Subsequent Martensitic Transformation Research Articles

Twin inheritance effect of thermomechanical-processing NiTiCu shape memory alloy

NiTiCu shape memory alloy undergoes thermomechanical processing which deals with local canning compression followed by annealing at 300, 450 and 600 °C, respectively. In particular, formation of twins is dependent upon size of grains. In the case of annealing at 300 °C, nanocrystalline NiTiCu specimen is fabricated, where a small number of nanotwins are observed and the smaller nanocrystalline grains contributes to suppressing the formation of twins. In addition, twin inheritance effect of NiTiCu shape memory alloy is revealed during thermomechanical processing which deals with local canning compression followed by annealing at 450 and 600 °C, respectively. {112} B2 austenite twin, (001) B19′ martensite twin and (100) B19′ martensite twin are found in NiTiCu specimen annealed at 450 °C. In addition to (001) B19′ martensite twin, {114} B2 austenite twin appears in NiTiCu sample annealed at 600 °C. Formation of {114} twin results from transformation of 201̄ B19′ martensite twin during reverse martensite transformation, whereas formation of {112} twin stems from transformation of {113} B19′ martensite twin during reverse martensite transformation. During subsequent martensite transformation, {114} and {112} twins are so stable that they are unable to be transformed into the corresponding 201̄ and {113} twins.

Effect of repeated-tempering induced martensite on the microstructural evolution in 17–7 PH stainless steel

Single long-time tempering and triple tempering processes have been applied to 17–7 PH stainless steel (chemical composition: Fe-0.09C-16.4Cr-7.7Ni-0.9Al, wt%) to investigate the effects of the stability of austenite on subsequent martensitic transformation and aging hardening. The as-received steel was composed of 0.55 austenite and 0.45 martensite (volume fraction). Scanning electron microscopy images showed that the volume fraction of martensite (up to 0.98) in triple-tempered specimens ((0.5 h @ 760 °C) × 3) was about 0.15 higher than that (0.83) in single-tempered ones ((1.5 h @ 760 °C) × 1). The results clearly indicate the effectiveness of the triple-tempering process in progressively improving martensitic transformation. Direct observation of the austenite/martensite interface has provided evidence that the preceding martensitic transformation induces accommodation strain into the adjacent austenite, which brings about the mechanical stabilization of austenite but will facilitate the succeeding martensitic transformation after three consecutive tempering operations. The relevant microstructural evolution has been explored and discussed. Furthermore, subsequent aging at 565 °C led to precipitation hardening associated with the formation of NiAl nanoparticles. The triple-tempered specimen yielded the higher peak hardness value (424 HV) in 20 min, significantly faster than the single-tempered specimen, which took 90 min to reach the lower one (357 HV). The corresponding number density of NiAl nano-precipitates and dislocation densities of martensite at various aging times have been examined by transmission electron microscopy to elucidate the benefits of the triple tempering process for precipitation strengthening.

In-situ heating-stage EBSD validation of algorithms for prior-austenite grain reconstruction in steel

Prior-austenite grain (PAG) structures play a critical role in determining steel microstructures and properties. Over recent years, the use of algorithms that reconstruct PAGs from electron backscatter diffraction (EBSD) measurements of martensitic or bainitic microstructures have become commonplace. These appear to work well, but there has yet to be a comparison made of the reconstructed PAG microstructure to the true PAG microstructure of a steel measured above Ae3 before transformation. Here, in-situ EBSD was used to record the austenite grain structure (at 1000 °C) before transformation, as well as the progress of subsequent martensitic and bainitic transformations. The resulting dataset, which we have made available to the community here: https://doi.org/10.5281/zenodo.8348372, was used to assess of two popular algorithms for PAG reconstruction. They were found to be accurate enough for PAG size measurement and general texture analysis, although the exact morphologies of some PAG and twin boundaries are sometimes lost.

Martensitic Induction Hardening of Nitrided Layers*

Abstract In this research a combination of nitriding and induction hardening is investigated, as this is expected not only to result in significant savings in process time and energy, but also to produce surface layer properties that cannot be set with one of the individual processes. The focus of the current investigations was on the dissolution of the compound layer during inductive heating and the resulting microstructure formation and the hardness profile. Furthermore, it was investigated how the absence of a compound layer affects the subsequent martensitic transformation. For this purpose, differently nitrided surface layers were martensitically hardened and the microstructure was investigated metallographically and physically. After the martensitic transformation of the nitrided layer porosity and retained austenite were observed due to the decomposition of the nitrides of the compound layer. The retained austenite could be reduced by higher temperatures during surface hardening and compound layer removal. The investigations showed, that the optimum initial condition for induction hardening is nitriding with compound layer and a mechanical removal of the latter prior to induction heat treatment.

Simultaneous enhancements of strength and ductility of a selective laser melted H13 steel through inoculation treatment

Recent studies have confirmed that the high brittleness and property anisotropy of the selective laser melting (SLM) fabricated H13 tool steel are attributed to the presence of elongated, unstable high carbon γ (austenite) thin films in between the columnar structures. To overcome this problem, TiN nanoparticles (0.5 wt.%) were used as an inoculator to promote heterogeneous nucleation of the prior γ-grains, converting the columnar structure into equiaxed and refined grains. As a result, the finer α’-blocks formed upon the subsequent martensitic transformation. This not only leads to the elimination of the property anisotropy, but also simultaneously increases the strength (tensile strength of 2051 ± 48 MPa) and ductility (elongation of 7.4 ± 0.7%) of the SLM-fabricated H13 steel. The present work forms a significant step towards to fabricating high-performance tool steels using the SLM technique.

Austenite Grain Refinement and Subsequent Martensite Transformation of Ternary Cu–Al–Ni Alloy Solidified Within Ultrasonic Field

Austenite Grain Refinement and Subsequent Martensite Transformation of Ternary Cu–Al–Ni Alloy Solidified Within Ultrasonic Field

Influence of spinodal decomposition on structure and thermoelastic martensitic transition in MnCuAlNi alloy

The effect of spinodal decomposition in Mn-19Cu-2Al-4Ni solid solution quenched from 830 °C after different ageing regimes is studied. After water quenching, face-centered cubic (f.c.c.) austenite is the only observed phase in the samples at room temperature, whereas, after different ageing regimes, the alloy has a two-phase state: face-centered tetragonal (f.c.t.) martensite and α-Mn body-centered cubic (b.c.c.) phase. Small-angle neutron scattering reveals that Cu-enriched clusters are formed in a Mn-enriched matrix due to spinodal decomposition during ageing in the austenitic state. With an increase in the Mn content in the matrix, temperatures of martensitic transition rise above room temperature, and a subsequent martensitic transformation upon cooling leads to the formation of f.c.t.-martensite. Temperatures for direct and reverse transitions are strongly dependent on ageing temperature and time, i.e., on the degree of chemical decomposition in austenite.

Hot workability of FeMnSiCrNi shape memory alloy based on processing map and martensitic transformation

Hot workability of Fe66Mn15Si5Cr9Ni5 (wt.%) shape memory alloy (SMA) with γ austenite and ε martensite at room temperature was investigated by establishing processing map and simultaneously considering martensitic transformation. All the FeMnSiCrNi samples subjected to hot deformation are composed of ε martensite and γ austenite. Furthermore, the higher temperature results in the larger fraction of ε martensite, whereas the higher strain rate leads to the smaller fraction of ε martensite. Simultaneously, the higher temperature or the lower strain rate leads to the larger grain size. In addition, dislocations, stacking faults and austenite twins appear in the deformed samples. According to processing map, the optimal hot processing parameters for the FeMnSiCrNi SMA is determined as the temperature range of 850–925 °C at the strain rate below 0.0008 s−1 and the temperature range of 925–1000 °C at the strain rate range of 0.001–0.03 s−1. Furthermore, the existence of ε martensite in the deformed FeMnSiCrNi samples has an adverse effect on shape memory effect of FeMnSiCrNi SMA. In addition, dynamic recrystallization (DRX) and subsequent martensitic transformation weaken the textures of the grains with γ austenite phase in the deformed FeMnSiCrNi SMA samples. Therefore, it is necessary to prevent FeMnSiCrNi SMA from being subjected to hot workability in the case of too high temperatures or too low strain rates.

The Effect of Stress on Bainite Transformation, Microstructure, and Properties of a Low‐Carbon Bainitic Steel

The effects of applying external stress on bainite transformation, microstructure, and mechanical properties are investigated by metallography, dilatometry, etc. Results show that compressive stress during bainite transformation accelerates the bainite transformation and increases the final bainite amount due to the additional mechanical driving force. The lath‐like bainite becomes shorter, and smaller blocky martensite/austenite islands form under stress during the bainite reaction. Different from the results with stress during bainite transformation, applying stress after bainite transformation during isothermal holding cannot increase the volume fraction of bainite, and no obvious change in bainite morphology is observed. In addition, the subsequent martensite transformation is promoted during the cooling process. Moreover, the less amount of retained austenite (RA) but more carbon content in RA is obtained with applying stress after the bainite reaction. Finally, the tensile strength increases, while the elongation decreases when the external stress is imposed after bainite transformation.

Significance of epitaxial ferrite formation on phase transformation kinetics in quenching and partitioning steels: modeling and experiment

Experimental and simulation methods were applied to analyze the transformation kinetics of epitaxial ferrite (EF) formed during a continuous cooling process and relevant carbon content heterogeneity in a low-carbon quenching and partitioning steel, with an emphasis on the influence of austenite carbon heterogeneity issued from EF formation on subsequent martensite and bainite transformation behaviors. It revealed that EF transformation possessed a kinetic curve with sigmoid shape and accelerated with decreasing cooling rate. With EF/γ interface under negligible partition local equilibrium condition in Dictra simulation, the simulation EF transformation kinetics can reproduce the experimental results well and the partial inheritance of Mn and Si from austenite into EF was also predicted. Furthermore, martensite transformation behaviors (one-stage or two-stage transformation) significantly depended on cooling rate and were explained by the austenite carbon heterogeneity issued from EF formation. The increase in initial martensite fraction with elevating cooling rate accelerated the subsequent bainite transformation and was favorable to austenite retention as well.

Interaction of martensite and bainite transformations and its dependence on quenching temperature in intercritical quenching and partitioning steels

A series of quenching temperatures (QT) were applied to investigate the interaction of bainite and martensite transformations in intercritical quenching and partitioning steels with multiple-stage martensite transformation. It reveals that the fractions of initial martensite, bainite, whole retained austenite and austenite respectively retained by bainite or martensite showed obvious three-stage variations, coincided with first stage, stagnant stage and second stage of martensite transformation. In the case of high QT that near Ms., the retained austenite was mainly ascribed to bainite carbon enrichment. As quenched to relatively low QT in stage-1 region or lower, the austenite retained by martensite carbon partitioning occupied the majority and the decrease of RA was mainly ascribed to inter-lath austenite and packet boundary austenite. In addition, an identical partitioning end was achieved with mean austenite carbon contents between T0 line and paraequilibrium condition, irrespective of initial martensite and bainite fractions. The heterogeneous carbon distribution in parent austenite deriving from ferrite formation significantly influenced the subsequent martensite transformation and corresponding martensite structure. The martensite block in high carbon region possessed much smaller size, while the corresponding packet size was still larger. Moreover, the significant martensite variant absence and variant combination were also observed.

Microstructural evolution of dissimilar welded joints between reduced-activation ferritic-martensitic steel and 316L stainless steel during the post weld heat treatment

In the present study, electron beam welding (EBW) was used to join reduced-activation ferritic-martensitic (RAFM) steel to 316L stainless steel. Microstructural evolution and mechanical property variation in the heat-affected zone (HAZ) and weld metal (WM) during the post weld heat treatment (PWHT) was investigated. The results suggested that the hardness of RAFM-HAZ decreased with increased PWHT temperature, while the hardness of WM decreased to minimum at 620 ℃ and then increased at higher PWHT temperatures. This was consistent with strength variation in the WM during PWHTs. Microstructural observations and corresponding phase diagram calculation indicated that blending of two types of base metals increased nickel content of WM, enlarged the two phase-region and lowered A1 (start of transformation of ferrite to austenite) point of WM, as compared with similar ferritic-martensitic steel weld joint. Thus, reversed austenite transformation and subsequent martensite transformation occurred in WM of sample subjected to higher temperature heat treatment, while there was no transformation in the HAZ. This effect explained the changes in hardness and strength of WM corresponding to PWHT. The optimized properties of both HAZ and WM in the dissimilar weld joint were obtained by a two-step post weld heat treatment.

Control of phase transformations and microstructure for optimum realization of one-way and two-way shape memory effects in removable surgical clips

The functioning of a newly developed “smart” clip for permanent or temporary clipping of the blood vessels is based on one-way and two-way shape memory effects (SME and TWSME, respectively) of Ti-Ni alloy. The clip remains in the ready-to-close position before it is locally heated up to 40–45°C. Once heated by a specially developed clip-holder device, it closes due to realization of the SME, thus softly clipping the vessel. Upon local cooling, the clip opens due to the TWSME realization and can be easily removed without trauma to the vessel. To solve the problem of realization of SME and TWSME in the required temperature ranges and with the best functional characteristics, a thermomechanical treatment of Ti-50.7at.%Ni alloy for controlled precipitation of Ti3Ni4 phase particles and subsequent martensitic transformations was developed. A special regime of the clip thermomechanical treatment, including hot rolling and post-deformation isothermal annealing (aging), SME training procedures, and special design of the clip, was developed to obtain the best combination of the recovery strain and recovery stress of the clip. The phase transformations and microstructure were studied using differential scanning calorimetry, X-ray diffractometry, and transmission electron microscopy. The functional properties of the clips were determined using a bending method for inducing strain. The bench tests of the clip confirmed the effectiveness of the optimized thermomechanical treatment and training regimes as well as the cross-shaped clip design.

Fine structure and mechanical properties of the shape-memory Ni50Ti32Hf18 alloy rapidly quenched by spinning

We have reported the results of investigations of the structure and chemical and phase compositions of the amorphous Ni50Ti32Hf18 alloy prepared by rapid quenching from melt by spinning and subjected to heat treatments. The specific features of the fine polycrystalline alloy structure formation depending on the heat-treatment mode have been studied by transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction analysis. According to the data on the temperature behavior of electrical resistivity, critical temperatures of devitrification and subsequent thermoelastic martensitic transformation B2 → B19′ have been determined. The mechanical properties in different heat-treatment modes have been investigated.

Investigation on bainite transformation and metastable omega/orthorhombic martensite phases in as-quenched Cu-bearing ferrite steel

The as-quenched microstructures of low-carbon Cu-bearing ferrite steel are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the granular bainite formation in Cu-bearing ferrite steel is attributed to a mixed mode of diffusional and displacive mechanisms. Besides, the particle-like or plate-shaped ω phases, along with thin-plate martensites consist of twin, are observed in the Mn1.4Cu-bearing steel, and the orientation relationship is determined as: [2 −1 −10]ω // [0 1 1]α and (0 1 −1 0)ω // (2 −1 1)α. The ultra-fine or ellipsoidal shaped nanoscale orthorhombic martensite phases, along with laths martensites consist of a high density of dislocations, are observed in the Mn0.66Cu-bearing steel, and the orientation relationship is determined as: [0 1 −1]o//[−1 1 1]α and (−1 1 1)o//(1 0 1) α. A model is proposed to describe the overall martensite transformation kinetics by taking into account the Mn content, heterogeneity of C content and dislocation reactions play a major role in the subsequent martensite transformation.

Low temperature magnetic properties of a Ni50Mn34In16 ball-milled metamagnetic shape memory alloy

The effect of the atomic disorder induced by ball-milling on the structure and the magnetic properties has been analyzed in a Ni50Mn34In16 metamagnetic shape memory alloy. The as-milled samples displayed an amorphous structure which crystallizes to a disordered B2 structure on annealing. On further annealing, several recovery processes leading to the austenitic L21 structure and the subsequent martensitic transformation are observed. As a result of the recovery processes, the magnetic order drastically varies concurrent with the long-range atomic order. In particular, the magnetism evolves from a frustrated magnetic state compatible with a canonical spin-glass, observed in the amorphous structure, to the well-developed ferromagnetic state.

Effect of Annealing in Magnetic Field on Ferromagnetic Nanoparticle Formation in Cu-Al-Mn Alloy with Induced Martensite Transformation.

The paper considers the influence of aging of high-temperature phase on subsequent martensitic transformation in Cu-Al-Mn alloy. The morphology of behavior of martensitic transformation as a result of alloy aging under annealing in a constant magnetic field with different sample orientation relatively to the field direction and without field was studied for direct control of the processes of martensite induction at cooling. Temperature dependences of electrical resistance, magnetic susceptibility, and magnetization, as well as field dependences of magnetization, and phase composition were found. The tendency to the oriented growth of precipitated ferromagnetic phase nanoparticles in a direction of applied field and to an increase of their volume fraction under thermal magnetic treatment of material that favors a reversibility of induced martensitic transformation is observed.

Microstructural evolution in a Ti – Ta high-temperature shape memory alloy during creep

Abstract Alloys based on the titanium–tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti70Ta30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol−1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations.

On the Stress Development in SA508 Autogenous Weld

Considering the significant role that residual stresses play in determining the lifetime-service of materials, it is mandatory to have a good understanding of and a means of predicting those that develop during welding processes. For this purpose, a User MATerial subroutine (UMAT) is developed to study the effects of various parameters that influence solid state phase transformations and residual stress evolution during welding of SA508 ferritic steel. The temperature dependent elastic and kinematic hardening parameters for each of the individual phases that can potentially develop during cooling from elevated temperatures are measured and are used for calculating stress development during low (75 mm/min) and high (300 mm/min) speed gas-tungsten arc welding (GTAW) on SA508 grade 3. These two speeds are selected to cover a wide range of cooling rates in the heat affected zone so that different phase proportions would be present. The results of the numerical simulations for residual stresses are compared against those measured by neutron diffraction. It is shown here that a low speed weld results in bainite formation whereas a high speed weld results in bainitic as well as subsequent martensitic phase transformations where each welding rate results in different residual stress development.

PHASE FIELD SIMULATION OF AUTO-CATALYTIC GROWTH EFFECT OF COHERENT Ni4Ti3 PRECIPITATE IN Ni 4 Ti 3 SHAPE MEMORY ALLOY

The precipitation behavior and distribution of Ni4Ti3 particles in NiTi alloys have a significant influence on the subsequent martensitic transformation, which may consequently affect shape memory effect and superelasticity of NiTi alloys. The latest experimental studies confirmed that in single crystal NiTi alloy, the Ni4Ti3 particles nucleate and grow as an autocatalytic way leading to a step–like configuration. The autocatalytic nucleation known as collective manner has been widely studied in martensitic transformation while causing relatively less attention in diffusion transformation such as precipitation. Previous studies have been launched to investigate the nucleation and orientation issues of Ni4Ti3 precipitate mainly by analyzing the stress or concentration field around the Ni4Ti3 particle. However, the coupling between stress and concentration field is necessary in * 50871039, 10151064101000017, 20110490881 ! 2012ZZ0014 : 2012–05–10, ! : 2012–07–22 $%! : , , 1981 , # &, DOI: 10.3724/SP.J.1037.2012.00264