Martensite Lattice Research Articles

Interface compatibility and hysteresis in shape memory materials are affected by lattice distortions from applied stresses

Significant effort has been put into designing shape-memory materials that can survive many cycles without functional or structural fatigue. A component of the design process is the condition defining perfect interface compatibility between the austenite and martensite lattices (λ2=1). In this paper, we evaluate the traditional mathematical theories of martensite under applied stresses, which distort the lattice compatibility through elastic strains. In NiTi we find that elastic distortions resulting from applied stresses influence the interface compatibility to a degree of impacting the material's functional abilities. Combining our results with empirical relationships connecting interface compatibility to transformation hysteresis we show that the model matches reasonably to a number of experimental results in the literature in which hysteresis changes under applied loads. We also apply these theories to a shape-memory ceramic (zirconia), which suggests a large orientation-dependence and asymmetric behavior in tension and compression. In both systems, we find that variant selection plays a large role in whether interface compatibility will improve or worsen under stress.

Generalized mechanism for the solid phase transition of M2O3 (M=AI, Ga) featuring single cation migration and martensitic lattice transformation

Generalized mechanism for the solid phase transition of M2O3 (M=AI, Ga) featuring single cation migration and martensitic lattice transformation

Hydrogen penetration into the NiTi superelastic alloy investigated in-situ by synchrotron diffraction experiments

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm.

On the habit planes between elastically distorted austenite and martensite in NiTi

The cubic B2 to monoclinic B19’ martensitic transformation in NiTi shape memory alloys proceeds by the nucleation and propagation of habit plane interfaces that remain undistorted and unrotated during the transformation. Due to incompatibility of austenite and martensite lattices in NiTi, habit plane interfaces are assumed to form between austenite and twinned martensite as described by Phenomenological Theory of Martensite Crystallography (PTMC). This view is currently widely spread in the literature in spite of the fact that it contradicts experimental observations of interfaces between austenite and single martensite lattice by TEM and EBSD methods frequently reported in the literature. On this account, we propose a different solution for the formation of strain compatible habit planes propagating during stress induced martensitic transformation by considering the effect of elastic deformation of both lattices caused by the external stress. Using modified PTMC theory, we evaluate the magnitudes of the uniaxial stress in tension and compression along a general austenite crystal direction, for which strain compatible habit plane interfaces can be established between austenite and single variant of martensite. The stress calculated in such a way is, however, too high. Nevertheless, considering the pre-transformation softening of austenite, the critical stress decreases to 500 MPa commonly observed in experiments. Three scenarios are simulated assuming softening of elastic constants of austenite. Orientation dependence and tension-compression asymmetry of predicted habit planes of stress induced martensitic transformation is compared with available experimental results.

Modulated martensite in NiTi shape memory alloy exposed to high stress at high temperatures

NiTi shape memory wire was deformed in martensite state at low temperature −125 °C up to 750 MPa stress and subsequently heated far above the Af temperature while the stress 750 MPa was held constant. Under such conditions, the heated B19’ martensite in the wire was driven towards its thermomechanical stability limits and eventually transformed into plastically deformed austenite. In order to evaluate the mechanism by which such highly constrained reverse martensitic transformation proceeds, martensite variant microstructures and lattice defects evolving in the heated wire were analyzed by TEM and HRTEM.It is found that, the oriented B19’ monoclinic martensite deformed plastically via kwinking deformation involving coordinated dislocation slip and twinning and simultaneously adopted a long period modulated crystal structure, before it transformed to austenite upon heating under applied stress. It is proposed that the modulated martensite structure forms from the B19’ structure via coordinated slip of partial dislocations with Burgers vector b = a/2 [100] on (001) crystal planes. Coordinated slip was enforced by the kwinking deformation and partial dislocations started to glide because the martensite was exposed to elevated temperatures. Plastic deformation of oriented B19’ martensite enabling the reverse martensitic transformation upon the constrained heating is discussed in view of its impact on NiTi functionality in engineering applications.

Effect of Heat Treatment on Corrosion and Mechanical Properties of M789 Alloy Fabricated Using DED

The directed energy deposition (DED) process offers potential advantages, such as a large building space, limited dilutions, narrow heat-affected zones (HAZ) and potentially improved surface properties. Moreover, heat treatments have been reported to significantly improve the properties of the as-built sample by modifying the microstructure. In this study, the influences of various combinations of heating and cryogenic treatments on the mechanical performance and corrosion resistance of DED M789 steel have been critically investigated. The microstructure and hardness were examined to discuss the characteristics of the M789 parts in the as-printed and heat-treated states. The corrosion rate was determined from the weight loss monitoring based on the seawater immersion condition. The microstructural results revealed the distortion of martensite lattice and the formation of nano-carbide precipitates after the cryogenic treatment. Moreover, the microhardness of the cryogenically treated M789 steel was found to be significantly higher which was attributed to the precipitate strengthening and elimination of retained austenite, resulting from the increased volume fraction of carbides due to cryogenic treatment. The corrosion characteristics were also modified by the heating/cryogenic treatments, and the substrate-to-deposit ratio of the corrosion sample also substantially affected the overall corrosion rate.

Wide tuning the carbon supersaturation within Fe-C lath martensite via high pressure martensitic transformation of Fe-0.45C alloy

The carbon supersaturation within lath martensitic lattice was tuned via thermal treatment of a Fe-0.45C alloy under hydrostatic pressure of 1–6 GPa. The content of supersaturated carbon increases linearly with high pressure till 84.4% of the nominal carbon content under 6 GPa. The strengthening of carbon supersaturation is remained proportional to the square root of its content but the contribution was underestimated.

Tensile deformation of NiTi shape memory alloy thermally loaded under applied stress

Constitutive behavior of engineering materials is typically characterized by stress–strain curves from isothermal tensile and/or compression tests until fracture. Strain reversible behavior of martensitically transforming shape memory alloys (SMA) is additionally characterized by cyclic stress–strain and strain-temperature curves limited to temperatures and stresses, at which the recorded strain responses are reversible in closed loop cycles. In this work focussing coupled martensitic transformation and plastic deformation of NiTi, we deformed nanocrystalline NiTi SMA wire in isothermal and isostress tensile tests beyond the temperature and stress limits stemming from the requirement on the strain reversibility in closed loop cyclic tests. Stress–strain-temperature responses of NiTi wire in such tests were recorded and analysed. To detect and characterize deformation mechanisms activated in performed thermomechanical loads, electric resistance and dynamic elastic modulus of the wire were evaluated in-situ during tensile tests. Martensite variant microstructures and lattice defects in austenite evolving upon heating deformed NiTi wire under 750 MPa stress were analyzed by post mortem transmission electron microscopy. Stress-temperature diagram showing critical stress–temperature conditions for activation of 5 different deformation/transformation processes in thermomechanically loaded NiTi was constructed from the results of isothermal and isostress tests and the recorded stress–strain-temperature responses were discussed based on this diagram

Hydrogen diffusion kinetics in dual-phase (DP 980) steel: The role of pre-strain and tensile stress

The current study investigates the influence of pre-strain (un-strained, 3 %, and 6 % engineering strain) and in-situ tensile stress (50 %, 110 %, and 125 % of the yield stress (YS)) on the hydrogen diffusion and trapping behaviour in DP 980 steel. Electrochemical hydrogen permeation tests were conducted using a custom-built Devanathan-Stachurski (DS) cell integrated with a horizontal tensometer. The results showed that the value of apparent diffusion coefficient (Dapp) decreased from 5.4 ± 0.1 to 3.8 ± 0.029 × 10−7 cm2 s−1, while trap density (NT) and sub-surface hydrogen concentration (Capp) increased from 1.08 ± 0.01 to 1.58 ± 0.012 × 1021 cm−3, 2.70 ± 0.13 to 3.27 ± 0.10 × 10−4 mol H cm−3, respectively for unstrained and 6 % pre-strain conditions. This is due to increased dislocation density in ferrite and martensite. Hydrogen permeation tests under tensile stress (in-situ) revealed that the applied elastic stress of 50 % of YS showed a slight increase in Dapp and a decrease in NT values. This is due to an increase in the interstitial space between the atoms at applied elastic load favouring hydrogen diffusion. The plastic tensile stress of 110 % of YS showed the highest value of steady-state permeation current density iss (169 ± 10.6 µA cm−2). It could be due to an expanded lattice of martensite accommodating more hydrogen because of its elastic deformation. The tensile stress of 125 % of YS results in a significant reduction in Dapp (2.86 × 10−7 cm2 s−1) and an increase in Capp (5.52 × 10−4 mol H cm−3) and NT (2.11 × 1021 cm−3) as a result of plastic deformation in ferrite and martensite. The decay transient studies revealed that the applied elastic tensile stress enhances the rate of desorption of diffusible hydrogen, increasing lattice diffusion coefficient DL. The plastic pre-strain and plastic tensile stress resulted in slower desorption, lower DL, and an increase in reversible trap density.

Time-resolved in-situ dislocation density evolution during martensitic transformation by high-energy-XRD experiments: A study of C content and cooling rate effects

Recent studies on the mechanical behavior of martensitic steels show the importance of considering lath martensite as a “polycrystalline aggregate” with a large distribution of the local yield strength. The latter depends on local microstructural features, such as lath sizes, carbon distribution (in solid solution, segregated to defects, in carbides) and density of defects Internal stresses have to be considered as an additional contribution to the yield strength participating to enlarge the distribution of the local flow stresses at the microstructure scale.This study focuses on one of these microstructural features, the dislocation density. Its evolution is followed in situ upon martensitic transformation by High Energy X-Ray Diffraction experiments on a synchrotron beamline. A previously introduced method, based on modified Williamson-Hall (mWH) analysis, is improved in order to take account of the martensite lattice tetragonality when applying the mWH method.The influence of the steel carbon content (0.11, 0.21 and 0.31 wt.% C steels) and of the cooling rate (-10, -50, -100 °C/s and water-quench) on the dislocation density evolution in martensite during the martensitic transformation is established. The effect of the cooling rate on the dislocation density is low between -10 and -100 °C/s, but becomes visible after the water-quench. The higher the C content, the higher the dislocation densities at the end of the transformation. The steels with higher C content also show a wider distribution of the local dislocation density and, therefore, of the local yield strength.

Nanotwinned (inter)martensite transformation interfaces in Ni50Mn25Ga20Fe5 magnetic shape memory single crystal foil

Using common and high resolution transmission electron microscopy (TEM), we study the magnetic and crystal phase structures as well as their evolution in Ni50Mn25Ga20Fe5 magnetic shape memory material with high Curie point. The particular alloying by Fe and changing thickness of the TEM foil enable us to observe all martensitic phases known in Ni-Mn-Ga Heusler alloy system and their respective transitions simultaneously. Starting from cubic austenite at about 10 nm foil thickness, the structure evolves via peculiar interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with a low density of stacking faults at 40 nm thickness. With further increasing thickness, the 10M phase transforms gradually to seven-layered modulated 14M martensite with an increased density of stacking faults. Finally, the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins forming the modulated phases. High resolution TEM further confirms that nanotwinning and stacking faults are inherent structure features tightly connected with the lattice modulation and intermartensite transformations. Overall, the evolution of average lattice shows the same general trend, an increasing simple shear with (11¯0) shuffling plane, across the whole phase sequence. Additionally, we found large local variation of lattice parameters in all phases, which is is ascribed to strong lattice softening in the vicinity of the martensitic transformation and high density of stacking faults in 14M martensite lattice.

Tailoring the anisotropic (positive/zero/negative) thermal expansion in shape memory alloys through phase transformation and martensite (re)orientation

Recent experiments demonstrated that the macroscopic thermal expansion (TE) response of bulk shape memory alloys (SMAs) can be tailored via martensite variant texturing by taking advantage of the significant intrinsic TE anisotropy of the martensite lattice. Here, a description of the TE response of SMAs is proposed, which relies on the volume fraction of oriented (detwinned) martensite and the orientation direction tensor. The model is validated by comparing simulations against available experimental data on the positive/zero/negative TE of a polycrystalline NiTiPd SMA upon deformation processing and applied to track the TE evolution due to phase transformation and reorientation induced under nonproportional loading.

In situ Investigation during Low Pressure Carburizing by Means of Synchrotron X-ray Diffraction*

Abstract In situ X-ray diffraction investigations during low pressure carburizing (LPC) processes were performed with a specially developed process chamber at the German Electron Synchrotron Facility (DESY) in Hamburg, Germany. Carbon saturation in austenite was reached in less than 20 seconds for all processes with different parameters and carbides formed at the surface. Therefore, the direct contribution of carbon donor gas to the carbon profile after 20 seconds was reduced to very low levels. After that point, further supply of carbon donor gas increased the amount of carbides formed at the surface, which will contribute to the carbon profile indirectly by dissolution in the following diffusion steps. During quenching, martensite at higher temperatures had a lower c/a ratio than later formed ones. This difference is credited to self-tempering effects and reordering of carbon atoms within the martensite lattice.

Neutron Diffraction Study of the Martensitic Transformation of Ni2.07Mn0.93Ga Heusler Alloy

The martensitic transition featuring the ternary Heusler alloy Ni2.09Mn0.91Ga was investigated by neutron diffraction. Differential scanning calorimetry indicated that structural transition starts at 230 K on cooling with a significant increase in the martensitic transformation onset compared to the classical Ni2MnGa. The low-temperature martensite presents the 5M type of modulated structure, and the structural analysis was performed by the application of the superspace approach. As already observed in Mn-rich modulated martensites, the periodical distortion corresponds to an incommensurate wave-like shift of the atomic layers. The symmetry of the modulated martensite at 220 K is orthorhombic with unit cell constants a = 4.2172(3) Å, b = 5.5482(2) Å, and c = 4.1899(2) Å; space group Immm(00γ)s00; and modulation vector q = γc* = 0.4226(5)c*. Considering the different neutron scattering lengths of the elements involved in this alloy, it was possible to ascertain that the chemical composition was Ni2.07Mn0.93Ga, close to the nominal formula. In order to characterize the martensitic transformation upon increasing the temperature, a series of neutron diffraction patterns was collected at different temperatures. The structural analysis indicated that the progressive change of the martensitic lattice is characterized by the exponential change of the c/a parameter approaching the limit value c/a = 1 of the cubic austenite.

Influence of High-pressure Quenching on the Microstructure, Martensite Transformation, and Mechanical Properties of 0.2 Mass% C Steel

Low-carbon steel (0.2 mass%) samples were austenitized and quenched at a cooling rate of 10°C/s under GPa level high pressure. The morphology, lattice constant, and order degree of C atom distribution of high-pressure quenching martensite were characterised and analyzed by TEM, EBSD, XRD, and Mössbauer. Besides, the transformation characteristics and strengthening mechanisms were discussed. The results show that the microstructure of 0.2 mass% C steel is fine hierarchical lath martensite with almost no residual austenite, and its laths mostly follow {112} <111> twin relationship, indicating the self-accommodation effect among martensite variants. Compared to atmospheric pressure, the order degree of carbon atom distribution increases in high-pressure quenched martensite, meanwhile the tetragonality (c/a) of martensite lattice increases from 1.009 at atmospheric pressure to 1.012 at 4 GPa. The significant promotion of hardness in 0.2 mass% C steel subjected to high-pressure treatment can be ascribed to a large number of dislocations in the structure, grain refinement strengthening caused by twin boundary, and solution strengthening caused by large distortion due to the increase of the order degree for C atom distribution and the decrease of lattice constant. These findings provide new insights into the carbon steel martensite transformation mechanism, and a new martensite transformation technique can be developed.

Evolution of martensitic microstructures in nanocrystalline NiTi wires deformed in tension

An experimental method allowing for reconstruction of martensite variant microstructures evolving during tensile thermomechanical loading test on nanocrystalline NiTi wire is introduced. The method is based on the determination of the location, size and orientation of all martensite variants and interfaces within a selected polycrystal grain using post mortem selected area electron diffraction with dark field image analysis in TEM.Martensitic microstructures in the NiTi wire deformed up to the end of reorientation (transformation) plateau up to ∼7% strain at room temperature (100°C), respectively, were found to contain single domain (001) compound twinned martensite filling whole grains of the deformed wire. This observation was rationalized by theoretical treatment of strain accommodation in <111> fiber textured NiTi wire deformed in tension predicting such singular microstructural state in grains of NiTi wire deformed up to 6.74% strain. Upon subsequent unloading and stress free heating above the Af temperature, this martensitic microstructure retransforms back to the parent austenite yielding recoverable strains typical for NiTi wires (∼6%) accompanied by very small unrecovered strain (∼0.6% - 1.5% depending on the test temperature).On further tensile loading up to 15% strain, plastic deformation of oriented martensite proceeds by coordinated (100) and (201¯) deformation twinning in martensite assisted by [1 0 0](0 0 1) dislocation slip giving rise to characteristic martensitic microstructures containing deformation bands frequently forming wedges within a single martensite lattice. Upon subsequent unloading and stress free heating above the Af temperature, this martensitic microstructure transforms to {114} twinned austenitic microstructure yielding very large recoverable strains (∼10%) accompanied by large unrecovered strains (∼5%). It is claimed that the dislocation slip assisted (100) and (201¯) deformation twinning in martensite renders NiTi excellent combination of strength and deformability and leads to refinement of austenitic microstructure accomplished via introducing {114} austenite twins into it.

Tailoring thermal expansion of shape memory alloys through designed reorientation deformation

Manipulating macroscopic linear thermal expansion (TE) through microstructure engineering is emerging as a new research topic of shape memory alloys (SMAs). Here, we perform a modelling-and-experiment combined study to design and tune the coefficient of TE (CTE) of polycrystalline martensitic SMAs based on reorientation deformation. Using Landau thermodynamic theory of first-order phase transition, we prove that the martensite lattices of most SMAs possess intrinsic negative (positive) TE along the directions of transformation elongation (contraction). As a combined result of such intrinsic lattice-level TE and the extrinsic reorientation-deformation texture, the overall linear CTEs of martensite polycrystals decrease (increase) with the amount of reorientation stretching (contraction). The deformation paths and strain magnitudes are designed to obtain zero linear TE in 17 types of bulk SMAs and are verified by the available experimental data. Furthermore, through designed cross-rolling on martensitic NiTi sheets, we obtain ultralow in-plane CTEs (+0.13 × 10−6 K−1 ∼ +1.4 × 10−6 K−1), which are smaller than the reported values of SMAs and are even smaller than the commercial FeNi Invar alloy (+2.0 × 10−6 K−1). Our work provides a theoretical base to design SMAs with desired linear CTEs for many applications.

Phase Transformation Law of Nb Microalloyed Steel at Different Cooling Rates

Through the Gleeble3500 thermal simulation test machine, the phase transformation law of Nb microalloyed steel was studied and tested. After the compression deformation, it was cooled to room temperature at different speeds. Obtain the dynamic continuous cooling transformation diagram and the scanning structure diagram of the test steel, and then analyze the phase composition under different cooling speeds through JMatPro material performance simulation. The results show that: at a lower cooling speed (0.1°C/s), austenite decomposition is a diffusion-type phase change that takes place in a high-temperature region, and carbon atoms can diffuse sufficiently. At a moderate cooling rate (1°C/s), the bainite phase transition is a semi-diffusion phase transition in which carbon atoms are displaced in a non-cooperative thermally activated transition mode. When the cooling rate is high (15°C/s), the martensitic transformation is a non-diffusion-type transformation carried out in the low temperature region, and the atoms are directly transferred from the austenite lattice to the martensite lattice. With the increase of the cooling rate and the decrease of the transition temperature, from low-speed cooling→medium-speed cooling→high-speed cooling, respectively, the diffusion type phase transition→semi-diffusion type phase transition→the non-diffusion type phase transition. At different cooling rates, the continuous cooling transition diagram simulated by JMatPro is basically the same as the phase transition in the dynamic continuous cooling transition diagram of the test steel, which proves that the simulation prediction of the dynamic continuous cooling transition of the test steel by the JMatPro software has high accuracy and applicability.

Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M 6 C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M 6 C and martensite caused by the inconsistency deformation, M 6 C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness. • The type of carbides and interface relationship was obtained. • Evolution of microstructure during heat treatment was studied. • The role of carbides in impact toughness was investigated. • The variation of impact toughness value has been explained by the characteristics of martensite.

An in-situ neutron diffraction investigation of martensitic transformation in a metastable β Ti-10V-2Fe-3Al alloy during uniaxial tension

The deformation behaviour of a metastable β Ti-10V-2Fe-3Al (wt%) alloy containing ~10 vol% α phase was investigated via single-peak fitting and Rietveld refinement of in-situ neutron diffraction data collected during uniaxial tension. The formation of the 021α″ fibre is related to martensite variant selection during β → α″ transformation under applied load. The β lattice parameter increases by ~0.08% during tension whereas α″ accommodates deformation through a decrease and increase in a and b lattice parameters, respectively. The origin of the variation in α″ martensite lattice parameters with strain for different Ti alloys is linked to their different rhombicity values.