing heat treatments, which have the potential to induce superelasticity in DED-LB samples and further enhance the application potential of FeMnAlNi alloys in advanced engineering fields.

Stress-Induced Martensitic Transformation in Na3YCl6.

Martensitic transformation with volume expansion plays a crucial role in enhancing the mechanical properties of steel and partially stabilized zirconia. We believe that a similar concept could be applied to unexplored nonoxide materials. Herein, we report the stress-induced martensitic transformation of monoclinic Na3YCl6 with an ∼3.4% expansion. In situ synchrotron X-ray diffraction and atomistic simulations showed that anisotropic crystallographic transformation from monoclinic to rhombohedral Na3YCl6 occurs exclusively under uniaxial pressure; no effect is observed under hydrostatic pressure conditions. The uniaxially pressed powder compact of monoclinic Na3YCl6 showed a large indentation impression and low Young's modulus, in contrast to its high bulk modulus, suggesting that these unique mechanical properties are induced by the martensitic transformation.

Regularities of Martensitic Transformations in New Medium-Entropy Single Crystals of CoNiAlFe Alloys

This paper deals with the martensitic transformation and functional properties in the quenched single crystals of the Co35Ni35Al28Fe2 medium-entropy alloy, oriented along the [001]B2-direction. The microstructure and chemical composition of the single crystals have been studied in detail using transmission and scanning electron microscopy. The {111}L10 martensite twins up to 10-20 nm width and γ/γ′-phase precipitations larger than 100 μm are detected. The thermoelastic B2-L10 martensitic transformation upon stress-free cooling/heating in single crystals of Co35Ni35Al28Fe2 alloy is characterized by the accumulation of elastic energy, which is the driving force of the reverse martensitic transformation, and the low dissipation energy. The reverse transformation starts at lower temperatures than the forward transformation Ms&gt;As. The regularities of the stress-induced B2-L10 martensitic transformation change due to an increase in the contribution of the dissipated energy and Msσ&lt;Asσ. There is shape memory effect with the reversible strain (3.2±0.3)% and high temperature superelasticity with the reversible strain (3.3±0.3)% in the temperature range from 323 K to ≥548 K in the [001]B2-oriented single crystals. These crystals withstand stress up to 1200 MPa in compression without destruction.

Achieving strength-ductility synergy in LPBF Ti-6Al-8 V alloy through in situ alloying

This study utilized in situ alloying during laser powder bed fusion (LPBF) to create a novel Ti-6Al-8V (wt. %, Ti68) alloy. Pure vanadium nanoparticles were incorporated into Ti-6Al-4V feedstock at a concentration of 4 wt.%. The resulting Ti68 alloy displayed exceptional mechanical properties, including a ultimate tensile strength of 1163 ± 34 MPa and a fracture elongation of 13.1 ± 2.0 %. These improvements can be attributed to the unique microstructure of this alloy, characterized by a heterogeneous α’+β phase resulting from the uneven distribution of vanadium. This microstructure enables sustained plastic deformation through dislocation slip and twinning in α’ phase, as well as dislocation slip and stress-induced martensitic transformation in β phase.

Effect of primary α phase fraction on deformation mechanism of Ti-1023 alloy at room temperature

The fraction and morphology of α phase have important effects on the mechanical properties and deformation mechanism of Ti-1023 alloy. Here, the tensile stress-strain curves and deformation mechanisms with the fraction of α phase are discussed in detail. As α phase fraction decreases from 67.5 % to 12.6 %, there is a negative correlation coefficient between the yield strength and α phase volume fraction of Ti-1023 alloy, and its fitting equation is obtained. When the fraction of α phase is decreased to 12.6 %, the elongation and ultimate tensile strength of the alloy are 31.09 % and 845.52 MPa, as well as a transition from single to double yielding phenomenon, which is caused by stress-induced martensitic transformation (SIMT). When the fraction of α phase is high, the deformation mechanism is dominated by dislocation slip and mechanical twinning of the α phase. When the fraction of α phase is low, the deformation mechanism is dominated by mechanical twinning of the β matrix and stress-induced martensitic transformation (SIMT). The relationship between α phase fraction and deformation mechanism and mechanical properties of Ti-1023 in α+β dual phase region is studied, which provides a template to guide the development of comprehensive mechanical properties of metastable β titanium alloy in α+β dual phase region.

Effect of laser energy density on the phase transformation behavior and functional properties of NiTi shape memory alloy by selective laser melting

NiTi shape memory alloy (SMA) was prepared via selective laser melting (SLM). Orthogonal tests were conducted with varying laser power and scanning speeds to explore the influence of laser energy density on microstructure, phase transformation behavior, mechanical properties and functional properties (superelasticity and shape memory effect). As energy density increases from 24.3 to 106.3 J/mm³, microstructure transforms from fine to columnar grains. Ni consumption intensifies, boosting transformation temperature and B2 content at room temperature. Post cyclic stretching, B2 content in alloy escalates further at room temperature. Tensile strength peaked at 53.6 J/mm³, while the elongation rate significantly increases with increasing energy density. During cyclic stretching, low or high energy densities in SLM NiTi induce high-density dislocations and stable residual martensite, respectively, significantly inhibiting martensitic transformation and resulting in extremely poor functional properties. The alloy produced at 53.6 J/mm³ boasts exceptional superelasticity due to stress-induced martensitic transformation and phase inversion post-unloading. Following heating past the critical transition, it nearly fully recovers (97.78 % recovery rate). Despite multiple shape memory fatigue tests, it maintains a recovery rate above 90 %. This study provides crucial theoretical insights for optimizing NiTi SMA phase transformation behavior and enhancing the performance.

Tension-compression asymmetry in Ti50Ni45Fe5 B2 intermetallic

This work investigates the asymmetry under tension and compression for the homogenized Ti50Ni45Fe5 B2 intermetallic. B2 matrix undergoes localized stress induced martensitic transformation (B2 → B19') under compression, resulting in a visible nonlinear elastic regime in the stress-strain curve. This was absent during deformation under tension; which could possibly be ascribed to lower resolved shear stress required for slip under tension than compression and a higher stress required for stress induced martensite transformation under tension than compression due to different martensite variants. The plastic deformation of B2 matrix under both tension and compression was marked by the presence and movement of stable elongated super intrinsic stacking faults, oriented along particular direction, which could be attributed to the activation of ⟨111⟩ screw dislocations.

Mechanical properties and corrosion resistance improvement of TiAlSnMo alloys via Mo addition

In this paper, Ti-5Al-2.5Sn-xMo (TASxM, x = 0,5,10,15 wt%) alloys were designed and prepared. Phase compositon and microstructure analysis results indicate that the addition of Mo reduces the phase transition point of TASxM alloys, stabilizes more β phase to room temperature, and refines the α grains. The strength of the TASxM alloys exhibits a positive correlation with the increasing Mo content within 10 wt%, and the highest ultimate tensile strength (UTS) can be obtained in TAS10M (1260 MPa), which can be primarily attributed to the synergistic effects of phase transition, solid solution strengthening, and fine grain strengthening. Interestingly, the UTS of TAS15M unexpectedly decreases to 724 MPa, which is strongly influenced by the altered slip system numbers resulting from the β/α phase ratio. Even more intriguing is the observation of stress-induced martensitic transformation in the deformed TAS15M alloy, often resulting in a substantial influence on elongation. The corrosion resistance of the TASxM alloy has also been investigated through electrochemical experiments. The findings reveal that the corrosion resistance of the TASxM alloy is enhanced with the elevation of Mo content, peaking at TAS15M. This improved resistance is attributed to the augmented β phase content and the formation of protective MoO2 and MoO3 oxide films on the surface of alloy.

Acoustic emission during approaching the critical point on stress-temperature diagram of martensitic transformation in Ni48Fe20Co5Ga27 (at.%) single crystal

Recently, the existence of martensite-austenite critical point, CP, on the stress-strain curves in the single crystalline Ni–Fe(Co)-Ga ferromagnetic shape memory alloys has been discovered by disappearance of hysteresis. It was also argued that the shear modulus at the CP and in the postcritical phase should be zero. In the present work we continue the investigation of the above new features on the “compression stress-temperature” phase diagram of the martensitic transformation (MT) of Ni48Fe20Co5Ga27 (at.%) single crystals in the vicinity of the CP by acoustic emission (AE). It is found that the AE activity (amount of events and their amplitudes) drastically dropped down by approaching the CP and tended to stabilize in a postcritical region. The above decrease is mainly due to the decrease of the number of large avalanches, while the exponents of the probability distribution of amplitudes and energies remained approximately the same, illustrating no change in the mechanism of acoustic emission events. The coordinates of the CP were estimated to be (340 ± 50) MPa and (327 ± 40) K. The elastic modulus was reduced with increasing temperature from 20 GPa down to 2 GPa in the critical region, whereby a huge total deformation of about 13 % was achieved. The stress-induced MT also shifted as a function of temperature and its slope decreased by about a factor of two upon approaching the CP. All features of the critical state determined have an obvious practical interest, as well as present a solid basis for theoretical understanding of critical and postcritical states in solids.

Near-linear deformation behavior of quasi-Gum Metal with composition Ti-18.4 Nb-1.4Zr-0.3Ta-4.3 O

A new type of Ti-18.4 Nb-1.4Zr-0.3Ta-4.3 O exhibiting near-linear deformation behavior was created to elucidate the core principles behind its near-linear deformation behavior in quasi-Gum Metals. The in situ simultaneous X-ray diffraction study shows that a β→α″ stress-induced martensitic transformation (SIMT) persists and covers a large stress range under the near-linear deformation of Ti-18.4 Nb-1.4Zr-0.3Ta-4.3 O. Within the SIMT process, the variant 5 of α″ martensite usually forms more readily as it can output the maximum phase transformation strains compared to the other equivalent variants. The experimental results indicate that near-linear deformability of quasi-Gum Metal is due to the combined influence of its inherent deformation (including elastic and trace plastic) together with a SIMT that operates over a large strain range. These findings have helped to develop a more comprehensive view of the principle behind the near-linear deformation in quasi-Gum Metals and Gum Metal.

Modeling the interaction between instabilities and functional degradation in shape memory alloys

Localization of the stress-induced martensitic phase transformation plays an important role in the fatigue behavior of shape memory alloys (SMAs). The phenomenon of return-point memory that is observed during the subloop deformation of a partially-transformed SMA is a clear manifestation of the interaction between localized phase transformation and degradation of the functional properties. The present study aims to demonstrate this structure–material interaction in the modeling of return-point memory. It seems that this crucial aspect has been overlooked in previous modeling studies. For this purpose, we developed a gradient-enhanced model of pseudoelasticity that incorporates the degradation of functional properties in its constitutive description. The model is employed to reproduce the hierarchical return-point memory in a pseudoelastic NiTi wire under isothermal uniaxial tension with nested subloops. Additionally, a detailed analysis is carried out for NiTi strip with a more complex transformation pattern. Our study highlights the subtle morphological changes of phase transformation under different loading scenarios and the resulting implications for return-point memory.

Sputter-deposited Ni-rich NiTi thin films: Mechanical behavior and composition sensitivity

In this study, we investigated the mechanical behavior of sputter-deposited Ni-rich NiTi thin films with various chemical compositions. Freestanding thin films were successfully produced by employing magnetron sputtering and microfabrication techniques. Notably, films containing Ni within the range of 52.6–57.6 at. % exhibited distinct stress-induced martensitic transformations. Among these, 53.3 at. % and 54.2 at. % Ni–Ti films demonstrated a remarkable combination of strength exceeding 1.5 GPa and elastic strain approaching 5%. Conversely, films with the highest Ni content displayed nearly linear elastic behavior, showcasing an exceptional tensile strength of 2.2 GPa. As the element composition deviated from the equiatomic ratio, the grain size of the deposited films underwent a transition from several micrometers to nano-scale. In addition, the critical stresses showed lower sensitivity to chemical composition compared to bulk alloys, indicating that superelastic behavior persists over a wider composition range in Ni-rich NiTi thin films. By comparing the NiTi matrix and precipitate morphology between bulk alloys and thin films, we discussed the variations in the mechanical behavior of thin films.