Martensite Stabilization Research Articles

Effect of Intercritical Annealing Parameters and Starting Microstructure on the Microstructural Evolution and Mechanical Properties of a Medium-Mn Third Generation Advanced High Strength Steel

A prototype medium-Mn TRIP steel (0.2 C–6 Mn–1.7 Si–0.4 Al–0.5 Cr (wt %)) with a cold-rolled tempered martensite (CR) and martensitic (M) starting microstructures was subjected to continuous galvanizing line (CGL) compatible heat treatments. It was found that the M starting microstructures achieved greater than 0.30 volume fraction of retained austenite and target 3G properties (UTS × TE ≥ 24,000 MPa%) using an intercritical annealing temperature (IAT) of 675 °C with an IA holding time of 60–360 s, whereas the CR microstructure required an IAT of 710 °C and annealing times of 360 s or greater to achieve comparable fractions of retained austenite and target 3G properties. This was attributed to the rapid austenite reversion kinetics for the M starting microstructures and rapid C partitioning from the C supersaturated martensite, providing chemical and mechanical stability to the retained austenite, thereby allowing for a gradual deformation-induced transformation of retained austenite to martensite—the TRIP effect—and the formation of nano-scale planar faults in the retained austenite (TWIP effect), such that a high work-hardening rate was maintained to elongation of greater than 0.20. Overall, it was concluded that the prototype steel with the M starting microstructure is a promising candidate for CGL processing for 3G AHSS properties.

Effects of quenching methods on shape memory properties of Cu-28Zn-3Al wt. % alloy produced by gravity casting

In recent years, Cu-Zn-Al alloys are studied intensively due to their excellence in shape memory effect (SME), good mechanical properties, along with accessible production process and cost. Cu-Zn-Al Shape Memory Alloy easily form stabilization in martensite phase that leads to SME properties reduction, but can be minimized by applying alternative quenching methods. In this research, several quenching methods are proposed to minimize the phase stabilization of martensite. The Cu-28Zn-3Al alloy was produced by gravity casting and homogenized at 800 ℃ for 2h followed by air cooling. Furthermore, as-homogenized plate was betatized at 850 ℃ for 30 minutes and then subsequently quenched using three different methods: direct quench (DQ), up quench (UQ), and step quench (SQ). Morphology, phase transformation, hardness, and strain recovery characterization are examined using an optical microscope, SEM-EDS, XRD, DSC, Vickers, and bending tests. The results showed that the DQ and UQ samples consist of V-shape βˈ[M18R] martensite as a matrix and retained α[A1] as the second phase. The volume fraction ratio of βˈ[M18R]: α[A1] is (98.4:1.6) and (92.9:6.1) for DQ and UQ, respectively. However, the SQ sample did not indicate the presence of the martensite phase and did not show any recovery rate. Bending test showed that DQ and UQ had 27.2% and 36.3% of recovery rate, respectively.

Моделирование эффекта мартенситной неупругости в плоских композиционных пружинах, выполненных из сплава с эффектом памяти формы

The purpose of the study was to develop a mathematical model of a flat composite spring operating in the region of small displacements and having a shape memory effect. To describe the behavior of the material, we applied a phenomenological approach based on the phase transition diagram; as the material model, we chose the Brinson phenomenological model. As a result, we proposed relationships for determining the reduced mechanical characteristics of a monolayer equivalent to a composite layer in the spring. The memory effect was taken into account by an additional internal force factor, i. e. the shape memory moment during bending, which resulted from the cross-section orientation of the martensite. To obtain the elastic characteristic of the spring, we used a finite element approach, in which the shape memory moment acts as an additional nodal load. Eventually, we introduced an algorithm for the dependence between the bending moment in the section and the shape memory moment for isothermal loading in the martensite stability zone and obtained the dependences of the shape memory moment during bending on the bending moment for various configurations of the cross-section of the spring.

Robust evidence for the stabilization of the premartensite phase in Ni-Mn-In magnetic shape memory alloys by chemical pressure

The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 K to around 5 K in a magnetic shape memory alloy (MSMA) Ni50Mn34In16 using magnetic susceptibility, synchrotron X-ray powder diffraction (SXRPD) studies and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for around 5% Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at and below 300 K without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as around 3% Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion/symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state.

Effects of plasticity-induced martensitic transformation and grain refinement on the evolution of microstructure and mechanical properties of a metastable high entropy alloy

This paper describes the main results from an experimental investigation into tailoring the phase content and grain structure for high strength of a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%), using rolling, friction stir processing (FSP), and compression. Optical microscopy, neutron diffraction, and electron backscatter diffraction were employed to characterize microstructure and texture evolution. The material upon rolling was found to have triplex structure consisting of metastable gamma austenite (γ), stable sigma (σ), and stable epsilon martensite (ε) phases. The adaptive phase stability exhibited by the selected HEA of very low stacking fault energy with strain, strain rate, and temperature was used along with grain refinement to enhance the strength. To this end, the complex structure was refined by FSP at 350 revolutions per minute (RPM) tool rotation rate, while increasing the fraction of γ and decreasing the σ and ε content. The strength was further enhanced by FSP at 150 RPM by further refinement of the grain structure and increasing the fraction of ε phase. The peak ultimate tensile strength of ~1850 MPa was achieved by double pass FSP (350 RPM followed by 150 RPM), the sequence which even more refined the microstructure and increased the fraction of σ phase. While the content of diffusion created σ phase remains constant during subsequent compression, the fraction of ε increases due to the diffusionless strain induced γ→ε phase transformation. The transformation facilitates plastic strain accommodation and rapid strain hardening, which has been attributed to the increase in highly dislocated ε phase fraction and transformation induced dynamic Hall-Petch-type barrier effect. Interestingly, a great deal of hardening ability was exhibited by the HEA even at very high strength. Roles of texture, grain size, and phase content on the transformation during compression have been rationalized and discussed in this paper.

Ab Initio Study of Martensitic Transformation in NiTiPt High Temperature Shape Memory Alloys

The crystal structures and martensitic transformation of Ti50Ni50−xPtx alloys (x = 0, 6.25, 8.33, 10.42, 12.5, 18.75, 25) were studied by means of density functional theory (DFT). The computational results indicate that the lattice parameters of Ti-Ni-Pt alloys continuously increase with increasing the Pt content. It is found that at ≤ 12.5 at.% Pt, the martensite structure is monoclinic B19′ phase, and the energy differences between parent and martensite phases (ΔE) decrease slightly with a minimum observed at 6.25 at.% Pt. However, when the Pt content is increased to around 15 at.%, the most stable martensite phase is the orthorhombic B19 structure, and the ΔE increases sharply with Pt concentration. It was found that the phase transition temperatures are closely related to the energy differences ΔE between parent and martensite phases. The electronic structures of martensite B19 and B19′ phases are also discussed.

Effect of Al Addition on Martensitic Transformation Stability and Microstructural and Mechanical Properties of CuZr Based Shape Memory Alloys

In this work, the effect of the Al content (x = 5, 10, and 15 at. %) on the martensitic transformation (MT) and microstructure and mechanical properties of Cu(50−x)Zr50Alx alloys was studied. The microstructure of the alloys was characterized at room temperature by means of scanning electron microscopy and X-ray diffraction. An increase in Al content reduces the amount of transforming CuZr phase, and consequently the secondary phase formation is favored. The evolution of the MT upon thermal cycling was investigated as a function of the Al content by differential scanning calorimetry. MT temperatures and enthalpies were found to be decreased when increasing the Al content. Al addition can induce a sudden, stable MT below 0 °C, while the binary alloy requires ten complete thermal cycles to stabilize. Finally, the mechanical properties were investigated through microhardness and compression testing. No linear dependence was found with composition. Hardness lowering effect was observed for 5–10 at. % of Al content, while the hardness was increased only for 15 at. % Al addition with respect to the binary alloy. Similarly, compressive response of the alloys showed behavior dependent on the Al content. Up to 10 at. % Al addition, the alloys indicate a superelastic response at room temperature, while higher Al content induced untimely failure.

5M and 7M martensitic stability and associated physical properties in Ni50Mn35In15 alloy: first-principles calculations and experimental verification

The excellent magnetic-driven properties of Ni-Mn-based ferromagnetic shape memory alloys are intrinsically related to the presence of the modulated martensite; however, the metastability of modulated martensite hinders the availability of these properties. Herein, the phase stability and associated physical properties of the 5M and 7M modulated martensites in Ni50Mn35In15 alloy are studied by the first-principles calculations and experiments. Results show that this alloy undergoes a magneto-structural coupling transformation (i.e., ferromagnetic austenite → ferrimagneitc 5M martensite → ferromagnetic 7M martensite). The stability of the possible phases can be understood based on both the density of states and differential charge densities analysis. A strong bonding ability between Ni and Mn atoms in the 7M martensite is responsible for it being the most stable phase.

Temperature Dependent Stress–Strain Behavior and Martensite Stabilization in Magnetic Shape Memory Ni51.1Fe16.4Ga26.3Co6.2 Single Crystal

The superelastic properties and stress-induced martensite (SIM) stabilization have been studied in a shape memory Ni51.1Fe16.4Ga26.3Co6.2 single crystal. The single crystal, characterized by a thermally induced forward martensitic transformation temperature around 56 °C in the initial state, has been submitted to compression mechanical testing at different temperatures well above, near and below the martensitic transformation (MT). After each mechanical test, the characteristic MT temperatures and the transformation enthalpy have been monitored by means of differential scanning calorimetry. At temperatures below MT, the stress–strain (σ–ε) curves show a large strain, around 6.0%, resulting from the detwinning process in the martensitic microstructure, which remains accumulated after unloading in the detwinned state of the sample as a typical behavior of the shape memory alloys (SMAs). After just two “σ–ε + heating” cycles the accumulation of strain was not observed any more indicating the formation of a two-way shape memory effect which consists in a spontaneous recovery of the aforementioned detwinned state of the sample during its cooling across the forward MT. Whereas the thermally induced shape recovery in conventional SMAs occurs at the fixed value of the reverse MT temperature, the heating DSC curves of the mechanically deformed martensite in the present work show a burst-like calorimetric peak at the reverse MT arising at temperatures essentially higher than the thermally activated one. This behavior is the result of the SIM stabilization effect. After a short thermal aging in the stress-free state, this effect almost disappears, showing a slight impact on the MT characteristic temperatures and the enthalpy. At temperatures higher than the transformation one, the SIM is not stabilized, as the mechanically induced martensite fully retransforms into austenite after the unloading. From the σ–ε curves, the critical stress, σc, as well as the values of Young’s moduli of martensite and austenite are determined showing linear dependences on the temperature with a slope of 3.6 MPa/°C.

Effect of chemical composition and quenching media on recoverable strain in Cu–Zn–Al alloys

The correlation of the thermomechanical parameters and chemical composition, which enables to determine the phases providing properties related to reliable, recoverable strain, was studied for three Cu-(21.6–25.4)Zn-(3.3–5.6)Al alloys. The conventionally cast samples were hot forged and cold-rolled, followed by quenching in different media and aging treatment. The alloy A with low aluminum content (3.3%) has exhibited better tensile strength and elongation than B and C samples, in addition to good low-temperature workability and refined grain. The stress plateau recorded on the stress–strain curve during tensile testing of quenched specimens was related to the recoverable strain of up to 4.2%. Most of the examined specimens have shown shape memory recovery above 90%. The apparent activation energy for microhardness increase in the examined alloys during aging at 200 °C–500 °C for up to 30 h was (55.3 ± 3.9) kJ mol−1 compared with (55.7 ± 4.6) kJ mol−1, which was recorded for the decrease in shape memory recovery. These results suggest a common origin for both changes. The precipitate was formed along the grain boundaries and the matrix. Neither the coarsening of α-precipitate nor γ-precipitate was observed during the aging of examined alloys for up to 30 h. Martensite stabilization was avoided in A and B alloys as a result of the step quenching treatment. A relatively short life expectancy was predicted for alloy C, while good shape memory properties may be expected for alloys A and B during their exposure to temperature up to 50 °C.

Deformation induced martensite stabilization in Ni45Mn36.7In13.3Co5 microparticles

A Ni45Mn36.7In13.3Co5 alloy has been deformed by hand crushing and ball milling, both in martensite and in austenite, to analyze the effect of the induced plastic deformation on the relative stability between structural phases. A huge martensite stabilization, with shifts on the temperature of the first reverse martensitic transformation of up to 100 K, is observed not only in the deformed martensite but also in the grinding-induced martensite. It is also found that, regardless of the degree of plastic deformation, the stabilized martensite displays a non-modulated tetragonal structure, which accommodates the generated internal stresses better than the thermal induced modulated monoclinic martensite. In all cases, the destabilization process is shown to finish around the Curie temperature of the austenite, which actually represents a high temperature limit for the mechanically-stabilized martensite. This phenomenon is analyzed and explained in terms of the effect of magnetic entropy contribution to the driving force for reverse MT.

Rotary friction welding of additively manufactured 17-4PH stainless steel

• Rotary friction welding (RFW) was employed to weld additively manufactured 17-4PH stainless steel. • Dynamic recrystallization caused grain refinement in FZ, TMAZ, and HAZ. • The formation of Cr-rich carbides and Cu-clustering resulted in sensitization in FZ and HAZ. The aim of the present study is to investigate the effect of rotary friction welding (RFW) on the microstructure, tensile properties, hardness, wear resistance and corrosion properties of the weld joint of an additively manufactured solid cylinder made of 17-4PH stainless steel. Significant grain refinement was observed in the welding zone (WZ) due to the plastic deformation and dynamic recrystallization. The morphology of the grains changed from columnar in the as-printed condition to equiaxed in the welding zone. The microhardness of the WZ is ≈10 % higher than the heat affected zone (HAZ) due to the grain refinement but still lower (≈27 %) than the as-printed microstructure which contains nano-sized carbides and less retained austenite (γ fcc ) due to the rapid solidification. The precipitation of Cr-rich carbides was observed in the WZ and HAZ leading to sensitization and decrease of corrosion resistance (up to ≈90 %). Cu-rich clusters were formed at the interface of Cr-rich carbides/matrix in the WZ and HAZ due to the cube-cube orientation relationship. The depletion of matrix from Cr and Cu in the welding zone decreased the stability of martensite after friction welding leading to the formation of 7–10 % retained austenite. At low load (2 N), the wear resistance of as-printed microstructure (base metal, BM) was found to be superior to WZ and HAZ. However, at intermediate and high loads (5 N and 10 N), the wear resistance of WZ was either better or comparable to the as-printed microstructure. Adhesion and Oxidative wear were the main wear mechanisms at all loads during the reciprocating action. Oxide films grow until they reach a critical thickness prior to becoming unstable and spalling off to form wear debris.

Microstructural stability of ODS steel after very long-term creep test

Microstructure of 9Cr oxide dispersion strengthened (ODS) tempered martensitic steel (TMS) cladding and 12Cr-ODS ferritic steel cladding after very long-term internal pressurized creep tests was observed. The creep test conditions were 700°C for 46,504 h and 750°C for 45,550 h for 9Cr-ODS TMS and 700°C for 30,116 h for 12Cr-ODS ferritic steel. The stability of martensite and ferrite matrix structure, including carbide distribution, dislocation density, and lath boundary, and that of nano-particle distribution were investigated. It was confirmed that the nano-particles were stably present during the high-temperature and very long-term stress loading environment, contributing to the prevention of matrix structure degradation. The results of observation are consistent with the fact that the 9Cr-ODS TMS and 12Cr-ODS ferritic steel exhibit excellent creep strength and show no reduction in the slope of the stress versus time to rupture curve over a significantly extended period of time in the creep test.

Stabilization of Martensite on Nanoprecipitates and Kinetics of Explosive Martensite Transition

Based on the theory of diffuse martensitic transitions (DMT), the physical mechanism of the influence of dispersed precipitate nanoparticles on the appearance of such features of the deformation behavior of alloys with the shape memory effect (SME) as the stabilization of martensite at temperatures significantly higher than the As and Af temperatures characteristic of an alloy in the absence of nanoparticles c-oherent with the matrix is analyzed. The reason for the stabilization of martensite is the internal elastic stress fields associated with the particles, which serve as sites for the heterogeneous nucleation of martensite. It has been shown theoretically that, with an increase in the volume concentration of nanoparticles, the recovery of shape memory deformation occupies an increasingly narrow temperature range and occurs at an ever higher temperature. After reaching a certain critical value of the particle concentration, the return of the deformation of SM loses its stability and becomes explosive.

Paramagnetism and martensite stabilization of tensile strained NiTi shape memory alloy

We present an experimental and theoretical study of Pauli paramagnetism and martensite stabilization in a near equiatomic NiTi shape memory alloy. We demonstrate a direct correlation between strain-induced shear of the B19′ NiTi lattice and its electronic and thermodynamical features. An increase in the monoclinic angle β from 97.4 to 98° induces a 7% decrease in the magnetic susceptibility because of a shift and deepening of a dip in B19′ density of states at the Fermi level. It also produces a decrease in the B19′ enthalpy, which translates into an increase in the martensite-to-austenite transition temperature by 60 K.

On the thermodynamic analysis of martensite stabilization treatments

It is shown that the broadenings of the forward and reverse martensitic transformations are proportional to the second derivatives of the elastic energy by the transformed martensite volume fraction. The shifts of the transitions, beside the effect of chemical contribution to the equilibrium transformation temperature, To, can depend on the change of the elastic and dissipative energies as well. The effect of mechanical stabilization (by making the surfaces roughened) in Ni49Fe18Ga27Co6 single crystal, where the transformation too place by single interface motion, can be well treated in thermoelastic approximation. In Ni50Mn28.5Ga21.5 single crystal (where the usual multivariant martensite structure developed) not only the dissipative energy, but the slope of the second derivative of the elastic energy for cooling has also been altered after mechanical stabilization and there can exist an elastic energy contribution to the width of the hysteresis too. Symmetry conforming short range ordering under uniaxial stress (SIM-aging) in Ni53Mn25Ga22 and Ni51Fe18Ga27Co4 single crystals caused an asymmetric change in the second derivatives of the elastic energy: the sharpening of the corresponding branches of the hysteresis was stronger for heating than cooling. The thermoelastic balance condition did not fulfill for the SIM-aged samples and the elastic energy contributions to the shift of the transformation temperatures could even compensate the increase of To (caused by the stabilization by symmetry conforming short range ordering). It is also illustrated that the dissipated energy per one cycle could decrease even if the area of the hysteresis has been increased.

Investigation on vibrational, electronic excitation entropy and magnetic moment contributions to phase stability of off-stoichiometric Ni50MnxIn50-x alloys at finite temperatures by first-principle calculations

The effects of Mn concentration and thermally excited contributions (including the vibrational, electronic excitation and magnetic contributions) on phase stability of austenite and martensite from 0 K to finite temperatures in Heusler typed Ni50MnxIn50-x shape memory alloys were studied by the first-principle calculations using exact muffin-tin orbitals with coherent potential approximation. Based on this, the martensitic transformation tendency was explored. Results show that at 0 K, the energy differences between the non-modulated martensite and the austenite become negative when extra Mn is added, indicating that the added Mn stabilizes the martensite and promotes martensitic transformation. The promoting effect increases with the increase of Mn content. At finite temperatures, the three thermal contributions (the vibrational, electronic excitation and magnetic contributions) were further calculated based on the equilibrium structure at 0 K. It was revealed that the vibrational entropies of the two phases increase with the increase of the temperature for all Mn contents. Under the two effects (temperature and Mn-content), the austenite has a larger vibrational entropy than the martensite, which indicates that the vibrational entropy contributes to promoting the martensitic transition. The Mn content and the temperature show a similar influence on the electronic entropies of the two phases. However, compared with the vibrational entropy, the contribution of the electronic entropy is much smaller. Furthermore, the influences of Mn content and temperature on the magnetic moment of both phases were simulated in their ferromagnetic state. The results show that the magnetic moments increase linearly with the Mn content, however, the influence of temperature is relatively small. Above 100 K, the magnetic moment of the austenite is higher than that of the martensite in ferromagnetic Ni50Mn29.25In20.75 alloy, suggesting that the magnetic entropy makes a similar contribution to promote the martensitic transformation, like the vibrational and electronic excitation entropies.

High temperature superelasticity realized in equiatomic Ti-Ni conventional shape memory alloy by severe cold rolling

Great interests on high temperature shape memory alloys (HTSMAs) are recently raised, driven primarily by the aerospace and automotive industries, for their potential to serve as solid state actuators at high temperatures over 373 K. Near equiatomic Ti-Ni conventional SMAs, as the most applied SMA system, were considered to possess appreciable shape memory effect and superelasticity only below 373 K, which owes to low martensitic transformation temperatures. Here we utilized the martensite stabilization effect and successfully expanded the viable zone of B19′ (monoclinic) martensite to 616 K in a 35% cold-rolled Ti-50Ni (CR35) SMA. After a training cycle, it exhibited quasi-linear superelasticity of narrow hysteresis, ~3% superelastic strain and high strength over 1.2 GPa in a wide temperature range up to 483 K. It is the first exploration of high temperature superelasticity associated with the stabilized martensite. Using transmission electron microscopy and in-situ heating/tensile synchrotron X-ray diffraction, we reveal the origin of martensite stabilization and the underlying mechanisms of unique superelasticity in the CR35. Our work provides an attractively facile approach to realize high temperature superelasticity in Ti-Ni conventional SMAs, as well as in other kinds of SMAs, for precise actuation within an expanded range of working temperature and stress.

Excellent shape recovery characteristics of Cu-Al-Mn-Fe shape memory single crystal

Shape memory alloys can recover the deformed shape due to their superelasticity or shape memory effect. In this study, a novel Cu-Al-Mn-Fe shape memory single crystal is reported. The results show that it has excellent superelasticity and shape memory effect simultaneously when deformed at room temperature, as well as tunably wide response temperature range with near-zero interval of reverse phase transformation. When deforming one single crystal at room temperature, it not only possesses full superelasticity of 7%, but also tunable shape memory effects up to 8.8 %. The full shape recovery during heating exhibits near-zero response interval and tunably wide response temperature range of 166 K depending on the deformation. The functional characteristics of the alloys result from the controllable reverse phase transformation hinging on the stabilization of stress-induced martensite. This class of Cu-Al-Mn-Fe alloy may be used as both superelastic materials, and shape memory materials with wide working temperature range as high-sensitive detector, driver or sensor.

Evolution of anisotropic and negative thermal expansion in rolled equiatomic nickel-titanium martensite

Martensitic phase of NiTi shape memory alloys has been shown to exhibit anisotropic coefficients of thermal expansion (CTEs) after severe plastic deformation; however, the evolution of this anisotropy has not been examined as a function of deformation. Here, equiatomic NiTi was incrementally rolled down to 30% thickness reduction and the evolution of CTEs was measured. NiTi samples exhibited maximum CTE anisotropy after only 5% rolling reduction with a range of -10 to 27 ppmK−1, as a result of rapid crystallographic texture evolution. Due to martensite stabilization, the CTE anisotropy persisted up to 125°C with CTE magnitude being enhanced by reverse transformation.