Martensitic Transformation Strain Research Articles

A novel Fe-Mn-Al-Ni-Ta-B superelastic polycrystalline alloy with high stability and low thermal hysteresis

The demand for advanced shape memory alloys, with good processability, high stability, and excellent superelasticity, is highly desirable in widespread practical applications. Here, we demonstrated a novel Fe-34Mn-12.5Al-7.5Ni-2.5Ta-0.05B (at%) polycrystalline alloy (FMAN-Ta-B) with good strain-recovery pseudoelastic behavior (5.68% at a compressive strain of 8%), ultra-low thermal hysteresis (∼81 K), and relatively high stability. In this alloy, highly ordered D03-structured precipitates (Ta-riched Fe3Al) (∼26.3 nm) coupled with B2 precipitates (NiAl) (∼19.6 nm), and dispersive distributed in the A2 (austenitic) matrix. On the one hand, D03-structured precipitates act as a connecting structure between B2 precipitates and the matrix, exhibiting low mismatch with the matrix and inducing a thermoelastic martensitic transformation and good strain recovery in the alloy. On the other hand, D03 nanoprecipitates with high structural and thermal stability limited the growth of B2 precipitates and reduced the size-dependence of the superelasticity for the B2 precipitate, thus elevating the aging temperature and enhancing the thermostability of superelasticity. The drastically improved superelasticity and low thermal hysteresis provide useful insights into the design of high-performance ferrous superelastic alloys.

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron backscatter diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the necklace-type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

Phase-field damage simulation of subloop loading in TiNi SMA

In practical applications, TiNi shape memory alloys (SMAs) exhibit behavior that can pose a challenge with current constitutive models and their implementations in finite element method (FEM) software. TiNi SMA devices typically operate in the forward or reverse martensitic transformation regime, which is known as subloop loading. During such cyclic loading–unloading, the hysteresis stress–strain loop changes because of material damage, which can be considered the fatigue of TiNi SMAs. During both the loading and unloading processes, the stress plateau decreases. At the same time, the accumulated (residual) martensitic transformation strain increases. In this study, the experimental investigation results and observations of the aforementioned phenomena are presented. Next, the phase-field damage model is employed, along with a modified Lagoudas constitutive model, to simulate the change in stress–strain hysteresis. Furthermore, a fatigue function is used to simulate the accumulation of martensitic transformation strain. The experimental stress–strain response is compared with the simulation results, and good quantitative and qualitative agreement is obtained. The damage and martensitic volume fraction with respect to strain are discussed for full-loop and subloop loading. The observations and conclusions, as well as open questions, are presented. Possible directions for future research are provided.

Experimental and computational study of the NiTi thin wires mechanical behavior

NiTi wires used for biological implants demonstrated ductile fracture. NiTi 40, 60, and 90 µm thick wires were tested in uniaxial tension to fracture and loading-unloading. Uniaxial stress-strain curves demonstrate superelastic behavior. The inelastic martensite transformation strain is completely recovered upon unloading, forming thermo-mechanical hysteresis. A mathematical model was developed to describe superelasticity effects in NiTi wires. Modeling results are qualitatively and quantitatively similar to experimental data, and capture elastic deformation of austenite, forward martensite phase transformation stress plateau before the onset of martensite elastic deformation and the entire unloading range. The elastic limit and strength increase with the wire thickness.

An outstanding synergy of high strength and ductility/toughness by combining gradient dislocation structure and DIM transformation

Constructing heterostructures is a feasible approach to achieving high strength and ductility/toughness in metals. Here, various gradient dislocation structures (GDS) with different dislocation density distributions were engineered in commercial 304 stainless steel by cyclic–torsion (CT) treatments. With increasing the twist angle of CT treatments, the evident enhancement of strength is along with a slow reduction of ductility following a positive deviation from the inverse linear rule, showing an exceptional strength–ductility synergy. This condition is validated by the simultaneous enhancement of yield strength and static toughness. The enhanced strength was attributed to the introduction of gradient distributed dislocations and the hetero–deformation induced (HDI) stress (300–500 MPa). The retention of high ductility and toughness was provided by the continuous work-hardening from the core to the surface, resulting from the HDI work-hardening and the promoted deformation induced martensite (DIM) transformation (γ → α′) and dynamical strain partitioning effect by the gradient dislocation structure. Moreover, the detwinning mechanism rather than twinning was active for the GDS samples in the tension and for the coarse–grained sample in both CT and tension, and the effect of detwinning on the mechanical properties was also discussed.

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.

Influence of annealing on the functional properties of the NiTi alloy produced by wire arc additive manufacturing

The influence of the annealing temperature on the recoverable strain variation on cooling and heating under a stress of 200 MPa was studied in the NiTi samples produced by wire arc additive manufacturing. The samples including the Ni-rich NiTi layer in the working length were annealed for 10 hours at various temperature from 450 to 600 °C. It is shown that an increase in annealing temperature leads to non-monontonic variation in the recoverable strain. This is caused by an increase in annealing temperature from 450 to 550 °C increases the volume fraction of Ni4Ti3 precipitates. As a result, the volume fraction of the NiTi phase undergoing the martensitic transformation and recoverable strain decrease. An increase in annealing temperature from 550 to 600 °C leads to a dissolving the Ni4Ti3 precipitates and formation of the Ni3Ti2 precipitates that increases the volume fraction of the NiTi phase and the recoverable strain.

Microstructure and properties of TiB2-reinforced Ti\u201335Nb\u20137Zr\u20135Ta processed by laser-powder bed fusion

The Ti–35Nb–7Zr–5Ta (wt%, TNZT) alloy was reinforced with TiB2 and synthesized by L-PBF. The relatively small TiB2 particles change the solidification structure from cellular to columnar-dendritic and lead to submicron TiB precipitation in the β matrix. This results in pronounced grain refinement and reduction of texture. However, the microstructure of the additively manufactured TNZT-TiB2 is still different from the as-cast, unreinforced TNZT, which contains equiaxed and randomly oriented grains. The β phase is less stable in the as-cast samples, leading to stress-induced martensitic transformation and recoverable strain of 1.5%. The TNZT with 1 wt% of TiB2 presents significantly higher compressive strength (σYS = 495 MPa) compared to unreinforced samples (σYS = 430 MPa), without sacrificing ductility or altering Young’s modulus (E ≈ 46 GPa). The addition of a small fraction of TiB2 to the TNZT alloy synthesized by L-PBF is a promising alternative for manufacturing sophisticated components for biomedical applications.Graphical abstract

Mechanical behavior and stability of dispersed retained austenite in thermomechanically rolled and isothermally-treated TRIP-aided multiphase steel

The effect of thermomechanical processing and cooling conditions on the microstructure and mechanical properties during multi-step interrupted tensile tests was investigated in 0.24C–1.55Mn-0.87Si-0.40Al multiphase steel. Two temperature variants of isothermal bainitic holdings at 450 and 350 °C were applied. The tensile tests were interrupted at strain levels of 5, 10, 15%. The microstructure was assessed using light and SEM microscopy. EBSD analysis was performed to determine the retained austenite (RA) distribution and size. Detailed calculations concerning the mechanical performance were performed including the Jaoul-Crussard's analysis. Multiphase steels with the high retained austenite fraction (ca. 15%) of different γ phase sizes below 4 μm in a ferritic-bainitic matrix (with an average size of <6 μm) have been obtained. The research showed that during martensitic transformation of RA, the grain undergoes fragmentation. It has been shown that for the proper use of the TRIP effect, it is necessary to produce the dispersed RA of different sizes in the structure instead of striving for maximum fragmentation, which is a commonly used practice. Obtaining too small grains in the microstructure blocks their transformation during deformation due to their too high stability. Obtaining grains of one size leads to the intensification of the TRIP effect at a given stage of deformation only, reducing the plasticity in the others. The corresponding different mechanical stability of the γ phase ensures a gradual martensitic transformation and beneficial strain hardening progress.

Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons.

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.

Microstructural characteristic and mechanical properties of titanium-copper alloys in-situ fabricated by selective laser melting

In this paper, Ti-Cu alloys were in-situ fabricated by selective laser melting (SLM) with 1.25, 2.5 and 5 wt% Cu contents, namely Ti-1.25Cu, Ti-2.5Cu and Ti-5Cu, respectively. The microstructural characteristic and mechanical properties of Ti-Cu alloys were systematically investigated. The results showed that the SLM-fabricated Ti-Cu alloys have a fine microstructure, and the morphology of martensite exhibited a transition from parallel-sided plates or laths to acicular plates with increasing Cu content. Due to the accommodation effect of the martensitic transformation strain, the 14–20% of the martensite partitioning interfaces was constituted of {101̅1} twin boundaries. The microhardness, yield strength and ultimate tensile strength were enhanced synergistically with increasing Cu content, which can be attributed to the combined effects of grain-boundary strengthening, solid-solution strengthening, dislocation strengthening and precipitation strengthening.

Stress-induced martensitic transformation in metastable austenite grains during nanoindentation investigation

ABSTRACT In general, the formation of martensite in the metastable austenite grains in steels during nanoindentation investigations is believed to be induced by strain as the plastic deformation of austenite takes place prior to the martensitic transformation. However, it is not clear whether the formation of martensite occurs without the prior initiation of plasticity (stress-induced martensitic transformation) during nanoindentation measurement. The present work demonstrates that the martensitic transformation can be triggered during elastic deformation of austenite under an ultralow load when the indenter is close to the annealing twin boundaries. The indentation pressure interacts with the martensite transformation strain, providing the mechanical interaction energy to overcome the nucleation barrier of the martensite embryo. The present work suggests that the annealing twin boundaries can also serve as the nucleation sites of martensite and the stress-induced martensitic transformation is possible during nanoindentation investigation.

Microstructure and Magnetic Field-Induced Strain of a Ni-Mn-Ga-Co-Gd High-Entropy Alloy.

The effect of a high-entropy design on martensitic transformation and magnetic field-induced strain has been investigated in the present study for Ni-Mn-Ga-Co-Gd ferromagnetic shape-memory alloys. The purpose was to increase the martensitic transition temperature, as well as the magnetic field-induced strain, of these materials. The results show that there is a co-existence of β, γ, and martensite phases in the microstructure of the alloy samples. Additionally, the martensitic transformation temperature shows a markedly increasing trend for these high-entropy samples, with the largest value being approximately 500 °C. The morphology of the martensite exhibits typical twin characteristics of type L10. Moreover, the magnetic field-induced strain shows an increasing trend, which is caused by the driving force of the twin martensite re-arrangement strengthening.

Finite element and experimental structural analysis of endodontic rotary file made of Cu-based single crystal SMA considering a micromechanical behavior model

Shape Memory Alloys (SMAs) are widely used in endodontics as instruments for root canal preparation namely endodontic rotary files. Despite their high performances, compared to stainless steel instruments, it is still possible to enhance their cutting efficiency by acting on their shape and material properties. As an example, increasing the exhibited reversible martensitic transformation strain makes them more flexible without decreasing their mechanical strength. It becomes possible with single crystal SMAs. Cu-based (Cu-Al-Be, Cu-Zn-Al, Cu-Al-Mn) single crystal SMAs start emerging and can reach about 12% of martensitic transformation strain, in addition to their interesting antimicrobial properties. This study investigates, both numerically and experimentally, the development of endodontic files made of Cu-based single crystal SMAs. The numerical analysis is carried out by the finite element method. The geometry of the instrument is parameterized. Depending on the applied boundary conditions, bending, torsion, or combined bending-torsion loadings are represented. A micromechanical constitutive law is implemented in Abaqus via the UMAT subroutine to describe the thermomechanical behavior of the Cu-based single crystal SMA. Following the thus obtained numerical results, Coltene-Micromega company has manufactured endodontic file prototypes made of Cu-Al-Be single crystal SMA. A specific setup applying the same boundary conditions of torsion-bending loading is used to characterize these SMA file prototypes. The good agreement between experimental and numerical responses, for a combined bending-torsion loading, proves the relevance of the proposed approach and the pertinence of considering Cu-based SMAs for endodontic file applications.

Strain glass and stress-induced martensitic transformation characteristics of Ti40Zr10Ni40Co5Cu5 multi-principal element alloy

The Ti40Zr10Ni40Co5Cu5 multi-principal element alloy has been demonstrated to be a strain glass alloy. It did not undergo martensitic transformation and retained a B2 structure even when the temperature was decreased to 15 K. Dynamic mechanical tests showed that its responses (elastic modulus and tan δ value) were frequency-dependent, with an ideal freezing temperature of 216.1 K. The Ti40Zr10Ni40Co5Cu5 alloy is a B19’ strain glass with recoverable strain of 4.4% and elastocaloric cooling of 14.8 K when tested at 233 K. Adding multiple alloying elements is an effective way to suppress martensitic transformation and design novel strain glass alloys.

Influence of hydrostatic pressure on martensitic transformation and strain behavior for Co52V29+xGa19−x Heusler alloys

In the present work, polycrystalline Co52V29+xGa19-x(0 ≤ x ≤ 2) Heusler alloys have been prepared. The related experimental results confirm that each sample undergoes a first-order martensitic transformation (MT) from the L21 cubic austenitic state to the D022 tetragonal martensitic state. With increase of V concentration, both transformation temperature and strain are heightened simultaneously, thus giving rise to an increment of transition entropy change $$\left( {\Delta S_{{{\text{tr}}}} } \right)$$ . In addition, the influence of hydrostatic pressure on MT and transition strain have been also carefully studied. With increase of pressure, the transformation temperatures of these samples move to high values apparently, but their reversible strain only exhibit a slight enhancement and quickly approach the saturated values. For different compositions, however, there is no obvious difference in the rate of transformation temperature change with pressure. Such a behavior opens up the potential applications of the functional properties associated with the hydrostatic pressure-induced MT for this system.

Design and fabrication of a Nb/NiTi superelastic composite with high critical stress for inducing martensitic transformation and large recoverable strain for biomedical applications.

A novel Nb/NiTi superelastic composite with a shell-core structure was designed and fabricated to achieve a combination of biocompatibility and superelasticity (large recoverable strain ε accompanied by high critical stress for inducing martensitic transformation σSIM). The good biocompatibility is mainly attributed to the outer non-cytotoxic Nb shell that prevents inner NiTi core from direct contact with cells. Meanwhile, the inner NiTi core endows the composite with superelasticity through a fully reversible stress-induced martensitic transformation between B2 parent phase and B19' martensite. These results might shed some light on design and development of novel superelastic composites for biomedical applications.

Finite strain constitutive modeling for shape memory alloys considering transformation-induced plasticity and two-way shape memory effect

This work presents a three-dimensional constitutive model for shape memory alloys considering the TRansformation-Induced Plasticity (TRIP) as well as the Two-Way Shape Memory Effect (TWSME) through a large deformation framework. The presented logarithmic strain based model is able to capture the large strains and rotations exhibited by SMAs under general thermomechanical cycling. By using the martensitic volume fraction, transformation strain, internal stress, and TRIP strain tensors as internal state variables, the model is capable to capture the stress-dependent TRIP generation when SMAs are subjected to a multiaxial stress state, as well as the TWSME for thermomechanically trained SMAs under load-free conditions. A detailed implementation procedure of the proposed model is presented through a user-defined material subroutine within a finite element framework allowing for solving different Boundary Value Problems (BVPs). Comprehensive instruction on calibrating the model parameters as well as the derivation of continuum tangent stiffness matrix are also provided. In the end, the simulated cyclic pseudoelastic and actuation responses by the presented model for a wide range of SMA material systems under both uniaxial and multiaxial stress states are compared against experimental results to validate the proposed modeling capabilities.

Interactions of negative strain rate sensitivity, martensite transformation, and dynamic strain aging in 3rd generation advanced high-strength steels

Medium Mn transformation-induced plasticity (TRIP) steels, a subgroup of third generation advanced high-strength steels (AHSS), often present a combination of both dynamic strain aging (DSA) and TRIP. It has been previously shown that TRIP coincides with the passage of strain bands due to DSA, but their interdependence has not been well-established. This study seeks to explore the interactions between the two phenomena as well as the commonly-observed negative strain rate sensitivity. Digital image correlation (DIC) and in-situ magnetic measurements of the retained austenite fraction were employed to demonstrate the variations in mechanical behavior with increasing strain rate. Strain rate jump tests provided measurements of the strain rate sensitivity and were combined with either DIC to characterize PLC bands or with magnetic measurements to measure variations in the martensite fraction during the jumps. Finally, the micromechanics defining the peculiar bursts of martensite transformation in response to strain rate jumps are discussed in terms of a competition between dislocation mobility and austenite stability.

To correlate the phase transformation and mechanical behavior of QP steel sheets

The Quenching and Partitioning steels (QP steel) have received attention due to their deformation-induced martensitic transformation (DIMT) behavior. However, this DIMT behavior makes the stress-strain response of QP steel more complex to predict. In this paper, a comprehensive study of the DIMT behavior of QP980 steels was carried out in which an extended martensitic transformation kinetics model was further developed to incorporate the stress state, temperature and strain rate. To further correlate the microstructure evolution and macromechanical behavior, a modified plastic hardening equation considering martensitic transformation strengthening, thermal softening and strain rate strengthening effects was proposed. The impact of temperature and strain rate on the stress-strain response (first-order effect) of QP steels has been decoupled into two parts: (i) the thermal and strain-rate effect on traditional strain hardening and (ii) the temperature- and strain rate- dependent DIMT behavior on martensitic transformation-induced hardening. The correlation between the microstructure volume fraction and yield surface evolution (second-order effect) in stress space was investigated. The kinematic hardening behavior was also studied to explore the Bauschinger effect with different phase transformation rates. This extended model, therefore, provides a good description of the complex stress state-, strain rate- and temperature-dependent mechanical behavior of QP steels.