Martensite Plates Research Articles

Microstructure Evolution in a GOES Thin Strip

This paper focuses on the evolution of the microstructure in a grain-oriented electrical steel (GOES) thin strip after casting. After solidification, the microstructure consisted of delta-ferrite. A small fraction of austenite was formed during the cooling of the thin strip in the two-phase region (gamma+delta). Fine Cr2CuS4 particles precipitated in the ferrite and along the delta/gamma interfaces. Laths of primary Widmanstätten austenite (WA) nucleated directly on the high-angle delta-ferrite grain boundaries. The formation of WA laths in both adjacent ferritic grains resulted in a zig-zag shape of delta-ferrite grain boundaries due to their local rotation during austenite nucleation. Based on the EBSD results, a mechanism of the formation of the zig-zag grain boundaries has been proposed. Besides the Widmanstätten morphology, austenite also formed as films along the delta-ferrite grain boundaries. Sulfide precipitation along the delta/gamma interfaces made it possible to prove that austenite decomposition upon a drop in temperature was initiated by the formation of epitaxial ferrite. Further cooling brought the decay of austenite to either pearlite or a mixture of plate martensite and some retained austenite.

Application of post-weld treatment (PWHT) in improving properties of martensitic heat-resistant steel thick plate by cold metal transfer plus pulse (CMT + P) welding

Application of post-weld treatment (PWHT) in improving properties of martensitic heat-resistant steel thick plate by cold metal transfer plus pulse (CMT + P) welding

Formation mechanism of stacking faults within α′′ martensite in Ti-7 wt%Mo alloy

Stacking faults commonly exist within martensite plates in α + β titanium alloys. However, their structure and formation mechanism remain unclear. In this study, stacking faults within α′′ martensite plates were systematically investigated in an as-quenched Ti-7 wt% Mo alloy using aberration-corrected scanning transmission electron microscopy and first-principles calculations. Two kinds of stacking faults were detected, lying on (001)α′′ and (110)α′′ planes, respectively. Across each of the faults, the arrangement of atoms is 180° rotation symmetrical. The stacking faults are all produced by a specific (112)[111¯]β shear and two opposite shuffles on every other (110)β plane during martensitic transformation. First-principles calculated results indicate that the occurrence of the two opposite shuffles is attributed to the decreased stacking fault energies near the Mo solute. Our findings contribute to a deep understanding of structural transformation during bcc-β to orthorhombic-α′′ martensitic transformation.

Role of martensitic interface on the discontinuous precipitation of U-Ti alloys at 400 °C

The microstructure evolution of U-0.8 wt%Ti (U-0.8Ti) during aging at 400 ºC was investigated through SEM and TEM techniques. Three kinds of decomposition routes were identified. One route is through continuous precipitation (CP) process while the other two are through discontinuous precipitation (DP) process. In the early stage of aging, CP reactions of U2Ti occurred at the boundaries of martensitic plates. DP reactions were discovered both around grain boundaries and inside large-sized martensitic plates. The colonies of the former one were found at prior grain boundaries but were soon suppressed in the later stage of aging. DP reactions inside large-sized martensitic plates did not take place until the final stage of aging. By applying the Hatt martensitic transformation theory to the electron backscatter diffraction results, it was identified that most of the boundaries of martensitic plates were incoherent high-angle boundaries. Such boundaries compensate the opposing energy of the nucleation of the U2Ti during CP process and offer extra surface energy to promote the DP process around grain boundaries. This paper provides an overview of the decomposition process and an undocumented understanding of the role of martensitic plate interfaces on the DP and CP processes during aging at 400 ºC in U-0.8Ti alloy, which is also worth considering in other systems with acicular martensitic structure, such as U-0.8 wt%Mo and U-1 wt%Nb alloys.

Electrochemical polarization behavior and superelastic properties of a Fe–Mn–Al–Ni–Cr shape memory alloy

AbstractThe present study investigates the corrosion behavior of a Fe–Mn–Al–Ni–Cr shape memory alloy in a 5.0 wt% NaCl solution in combination with the functional properties. In a single crystalline condition, the investigated alloy shows a superior superelastic response with a maximum recovery strain of 5.2%. Furthermore, the results of the potentiodynamic polarization and impedance spectroscopy show that the addition of chromium generally improves corrosion resistance by increasing the resistance to localized corrosion. The single crystalline condition, which is characterized by an austenitic matrix and scattered martensite plates, shows an enhanced, however, still unstable passive behavior due to the formation of corrosion pits along the martensite plates.

Unusual precipitation induced by solute segregation in coherent twin boundary in titanium alloys

Heterogeneous precipitation and hence the distribution of α and β phase in α+β titanium alloys can impact strongly on alloy mechanical properties. The β phase is commonly observed to precipitate heterogeneously at boundaries between α′ martensite plates and dislocations inside the martensite, especially in the additively manufactured α+β titanium alloys, with a Burgers orientation relationship with the surrounding matrix phase. Here, we report an interesting phenomenon in a Ti-4wt%Mo alloy where the β phase precipitates heterogeneously on Mo-segregated {101¯1} fully coherent twin boundaries (CTBs) within α′ martensite, with an unusual Potter orientation relationship with α′. We find that the CTB has a structure resembling that of the β phase and that Mo segregation in the CTB leads to an unusual out-of-plane shift of the segregated atomic columns that makes the local structure almost identical to the β structure, hence serving as a template for β precipitation with the unique orientation relationship. Our findings reveal a novel mechanism via which the distribution of β precipitates can be manipulated effectively for better mechanical properties and could be extended to a group of Ti alloys including the current workhorse alloy Ti-6Al-4V and more broadly to other engineering alloys.

Shear-band blunting governs superior mechanical properties of shape memory metallic glass composites

Metallic glass composites (MGCs) containing crystals can display tensile ductility, overcoming the catastrophic failure of bulk metallic glasses. However, MGCs containing crystals with dislocation-mediated plasticity generally undergo strain-softening, but MGCs containing shape memory crystals can exhibit strain-hardening. The origin of large ductility and strain-hardening of shape memory MGCs as well as the interactions of shear bands (SBs) with crystals remains elusive. Here, two kinds of MGCs containing crystals with dislocation-mediated plasticity or shape memory crystals with martensitic transformations are investigated. It is found that the behavior and properties of SBs in glass matrix can be significantly altered by deformation characteristics of crystals. If crystals deform via dislocations, SBs are narrow, sharp and become mature. In comparison, SBs in shape memory MGCs continuously broaden without maturing by following the growth of thick martensitic plates. Broad SBs tend to bifurcate during propagation, and bifurcated SBs further induce the formation of more martensitic variants in crystals, benefiting strain delocalization. Broadening and bifurcation of SBs cause the SB blunting, which governs the superior mechanical properties of shape memory MGCs. These findings not only deepen the understanding of SBs in glass materials, but also provide a fundamental basis to enhance their mechanical properties by engineering of blunt SBs.

Thermal analysis of thin P91 steel using FlexPDE and SYSWELD

In this paper investigation of FEM based thermal analysis of Gas Tungsten Arc welding of 2 mm thick plate ferritic martensitic steel plate was carried out. Three dimensional thermo-elastic model based on FEA was carried using two different softwares SYSWELD and FlexPDE. In the initial part of the work, heat source fitting (HSF) was carried out using Grade 91 steel bead on plate welding. Predicted weld profile using heat source fitting tool was compared with predicted models. In both the softwares step-by-step modeling has been carried out using HSF analysis tool. In the second step square butt joint welding of Grade 91 steel was carried.

Interplay of Fracture and Martensite Transformation in Microstructures: A Coupled Problem.

We are witnessing a tremendous transition towards a society powered by net-zero carbon emission energy, with a corresponding escalating reliance on functional materials (FM). In recent years, the application of FM in multiphysics environments has brought new challenges to the mechanics and materials research communities. The underlying mechanism in FM, which governs several fundamental characteristics, is known as martensitic phase transformation (MPT). When it comes to the application of FM in the multiphysics context, a thorough understanding of the interplay between MPT and fracture plays a crucial role in FM design and application. In the present work, a coupled problem of crack nucleation and propagation and multivariant stress-induced MPT in elastic materials is presented using a finite element method based on Khachaturyan’s microelasticity theory. The problem is established based on a phase-field (PF) approach, which includes the Ginzburg–Landau equations with advanced thermodynamic potential and the variational formulation of Griffith’s theory. Therefore, the model consists of a coupled system of the Ginzburg–Landau equations and the static elasticity equation, and it characterizes evolution of distributions of austenite and two martensitic variants as well as crack growth in terms of corresponding order parameters. The numerical results show that crack growth does not begin until MPT has grown almost completely through the microstructure. Subsequent to the initial formation of the martensite variants, the initial crack propagates in such a way that its path mainly depends on the feature of martensite variant formations, the orientation and direction upon which the martensite plates are aligned, and the stress concentration between martensite plates. In addition, crack propagation behavior and martensite variant evaluations for different lattice orientation angles are presented and discussed in-detail.

Microstructure characterization of Au7Cu5Al4 shape memory alloys

The crystal structures and microstructures of parent and martensite phases of Au7Cu5Al4 alloy were investigated using X-ray diffraction and transmission electron microscopy. Premartensitic tweed striations with 6 M modulated structures were observed in the austenitic phase. Non-modulated martensite variants and partially modulated (8 M) martensite variants were found in the alloy that was slowly cooled (10 K/min) to –10 ℃. By contrast, in addition to non-modulated martensite plates, perfect periodic modulated (8 M) martensites were observed when the alloy was quenched into liquid nitrogen. The crystal structures of martensites with or without modulation can be basically indexed with a body-centered tetragonal (bct) structure. Whereas the tetragonality ratio (c/a) for the martensite was found to be affected by the modulation structure. These results resolve the controversy regarding the structure of martensite.

In-situ synchrotron x-ray diffraction texture analysis of tensile deformation of nanocrystalline superelastic NiTi wire at various temperatures

According to the state-of-the-art view, superelastic deformation of NiTi wires at room temperature proceeds via stress induced martensitic transformation from B2 cubic austenite to B19′monoclinic martensite. With increasing test temperature, the stress induced martensitic transformation is substituted by plastic deformation of austenite at martensite desist temperature MD. However, there are many unsolved problems with this widely accepted view. What are the texture and martensite variant microstructure in stress induced martensite and do they depend on test temperature? Does the austenite transform to martensite completely within the transformation plateau range? How the superelasticity changes into plastic deformation of austenite with increasing temperature - is it stepwise or gradual change? How the wire deforms plastically at various temperatures? Does plastic deformation occur in austenite or in martensite, via dislocation slip or deformation twinning? Are the deformation/transformation processes in nanocrystalline NiTi wires same as in large grain polycrystals? We have addressed these long standing but unsolved questions by performing series of in-situ synchrotron x-ray diffraction experiments on superelastic nanocrystalline NiTi wire subjected to tensile tests at 20, 90 and 150 °C until fracture supplemented by post mortem TEM analysis of lattice defects created by the tensile deformation.It was found that, in case of conventional superelasticity at 20 °C, austenite transformed almost completely to stress induced martensite within the transformation plateau range. The stress induced martensite displayed a sharp two fibre texture reflecting its (001) compound twinned microstructure. Stress induced martensite transformed back to the parent austenite without leaving any significant unrecovered strains and lattice defects in the austenitic microstructure. When loaded further into the plastic deformation range, this martensite deformed via combination of (20-1) and (100) deformation twinning and kinking assisted by [100](001) dislocation slip in martensite. Recoverability of tensile strains on unloading and heating remained surprisingly large (∼10%) up to wire fracture at 62% strain.The superelasticity at elevated temperatures 90 °C (150 °C) was found to be very different. The austenite transformed into a mixture of phases containing 40% (10%) volume fraction of stress induced martensite within the transformation plateau range. The stress induced martensite displayed four fibre texture, which further evolved with increasing strain. The original <111> fibre texture of austenite evolved with increasing strain towards random orientation distribution. The recoverability of tensile strains on unloading and heating sharply decreased with increasing temperature. Two alternative deformation mechanisms are proposed to explain these changes. The first mechanism assumes that martensite stress induced at elevated temperatures appears in a form of thin internally twinned CVP martensite plates surrounded by austenite deforming via dislocation slip. Requirement for strain compatibility at habit plane interfaces affects selection of martensite variants under stress and texture. The second mechanism is based on the idea that martensite stress induced at elevated temperatures immediately deforms plastically and undergoes reverse martensitic transformation to austenite leaving behind unrecovered plastic strain, slip dislocations and {114} austenite twins in the austenitic microstructure of the wire. Since the second mechanism explains the experimental observations better, it is considered to be more realistic.

A strengthening strategy for metastable β titanium alloys: Synergy effect of primary α phase and β phase stability

Increasing strength and preserving high strain hardening rate are of importance to widen the application of the metastable β titanium alloys. In this study, Ti–4Mo–4Cr–3Al–2Nb–1.2V–1Zr–1Sn (wt.%) alloy with four different primary α phase contents were prepared by solution treatment at different temperatures. The differences in β phase stability, mechanical properties, and deformation modes in these samples were investigated systematically. With the decrease of solution treatment temperature, the volume fraction of α phase, the stability of β matrix, and the tensile strength of the alloy increase, and a high strength and high strain hardening rate can be achieved simultaneously when the content of the primary α phase is less than 20%. When the applied stress exceeds the activated stress of martensitic transformation, martensite plates can be generated at large β domains and caused a dynamic Hall-Petch effect in these samples. With the further increase of strain, substructures are generated within the martensite to accommodate the deformation. Our findings provide a possible route for strengthening the metastable β titanium alloys and are beneficial to the microstructure design of the alloys with a high strength-ductility combination.

Powder bed fusion of two-functional Cu-Al-Ni shape memory alloys utilized for 4D printing

Transforming a 3D-printed part into a pre-programmed shape over time is called four-dimensional (4D) printing. Shape memory alloy (SMA) is most commonly used material for 4D printing. However, metal-based SMAs exhibit lower recovered strain during 4D printing compared to shape memory polymers, thereby requiring enhanced recovery in metal-based SMAs. In this study, a novel two-functional Cu-Al-Ni SMA consisting of two separate structures fabricated by powder bed fusion process was investigated to enhance the recovery in SMAs. The binding morphology of the two structures and subsequent consequences on the 4D printing behavior were analyzed. The first structure recovered at a lower temperature, whereas the second structure recovered at a higher temperature. In the case of the two-functional material, the difference in the hardness values and recovery temperature favored the recovery ratio due to the damping behavior of the harder structure and the generation of a temperature gradient across the deformation section. Additionally, the higher residual stresses obtained in the harder structure by simulation proved the mismatch between the two structures, leading to resistance against plastic deformation. Meanwhile, the formation of secondary martensite plates was observed in the second structure of the two-functional material. These secondary martensite plates enhanced the overall recovery by (i) the attenuation of thermal conductivity across the deformation section and generation of the temperature gradient, and (ii) the increase in residual stresses and mismatch between the two joined structures.

Structure and substructure characterization of solution-treated Ni50.3Ti29.7Hf20 high-temperature shape memory alloy

We investigate the structure and substructure of the solution-treated martensitic Ni50.3Ti29.7Hf20 high-temperature shape memory alloy (SMA) by transmission electron microscopy and precession electron diffraction (PED). Most martensite plates contain high-density internal (0 0 1) compound twins. Four martensite variants (i.e., A, B, C, and D) were observed using PED. The orientation relationships were determined to be (1 1 0.64) Type II twins between A and B, and (1 1 1) Type I twins between C and D. B and C are divided by junction planes, in which (1 1 1) in C is parallel to (1 1 0.64) in B. These orientation relationships are rarely observed in other NiTi or NiTiHf SMAs, which offers new insight to understand the phase transformation behavior in Ni50.3Ti29.7Hf20 SMAs. Several low-angle grain boundaries were observed near the variant interfaces, which may explain the difficulty of martensite reorientation and detwinning in NiTiHf SMAs.

Structural, mechanical and electrochemical properties of spark plasma sintered Ti-30Ta alloys

Ti-Ta alloys are choice materials for bio-medical applications, but obtaining tailormade material properties through using spark plasma sintering (SPS) fabrication exhibits potential challenges. The effects of sintering temperature, pressure and holding time on the microstructural, crystallographic, mechanical and corrosion behaviour of spark plasma sintered Ti-30Ta alloys were investigated. Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectroscopy (EDS), X-Ray Diffractometry, microhardness and instrumented nanoindentation were used for the investigation. Microstructures obtained include martensite plates (alpha-Ti phases), beta-Ti phases, as well as significant volume of beta-Ta phases. Notable variation in crystal structure, mechanical properties and electrochemical behaviour were recorded. Generally, the materials exhibited high densification (>98%), improved micro-and nano-indentation hardness (>500 Hv), and elastic moduli (>40 GPa). Average Oxygen content of the sintered samples was 0.0106%. The measured properties of the sintered Ti-30Ta alloys suggest they can be efficient for applications in biomedical technologies.

STRUCTURAL CHANGES IN A POWDER METALLURGY 66FE-14MN-6SI-9CR-5NI (MASS. %) SHAPE MEMORY ALLOY SUBJECTED TO BENDING CREEP

Fe-14Mn-6Si-9Cr-5Ni (mass. %) shape memory alloy (SMA) has been developed as a commercial grade of Fe-Mn-Si based SMAs with excellent formability and corrosion resistance. Producing FeMnSiCrNi SMAs by powder metallurgy enables a better control of chemical composition and grain size. This paper discusses the variation of bending creep deformation, as a function of time, temperature and force, in the case of a powder metallurgy (PM) Fe-14Mn-6Si-9Cr-5Ni SMA with 50 % mechanically alloyed powder volume. Creep test temperatures were selected based on phase transitions determined by dynamic mechanical analysis (DMA), during heating. The results suggested the tendency of martensite plate variants to reorient along a common direction, after creep.

Structural-Functional Changes in a Ti50Ni45Cu5 Alloy Caused by Training Procedures Based on Free-Recovery and Work-Generating Shape Memory Effect.

Active elements made of Ti50Ni45Cu5 shape memory alloy (SMA) were martensitic at room temperature (RT) after hot rolling with instant water quenching. These pristine specimens were subjected to two thermomechanical training procedures consisting of (i) free recovery shape memory effect (FR-SME) and (ii) work generating shape memory effect (WG-SME) under constant stress as well as dynamic bending and RT static tensile testing (TENS). The structural-functional changes, caused by the two training procedures as well as TENS were investigated by various experimental techniques, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), and atomic force microscopy (AFM). Fragments cut from the active regions of trained specimens or from the elongated gauges of TENS specimens were analyzed by DSC, XRD, and AFM. The DSC thermograms revealed the shift in critical transformation temperatures and a diminution in specific absorbed enthalpy as an effect of training cycles. The DMA thermograms of pristine specimens emphasized a change of storage modulus variation during heating after the application of isothermal dynamical bending at RT. The XRD patterns and AMF micrographs disclosed the different evolution of martensite plate variants as an effect of FR-SME cycling and of being elongated upon convex surfaces or compressed upon concave surfaces of bent specimens. For illustrative reasons, the evolution of unit cell parameters of B19′ martensite, as a function of the number of cycles of FR-SME training, upon concave regions was discussed. AFM micrographs emphasized wider and shallower martensite plates on the convex region as compared to the concave one. With increasing the number of FR-SME training cycles, plates’ heights decreased by 84–87%. The results suggest that FR-SME training caused marked decreases in martensite plate dimensions, which engendered a decrease in specific absorbed enthalpy during martensite reversion.

Formation and Thermal Stability of the ω-Phase in Ti-Nb and Ti-Mo Alloys Subjected to HPT.

This paper discusses the features of ω-phase formation and its thermal stability depending on the phase composition, alloying element and the grain size of the initial microstructure of Ti–Nb and Ti–Mo alloys subjected to high-pressure torsion (HPT) deformation. In the case of two-phase Ti–3wt.% Nb and Ti–20wt.% Nb alloys with different volume fractions of α- and β-phases, a complete β→ω phase transformation and partial α→ω transformation were found. The dependence of the α→ω transformation on the concentration of the alloying element was determined: the greater content of Nb in the α-phase, the lower the amount of ω-phase that was formed from it. In the case of single-phase Ti–Mo alloys, it was found that the amount of ω-phase formed from the coarse-grained β-phase of the Ti–18wt.% Mo alloy was less than the amount of the ω-phase formed from the fine α′-martensite of the Ti–2wt.% Mo alloy. This was despite the fact that the ω-phase is easier to form from the β-phase than from the α- or α′-phase. It is possible that the grain size of the microstructure also affected the phase transformation, namely, the fine martensitic plates more easily gain deformation and overcome the critical shear stresses necessary for the phase transformation. It was also found that the thermal stability of the ω-phase in the Ti–Nb and Ti–Mo alloys increased with the increasing concentration of Nb or Mo.

The role of primary α grains in the degree of transformation texture obeying the Burgers orientation relationship in Zr alloys

The microstructure and texture evolution of Zr-xNb-0.4Mo (x = 0, 0.2, 0.4, 0.6 and 1.0 wt%) alloys are characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique. The aim is to elucidate the role of primary α grains in the degree of transformation texture obeying the Burgers orientation relationship (OR) in Zr alloys. The primary α grain is produced by adjusting Nb content in Zr alloys. The crystallographic orientations of Zr-xNb-0.4Mo alloys show that the degree of transformation texture obeying the Burgers OR increases with the proportion of primary α grains decreasing. Until no primary α grains exist, the transformation texture obeys the Burgers OR strictly. The primary α grains can exert great influence on variant selection by restricting the growth of high-temperature β grains and acting as nucleation sites. The fact that primary α grains are the origins of variant selection is observed. Meanwhile, variant selection occurs in both the duplex microstructure and full martensite plates during the water-quenching process. The texture weakening of the duplex microstructure is attributed to the new texture components given by martensite plates. In terms of microstructure, a series of precipitates, Zr3(Nb, Mo, Fe), are found in a primary α grain.

Enhancement of mechanical properties of modified 9Cr–1Mo (P91) steel using the thermomechanical processing and smart heat treatment protocol

An effort has been made to enhance the mechanical properties to a great extent by tuning the process parameters of the modified 9Cr–1Mo steel by carrying out its transformation into bainitic microstructure at isothermal temperatures of 460 °C for varying isothermal holding durations after austenitization at 1000 °C for an hour. Two sets of austempered specimens were made. One set was water quenched (Set-I), and the other set was air-cooled (Set-II) after austempering. The microstructural characterization of the processed steels was systematically carried out by optical and scanning electron microscopy (SEM). The Vickers hardness of the heat-treated specimens was found to be almost 1.5–2 times the hardness of the as-received tempered martensitic steel plates. The tensile strength was found to increase dramatically from ∼700 MPa to ∼1300 MPa after austempering as compared to the as-received tempered martensitic steel. Though the % elongation reduced slightly, a good amount of ductility was still maintained in the austempered steel specimens, allowing the steels to achieve an excellent combination of strength as well as ductility.