Austenite-martensite Phase Transformation Research Articles

A data-driven approach to model the martensitic transformation temperature in strain-induced metastable austenitic steels

Strain-induced metastable austenitic stainless steels form an important class of materials in metallurgical industries for their wide range of applications. These steels undergo austenite-martensite phase transformation at temperatures above martensite start temperature, Ms, at which martensite is formed on mechanical deformation, also known as the Md temperature. Md temperature depends on several factors related to the steel and is important from alloy design perspective. In the literature, there are quite a few equations based on composition of the steels for the prediction of Md temperature. However, it is well known that the transformation from austenite to martensite is dependent on the austenite grain size as well as deformation conditions i.e. strain, strain rate and temperature of deformation. In the present work, the role of those parameters has also been considered. The model is implemented using fourteen input parameters viz., composition, grain size, amount of strain, temperature of deformation, and strain rate. The architecture of the neural network model is optimized rigorously to predict the Md temperature on a par with actual value. It has been shown that grain size and strain rate have very negligible influence whereas strain and temperature of deformation have quite strong role. Md temperature is increased with increasing strain whereas the temperature of deformation shows opposite dependence on it. An empirical equation thus, has been established to calculate the Md temperature of a steel as a function of its composition, grain size, temperature of deformation and strain. The final optimized model is then deployed to predict the Md temperature of different steels and predictions are found to be in close agreement to the experimentally measured Md temperatures. The developed model is general and can be extended to include other parameters as well as various other steel alloys.

Topology Optimization of Shape Memory Alloy Actuators for Prescribed Two-Way Transforming Shapes

This paper proposes a new topology optimization formulation for obtaining shape memory alloy actuators which are designed with prescribed two-way transforming shapes. The actuation behaviors of shape memory alloy structures are governed by austenite-martensite phase transformations effected by thermal-mechanical loading processes; therefore, to realize the precise geometric shape variations of shape memory alloy actuators, traditional methods involve iteration processes including heuristic structural design, numerical predictions and experimental validation. Although advanced structural optimization methods such as topology optimization have been used to design three-dimensional (3D) shape memory alloy actuators, the maximization/minimization of quantities such as structural compliance or inaccurate stroke distances has usually been selected as the optimization objective to obtain feasible solutions. To bridge the gap between precise shape-morphing requirements and efficient shape memory alloy actuator designs, this paper formulates optimization criteria with quantitatively desired geometric shapes, and investigates the automatic designs of two-way prescribed shape morphing shape memory alloy structures based on the proposed topology optimization method. The super element method and adjoint method are used to derive the analytical sensitivities of the objective functions with respect to the design variables. Numerical examples demonstrate that the proposed method can obtain 3D actuator designs that have the desired two-way transforming shapes.

Significantly enhanced fatigue resistance and mechanisms of hypoeutectic Al-Si composite calibrated using trace in-situ nanocrystals

Fatigue resistance under extreme stress conditions is an important indicator to evaluate industrial application potential of Al-Si-Mg alloys, but their fatigue resistance is inherently weakened by large grain size, long-needle Si phases and coarse precipitates. This work reports significantly optimized microstructure configuration and fatigue resistance of hypoeutectic Al-Si composite optimized by the nanocrystallization products of a NiNbTi metallic glass. It was proved the in-situ NiTi (B2) nanocrystals can serve as the heterogeneous nucleation sites of α-Al and β-Mg2Si and the growth retarder of eutectic Si. Particularly, the optimized composite exhibited nearly 5 times and 6 times longer fatigue life than the unoptimized alloy at 120 MPa (60 Hz) and 240 MPa (20 Hz), respectively. The fatigue strength (N = 107) was increased by 21.7 % and 25.9 % at 60 Hz and 20 Hz, respectively. The α-Al refinement enhanced the resistance to fatigue crack initiation. The refinement and spheroidization of eutectic Si phases reduced stress concentration. Also, the austenite-martensite phase transformation of NiTi afforded more energy for precipitation. Refined β-Mg2Si and β'' phases were facilitated to interact with dislocations, improving the resistance to dislocation slip during cyclic strain through the dislocation shearing effect. This work provides a theoretical basis for the future development of Al-Si alloys and composites with excellent fatigue resistance for expanding their industrial application scope.

Giant magnetoelectric effect of Ni–Mn-Ga/piezopolymer composites tailored by a martensitic transformation

Polymer-based magnetoelectric (ME) composites, composed of magnetostrictive and piezoelectric phases, in addition to exhibiting an effective coupling between the magnetic and electric orders of the matter, also offer important advantages for the Internet-of-Things, digitalization, and 4.0 revolution environments: light weight, flexibility, wearability, environmental friendliness, printability, and biocompatibility. Nevertheless, their successful implementation on applications such as sensors, actuators, energy harvesters, biomedical devices and spintronics strongly depends on the increase of its ME voltage response.This work explores, both experimentally and theoretically, the magnetostrictive Ni–Mn-Ga's martensite ‒ austenite phase transformation to shift/tailor/design the ME resonance peak exhibited by Ni–Mn-Ga/P(VDF-TrFE) piezoelectric composites. In the austenite phase, the determined peak value of the ME coefficient of 18.1 V cm−1 Oe−1 was much higher than the value of 6.05 V cm−1 Oe−1 obtained for the martensite phase, whereas the estimated theoretically value of magnetoelastic constant in the austenite is much smaller. The ways to further increase the magnetically induced ME response are also outlined.

Regularities of structure and texture development in austenitic steel during cold rolling and heat treatment

The paper studies the regularities of the structure and texture formation in austenitic steel AISI 304 under cold rolling and subsequent heat treatment. Regularities of structure and texture formation depending on the rolling type and deformation degree were established. The ratios of main texture components, the volume fractions and the distortion of the martensite's crystalline lattice were analyzed. It is shown that mechanisms of the austenite martensite phase transformation in the course of cold rolling may change depending on various deformation schemes, as well as mechanisms of the reverse phase transformation variate under subsequent annealing. Herewith depending on the rolling type and deformation degree, the crystallographic texture formation of the nucleating austenite under the heat treatment can have single component or multi-component character and follows different orientation relations. The more scattered martensite texture after deformation results in the formation of fine-dispersed austenite, which hardens the material keeping a sufficient level of plasticity. The work was carried out with the financial support of the Ministry of Science and Higher Education of Russian Federation (Agreement No. 075-15-2021-1352).

Investigation of the Thermomechanical Response of Cyclically Loaded NiTi Alloys by Means of Temperature Frequency Domain Analyses.

Nickel–Titanium (NiTi) shape memory alloys subjected to cyclic loading exhibit reversible temperature changes whose modulation is correlated with the applied load. This reveals the presence of reversible thermomechanical heat sources activated by the applied stresses. One such source is the elastocaloric effect, accounting for the latent heat of Austenite–Martensite phase transformation. It is, however, observed that when the amplitude of cyclic loads is not sufficient to activate or further propagate this phase transformation, the material still exhibits a strong cyclic temperature modulation. The present work investigates the thermomechanical behaviour of NiTi under such low-amplitude cyclic loading. This is carried out by analysing the frequency domain content of temperature sampled over a time window. The amplitude and phase of the most significant harmonics are obtained and compared with the theoretical predictions from the first and second-order theories of the Thermoelastic Effect, this being the typical reversible thermomechanical coupling prevailing under elastic straining. A thin strip of NiTi, exhibiting a fully superelastic behaviour at room temperature, was investigated under low-stress amplitude tensile fatigue cycling. Full-field strain and temperature distributions were obtained by means of Digital Image Correlation and IR Thermography. The work shows that the full field maps of amplitude and phase of the first three significant temperature harmonics carry out many qualitative information about the stress and structural state of the material. It is, though, found that the second-order theory of the Thermoelastic Effect is not fully capable of justifying some of the features of the harmonic response, and further work on the specific nature of thermomechanical heat sources is required for a more quantitative interpretation.

Phase Transformation and Incompatibility at Grain Boundaries in Zirconia-Based Shape Memory Ceramics: A Micromechanics-Based Simulation Study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite-martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is developed to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress-strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars, prior to being used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bicrystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.

Viscoelastic Deformation and Fracture of Porous Nickel Titanium after Tension and Cyclic Bending

The paper deals with the deformation curves and fatigue tests of the porous nickel–titanium (NiTi) alloy produced with self-propagating high-temperature synthesis. Deformation curves obtained for tension and three-point bending of porous NiTi plates demonstrate their viscoelastic deformation due to the austenite-to-martensite phase transformation. Scanning electron microscopy observations of the fractured surface show the regions of quasi-brittle fracture of martensite and ductile fracture of austenite. Brittle fracture is observed for nonmetallic inclusions and the porous structure coating of the intermetallic compound. Fatigue tests show that the brittle phases and inclusions have no critical negative effect on the strain and fatigue properties of the NiTi alloy. It is determined that 70% of the porous alloy specimens withstand 106 cyclic bending owing to the reversible austenite-martensite phase transformation in the parent phase which is one the components of the multi-phase porous alloy.

Unravelling thermal history during additive manufacturing of martensitic stainless steel

In-situ thermal cycling neutron diffraction experiments were employed to unravel the effect of thermal history on the evolution of phase stability and internal stresses during the additive manufacturing (AM) process. While the fully-reversible martensite-austenite phase transformation was observed in the earlier thermal cycles where heating temperatures were higher than Af, the subsequent damped thermal cycles exhibited irreversible phase transformation forming reverted austenite. With increasing number of thermal cycles, the thermal stability of the retained austenite increased, which decreased the coefficient of thermal expansion. However, martensite revealed higher compressive residual stresses and lower dislocation density, indicating inhomogeneous distributions of the residual stresses and microstructures on the inside and on the surface of the AM component. The compressive residual stresses that acted on the martensite resulted preferentially from transformation strain and additionally from thermal misfit strain, and the decrease in the dislocation density might have been due to the strong recovery effect near the Ac1 temperature.

Microstructure evolution and adiabatic heating during dynamic biaxial deformation of a 304 stainless steel

This study focuses on the microstructure evolution and adiabatic heating during high strain rate biaxial deformation of a 304 stainless steel. Adiabatic heating is measured in situ during drop weight impact testing of the studied material. The considerable temperature rise induced by adiabatic heating (up to 184.3 °C when impacted with 140 J) is detected. The microstructure was analyzed by X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM) techniques. Dislocation glide, twinning and austenite-martensite phase transformation are operating during high strain rate plastic deformation. The XRD analysis shows a steady increase of α′-martensite fraction (up to 36%) with plastic strain (up to 0.8), whereas the fraction of intermediate ε-martensite remains negligible (<0.5%). The biaxial stress compensated the possible impediment of martensitic transformation caused by adiabatic heating and the increasing stacking fault energy (SFE). Nanotwins having a width of ~5 nm–~20 nm are inhomogeneously distributed in the microstructure. Interrelationship between adiabatic heating, deformation mechanisms and microstructure evolution is discussed.

Microstructure, mechanical properties and transformation behavior of friction stir welded Ni50.7Ti49.3 alloy

To confirm the factors of deteriorating the mechanical properties of joints and the law of transformation behavior along welds thickness, different rotation speeds were conducted for friction stir welding Ni50.7Ti49.3 alloy. Results showed that intermetallic compounds were deteriorated the mechanical properties of joints, and the joint failed before the martensitic platform appeared. W13Re7 and Ti2Ni granular shaped were distributed at the shoulder affected zone (SAZ), and this region was embrittled. Removing the brittle layer of welds surface via post-welded pickling (PWP), the tensile strength of joint at a rotation speed of 600 rpm reached 751 MPa, which reached 79.1% compared with that of the base metal (BM). After the friction stir welding (FSW), the DSC was exhibited the martensite–austenite phase transformation (B19′–B2) in the pin affected zone (PAZ) and SAZ, shown shape memory effect. Phase transformation temperatures (TTs) drifted forward to positive direction about 20 K after welding. Transformation hysteresis in SAZ decreased from 19.46 °C to 12.36 °C with increasing rotation speed from 475 rpm to 950 rpm under heat and deformation of FSW process.

Series Ni–Ti shape memory alloy wires with different martensitic–austenitic phase transformation temperatures as an actuator for input shaping control

This study presents series shape memory alloy (SMA) wire actuators featuring different martensitic–austenitic phase transformation temperatures for input shaping control of flexible structures, such as compliant robot manipulators. Similar to conventional electric motors, the proposed actuator can generate stair-wise input shaping commands by simply applying a step current input. A practical input shaping control of flexible structures (e.g. beam manipulator) actuated by a series SMA wire actuator with different martensitic–austenitic phase transformation temperatures was proposed, and its ability to suppress residual vibration was successfully validated through simulation and laboratory experiments.

Frequency-tunable nickel-titanium substrates for magnetoelectric sensors

We show the use of nickel-titanium (NiTi) metal thin films as functional substrates of magnetoelectric composites allowing the composites’ mechanical resonance to be tuned towards the frequency range of interest by employing the gradual change of the Young’s modulus during the martensite-austenite phase transformation. Such composites are successfully fabricated by layering functional piezoelectric aluminum nitride and highly magnetostrictive FeCoSiB on sputtered NiTi-films. Subsequent characterizations reveal high magnetoelectric response. Applied as magnetic field sensors a high sensitivity (with a magnetoelectric coefficient αME=290 V/cm Oe) and low limit of detection of approximately 110 pT/Hz1/2 is observed for AC magnetic fields matching the mechanical resonance frequency. This mechanical resonance of the sensor can be altered with a Δf of 12% in the temperature interval between room temperature and 130 °C. This offers the possibility to adjust the sensor’s mechanical resonance frequency featuring the highest sensitivity to the magnetic field frequency to be measured.

Exploiting thermal strain to achieve an in-situ magnetically graded material

Spatially resolved functional grading is a key differentiator for additive manufacturing, achieving a level of control that could not be realised by conventional methods. Here we use the rapid solidification and thermal strain associated with selective laser melting to create an in-situ microstructurally and magnetically graded single-composition material, exploiting the solid-state austenite-martensite phase transformation. The fine grain sizes resulting from high cooling rates suppress the thermal martensite start temperature, increasing the proportion of retained austenite. Then the thermal strain accrued during the build causes in-situ deformation-driven martensitic transformation. By controlling the thermal strain, through appropriate selection of build parameters and geometry, we have been able to control the final ratio of austenite to martensite. Fully austenitic regions are paramagnetic, while dual-phase regions show increasingly ferromagnetic behaviour with an increasing proportion of martensite. We exploit this to build a magnetically graded rotor which we run successfully in a synchronous motor.

Investigations of morphological and crystallographic properties of thermally and deformation-induced martensitic phase transformations in the Fe-31.5Ni-10Mn alloy

Martensitic transformations are diffusionless shear transformations. The transformation is usually driven by mechanical deformation or by a change in temperature. This study investigated morphological and crystallographic properties of thermally and deformation-induced austenite–martensite phase transformations in the Fe-31.5Ni-10Mn (wt%) alloy. Microstructural characterization was carried out by using a transmission electron microscope (TEM). It was seen that austenite–martensite phase transformation was $$\gamma \to \alpha ^{\prime}$$ (fcc→bcc) and the morphology of martensite was lath type. It was also determined that the habit plane of martensite was $${\left( {2~5~9} \right)_\alpha }$$ and the orientation relationship between the main and product crystal structures was Kurdjumov–Sachs type. The dilatation parameter of martensite was calculated to be $$\delta ^{\prime} \cong 0.91$$ . Accordingly, it was found that martensitic phase transformations in the Fe-31.5Ni-10Mn alloy had the isothermal kinetics. Moreover, it was revealed that thermally and deformation-induced martensitic transformations had similar characteristics in terms of morphological and crystallographic aspects.

Characterization of reverse martensitic transformation in cold-rolled austenitic 316 stainless steel

ABSTRACT In this work the reversion of the strain-induced ?’-martensite to ?’-austenite was studied in a series of AISI 316 stainless steels (SS) specimens. The samples were submitted to 63% reduction in thickness by rolling at a temperature of -70oC, to achieve high martensite content. The reversion of martensite to austenite was made by means of isochronic, isothermal heat treatments at temperatures between 200 and 900oC, so as to induce partial martensite-austenite phase transformation. The samples were studied by optical and electronic microscopy, X-ray diffraction, magnetization measurements, microhardness, and electromagnetic non destructive methods: feritscope, conductivity measurements using Van der Pauw’s technique, and magnetic permeability assessments by an eddy current inverse method. The metalography study showed an ?’ lath martensite type structure and a similar one for the reverted austenite (?’). Magnetization measurements and magnetic permeability showed that the reversion is active between 400 and 800°C. From the magnetic saturation measurements, a new calibration curve for the assessment of %?’ through the feritscope readings was obtained. The results of the different techniques were com-pared, in order to assess the scope of the studied techniques in the characterization of rolled SS products, and in the evaluation of progress of reversion reaction.

Mechanical and microstructural characteristics of as-sintered and solutionized porous 60NiTi

60NiTi is a Ni-rich NiTi alloy containing ∼55 at.% (60 wt%) Ni and ∼45 at.% (40 wt%) Ti. This paper covers the use of Ni and Ti blends to fabricate porous 60NiTi parts by conventional press-and-sinter method. In this study, critical processing parameters such as heating rate and sintering holding time were altered to process porous parts with different open porosity levels. In the next step, as-sintered samples were heated at ∼1050 °C and then quenched (solutionized) to obtain hard and homogenized 60NiTi parts. Effects of these processing factors on the microstructure and mechanical properties of porous 60NiTi parts were investigated. It was found that processing the parts under a faster heating rate or a shorter sintering holding time resulted in as-sintered and solutionized samples with lower relative density and a higher amount of open porosity volume. However, a lower hardness and compressive strength was achieved as compared to the situations where a slower heating rate or a longer sintering holding time was applied. It was observed that the effect of heating rate is more pronounced compared to the sintering holding time. Final solutionizing treatment resulted in solution of soft Ni-rich phases such as Ni3Ti and Ni3Ti2 in the parent NiTi phase. This consequently resulted in a drop of austenite-martensite phase transformation to well below room temperature. This final heat treatment was beneficial in terms of increasing the amount of open porosity. Solutionized parts exhibited higher hardness, approximately the same compressive strength but more embrittlement compared to as-sintered specimens.

Thermal effects on high-frequency magnetic-field-induced martensite reorientation in ferromagnetic shape memory alloys: An experimental and theoretical investigation

Ferromagnetic Shape Memory Alloys (FSMAs) exhibit large strains by the magnetic-field-induced martensite reorientation. But, due to the high-frequency field-induced cyclic frictional martensite twin boundary motion in FSMAs, the dissipation heat can cause a large temperature rise. Thus, the output strain amplitude of FSMAs would decrease significantly if the temperature increases to be high enough to trigger the Martensite-Austenite phase transformation. Such thermal effects on the dynamic responses of FSMAs are unclear in literature because most existing dynamic experiments were performed only for a short-time period (a few seconds) to avoid the temperature rise. In this paper, systematic long-time experiments (>100 s) on a Ni-Mn-Ga single crystal are conducted at various levels of magnetic field frequency, initial compressive stress and ambient airflow velocity. It is found that, during the long-time actuation, the specimen temperature increases and then saturates at a certain level (stable temperature) while the strain oscillation evolves to a stable cycle; both the stable temperature and the stable strain amplitude depend on the frequency, the stress level and the heat exchange condition (i.e., ambient airflow velocity). Particularly, when the specimen temperature reaches a critical level to partially transform the martensite to the austenite, the output strain amplitude reduces suddenly because of less martensite reorientation. Changing the ambient heat-exchange condition (by the airflow) can modify the specimen temperature evolution to avoid the phase transformation, but it also changes the behaviors of the martensite reorientation that is sensitive to temperature. Eventually, the output strain amplitude depends on the airflow velocity non-monotonically, i.e., there exists a critical heat exchange condition to achieve the maximum stable strain amplitude. Based on the systematic experiments and a simplified one-dimensional heat-transfer model, the critical condition can be determined. The new experimental phenomena of the thermal effects can be well understood and described by the heat-transfer model. Further, instead of avoiding the temperature rise and the phase transformation, we propose to take advantage of the interaction between the temperature-induced phase transformation and the magnetic-field-induced martensite reorientation to develop a special “isothermal” FSMA actuator with a tunable output strain amplitude and a constant working temperature. This paper provides systematic experimental data and theoretical analysis for understanding the thermo-magneto-mechanical coupling in FSMAs and developing reliable high-frequency long-time running FSMA-actuators.

A unified momentum equation approach for computing thermal residual stresses during melting and solidification

In this article, we present a novel numerical method for computing thermal residual stresses from a viewpoint of fluid–structure interaction (FSI). In a thermal processing of a material, residual stresses are developed as the material undergoes melting and solidification, and liquid, solid, and a mixture of liquid and solid (or mushy state) coexist and interact with each other during the process. In order to accurately account for the stress development during phase changes, we derived a unified momentum equation from the momentum equations of incompressible fluids and elastoplastic solids. In this approach, the whole fluid–structure system is treated as a single continuum, and the interaction between fluid and solid phases across the mushy zone is naturally taken into account in a monolithic way. For thermal analysis, an enthalpy-based method was employed. As a numerical example, a two-dimensional laser heating problem was considered, where a carbon steel sheet was heated by a Gaussian laser beam. Momentum and energy equations were discretized on a uniform Cartesian grid in a finite volume framework, and temperature-dependent material properties were used. The austenite–martensite phase transformation of carbon steel was also considered. In this study, the effects of solid strains, fluid flow, mushy zone size, and laser heating time on residual stress formation were investigated.

Structure and Martensitic Transformation in Rapidly Solidified CoNiAlFe Alloy

Housler based magnetic controlled shape memory alloys are characterized by a large magnetic field induced strain. The strain was dependent on the twin martensite structure rearrangement, and the rapid solidification technology had a significant influence on the microstructure, physical, and chemical properties of the alloy. Thus, the structure and the martensitic transformation changes of Co33Ni31Al27Fe9 during the rapidly solidified process were studied. The microstructure of Co33Ni31Al27Fe9 with furnace cooled and rapid solidification (RS) constitutes a dual-phase structure, β phase and γ phase in a low cooling rate and martensite and γ phase in a high cooling rate. The γ phase at the grain boundaries reduced and became more fragile by raising the RC value. The one-step austenite-martensite phase transformation occurred during the process of heating and cooling. The phase transition temperature presents an increasing trend by rising the cooling rate, even to over the room temperature. Moreover, the martensite structure in Co33Ni31Al27Fe9 constitutes a typical L10-type twinning structure.