Plastic Deformation Of Martensite Research Articles

Physical mechanisms for dependence of temperature-induced phase transition and shape memory effect on grain size in nanocrystalline NiTi shape memory alloys

Physical mechanisms for dependence of temperature-induced phase transition and shape memory effect (SME) on grain size (GS) of nanocrystalline NiTi shape memory alloys (SMAs) were revealed by combining experiment and molecular dynamics simulation. Influences of GS on phase transition temperature and stress level during tension loading below martensite transition finish temperature Mf are interpreted by core-shell theory that energy of shell (grain boundary (GB)) is higher than that of core (grain interior), so shell becomes an obstacle to deformation of the core and the obstacle increases with reducing GS because the fraction of GB increases with decreasing GS. Due to the constraint of shell, martensite transition during cooling is so incomplete that stress-induced martensite occurs in nanocrystalline NiTi SMAs deformed below Mf. After unloading, some stress-induced martensite phases are retained due to the lack of driving force for reverse martensite transition at low temperature. This is why SME occurs in the alloy with GS below 10 nm when no temperature-induced phase transition happens. Under the same tension strain, inelastic strain after unloading increases with increasing GS. The increase of GS causes the increase of SME strain, and the residual strain after heating the deformed nanocrystalline NiTi SMAs stems from the plastic deformation of martensite and austenite in grain interior during tension.

Effect of Austenitizing Temperature on the Work Hardening Behavior of Air-Hardening Steel LH800

In this paper, we present the effect of austenitizing temperature on the work hardening behavior of air-hardening steel LH800 by evaluating the influence of austenitizing temperature on microstructure evolution and mechanical properties, using Hollomon, Differential Crussard–Jaoul (DC-J), and Modified C-J (MC-J) work hardening models. The results reveal that with an increase in austenitizing temperature, there is an increase in the percentage of martensite, along with an increase in the strength and hardness of the LH800 steel; on the other hand, there is a decrease in the plasticity. Austenitized at 825 °C, LH800 steel exhibits its highest strength and good plasticity, with a tensile strength of 897 MPa and an elongation of 13.6%. The comparison between the three strain hardening models revealed that the Hollomon model was the finest fit for the experimental data utilized and could illustrate the work hardening behavior of LH800 steel most suitably. This model manifests a two-stage work hardening mechanism; the first stage is related to the plastic deformation of ferrite phase, while the second stage deals with the co-deformation of ferrite and martensite/bainite phase. As austenitizing temperature increases, the work hardening ability of LH800 steel diminishes at each stage, the transition strain decreases, and the plastic deformation of martensite starts earlier.

In-situ synchrotron X-ray diffraction texture analysis of tensile deformation of nanocrystalline NiTi wire in martensite state

Tensile deformation of nanocrystalline superelastic NiTi wire in the martensite state at -90 °C until fracture was investigated by in-situ synchrotron X-ray diffraction texture analysis. The results were interpreted in terms of the activity of various transformation and deformation twinning modes. The strain components into the load axis as well as discontinuous rotations of the martensite crystal lattice due to activation of various twinning modes were calculated using an abstraction of “ideal <hkl> fibre textured polycrystal” and used to estimate the evolution of the martensite texture during the tensile test.It is found that the four fibre texture of selfaccommodated martensite transforms during the martensite reorientation stage in the tensile test into the sharp two fibre texture of the reoriented martensite reflecting its (001) compound twinned microstructure. Plastic deformation of martensite is claimed to proceed via peculiar deformation mechanism involving deformation twinning and kinking assisted by [100]/(011) dislocation slip. This deformation mechanism enables the nanocrystalline NiTi wire to deform at ∼1 GPa engineering stress up to very large plastic strains ∼50% with small strain hardening and brings about refinement of the austenitic microstructure of the NiTi wire down to nanoscale.

3D Reconstruction of Martensitic Microstructures in Grains of Deformed Nanocrystalline NiTi Wires by TEM

A novel experimental method allowing for 3D 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. It is found that martensitic microstructures in grains of nanocrystalline NiTi wire evolving during tensile thermomechanical loading tests (involve martensite reorientation, stress induced martensitic transformation and plastic deformation of martensite) are rather different than currently assumed in the SMA field. 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. This was rationalized by the theoretical treatment of strain accommodation in grains of the fiber textured NiTi wire deformed in tension predicting such singular microstructural state for NiTi wire deformed up to 6.74% strain. Upon reverse martensitic subsequent stress free heating above the Af temperature, this martensitic microstructure retransforms back to the parent austenite yielding recoverable strains typical for NiTi (~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 starts by coordinated (100) and (201‾) deformation 1 twinning in martensite assisted by [1 0 0](0 0 1) dislocation slip giving rise to characteristic wedge type martensitic microstructure. Upon subsequent stress free heating above the Af temperature, this martensitic microstructure transforms to twinned austenitic microstructure yielding large recoverable strains (~10%) accompanied by large unrecovered strains (~5%). 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.

TEM Study of Martensite Microstructure in the Junctions of Laths in Low-Carbon Chromium Steel after LCF III

TEM study of junctions between martensite packets (laths) and their microstructure in low-carbon martensitic steel were studied. It was revealed that formation of slip bands commences at junctions between laths. Heavy changes in microstructure occur at junctions of packets and in the near-boundary laths because of plastic deformation of martensite. A number of typical junctions between the laths after LCF are considered. Deformation process within the individual packet occurs inhomogeneously, some of the laths deform more heavily than the others. The coarser slip bands group within large laths and run along their whole lengths. It was shown that the above microstructure changes strongly affect the cyclic fracture of the steel. Some concrete sites of microcrack initiation are indicated.

Mechanical behavior of DP780 dual phase steel at a wide range of strain rates

The mechanical behavior of DP780 cold-rolled dual phase steel was studied by means of high-speed tensile testing machine. The constitutive relation and strain hardening behavior was analyzed by modified Johnson-Cook (JC) model and modified Crussard-Jaoul (C-J) method, respectively. Results indicated that strain rates have significant effect on mechanical properties and plastic deformation of dual phase steel. Moreover, the modified JC model can well describe the experimental results. The plastic deformation of DP780 can be divided into two stages: the first low strain stage with stronger strain hardening capacity and the second high strain stage with weaker strain hardening capacity. Strain hardening behavior was influenced by strain rate, dislocations, plastic deformation of martensite and thermal softening; the strain hardening rate of DP780 steel was not sensitive to the strain rates. Micro-voids in the tensile samples were formed at ferrite/martensite interface or formed by the fracture of martensite.

Superelastic behavior of nanostructured Ti 50 Ni 48 Co 2 shape memory alloy with cold rolling processing

Effects of cold rolling followed by annealing on microstructural evolution and superelastic properties of the Ti50Ni48Co2 shape memory alloy were investigated. Results showed that during cold rolling, the alloy microstructure evolved through six basic stages including stress-induced martensite transformation and plastic deformation of martensite, deformation twinning, accumulation of dislocations along twin and variant boundaries in martensite, nanocrystallization, amorphization and reverse transformation of martensite to austenite. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. The value of stress level of the upper plateau in this nanocrystalline alloy was measured as high as 730 MPa which was significantly higher than that of the coarse-grained Ni50Ti50 and Ti50Ni48Co2 alloys. Moreover, the nanocrystalline Ti50Ni48Co2 alloy had a high damping capacity and considerable efficiency for energy storage.

Metastable austenite driven work-hardening behaviour in a TRIP-assisted dual phase steel

The mechanically-induced transformation behaviour of the metastable austenite phase in a high-strength industrial TRIP-assisted Dual Phase steel was monitored in situ using high-energy synchrotron diffraction under uniaxial loading. This allowed direct quantification of the impact of the transformation of the metastable austenite phase (16 vol %), embedded in a ferrite-bainite-martensite matrix, on the work hardening behaviour of this steel. Our results show that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding. We provide experimental evidence which demonstrates for the first time that the austenite transformation increases the work-hardening contribution, σw thereby supporting a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding and continues to offset the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Further we show conclusively that martensite yielding does not occur until the completion of the mechanically induced transformation of austenite. Plastic deformation of martensite is immediately followed by local plastic instability leading to necking and ultimate failure of this material.

Effect of temperature on the wear behavior of NiTi shape memory alloy

Abstract

The effect of thermomechanical processing on the microstructure and mechanical properties of the nanocrystalline TiNiCo shape memory alloy

The effect of thermomechanical processing comprising cold rolling followed by annealing on the microstructural evolution and mechanical behavior of the Ti50Ni48Co2 shape memory alloy was investigated. The annealed specimens were subjected to cold rolling at room temperature with various thickness reductions up to 70%. Transmission electron microscopy revealed that the initial deformation mechanism of Ti50Ni48Co2 alloy during cold rolling was stress-induced martensitic transformation followed by plastic deformation of martensite via dislocation slip and subsequent martensite to austenite transformation via the reverse transformation after unloading. Microstructural investigations showed that by increasing the cold deformation, a high density of dislocations is accumulated, leading gradually to nanocrystallization and amorphization. After annealing at 400°C for 1h, the amorphous phase formed in the cold rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. Results showed that the stress–strain curves of the cold rolled specimens exhibited plastic deformation of austenite without the stress plateau region. However, the stress plateau appeared in the stress–strain curves of the annealed specimens, whose stress level and length were increased with increasing thickness reduction.

Effect of hardness of martensite and ferrite on void formation in dual phase steel

The influence of the hardness of martensite and ferrite phases in dual phase steel on void formation has been investigated by in situ tensile loading in a scanning electron microscope. Microstructural observations have shown that most voids form in martensite by evolving four steps: plastic deformation of martensite, crack initiation at the martensite/ferrite interface, crack propagation leading to fracture of martensite particles and void formation by separation of particle fragments. It has been identified that the hardness effect is associated with the following aspects: strain partitioning between martensite and ferrite, strain localisation and critical strain required for void formation. Reducing the hardness difference between martensite and ferrite phases by tempering has been shown to be an effective approach to retard the void formation in martensite and thereby is expected to improve the formability.

Mechanical properties of fine-grained dual phase low-carbon steels based on dynamic transformation

The fine grained dual phase (FG-DP) steel with ferrite grains of 2-4.5 μm and martensite islands smaller than 3 μm was obtained through the mechanism of deformation-enhanced ferrite transformation (DEFT). Mechanical properties of the steel were tested at room temperature. The results indicated that with a similar volume fraction of martensite (about 20vol%), FG-DP steel exhibited a superior combination of higher strength and more rapid strain hardening at low strains compared with the coarse-grained dual phase (CG-DP) steel obtained by critical annealing. The combination of higher strength, large elongation, and more rapid strain hardening of FG-DP steel can be attributed to the fine ferrite grain and finely dispersed martensite islands. In addition, the uniformly distributed martensite islands in FG-DP steel have smaller interspacing compared with that of CG-DP steel. So, at the initial plastic deformation stage, the plastic deformation of ferrite was restrained and more pronounced load was transferred from ferrite to martensite. The plastic deformation of martensite in FG-DP steel started earlier.

Micromechanical modelling of the effect of plastic deformation on the mechanical behaviour in pseudoelastic shape memory alloys

Except for the recoverable strain induced by phase transformation, NiTi alloys are very ductile even in the martensite phase. The purpose of the present paper is to study the influence of permanent deformation, which results from plastic deformation of martensite, on the mechanical behaviour of pseudoelastic NiTi alloys. Based on phenomenological theory of martensitic transformation and crystal plasticity, a new three dimensional micromechanical model is proposed by coupling both the slip and twinning deformation mechanisms. The present model is implemented as User MATerial subroutine (UMAT) into ABAQUS/Standard to study the influences of plastic deformation on the stress and strain fields, and on the evolution of martensite transformation. Results show that with the increasing of plastic deformation the residual strain increases and the phase transformation stress–strain curves from the martensite to austenite become steeper and less obvious. Both characteristics, stabilisation of martensite and impedance of the reverse transformation, due to plastic deformation are captured.

Deformation Behaviour of Martensite in a Low-Carbon Dual-Phase Steel

The deformation behaviour of martensite and its effect on tensile properties of a lowcarbon dual-phase (DP) steel were investigated. DP steel samples with different martensite contents and morphologies were produced after intercritical annealing at different temperatures using low and high heating rates. Two distinct martensite morphologies were obtained for low and high heating rates. The investigated steel showed the unusual results that the true fracture stress and strain were found to increase with the martensite volume fraction. The plastic deformation of martensite was considered to be responsible for these results. Experimentally, it was observed that the martensite in DP steels with greater than 25-30% martensite can deform plastically during tensile straining. Finally, the effect of tempering on the martensite plasticity was also evaluated. It was found that the tempering process and an increase in the martensite content have a similar effect on promoting martensite plasticity.

Constrained phase-transformation of a TiNi shape-memory alloy

The phase-transformation behavior of a TiNi shape-memory alloy (SMA) under constraint of a constant strain is experimentally investigated by means of mechanical testing and differential scanning calorimetry (DSC) measurements. It is indicated that the reverse-transformation-temperature span under constraint is much larger than that of the unconstrained state. After an incomplete constrained transformation cycle, a two-stage recovery-stress in the constrained state as well as a two-stage recovery-strain in the unconstrained state emerges upon subsequent heating. This is rationalized on the basis of a mechanism which takes into account the influence of stress on the formation of the austenite and the plastic deformation of martensite and austenite during constrained heating. Most importantly, the constrained reverse and forward transformations corresponding to the redeformation of the oriented martensite and the formation of the stress-induced martensite and thermal martensite, respectively, lead to the subsequent two-stage recovery-strain and recovery-stress characteristics. Both the prestrain level and the constrained heating temperature play important roles in the phase transformations and thermomechanical characteristics of the TiNi SMA.

On the Influence of Thermomechanical Treatments on Shape Memory Alloys

Thermomechanical treatments (TMT) are used to modify the microstructure of austenite or martensite by introducing lattice defects and/or particles of a second phase. The temperature range extends between T ≥ M d (pure ausforming) and T ≤ M f (pure marforming). In the intermediate range (M f < T < M d ) a more complex behaviour is found. TMT are discussed in a systematic manner using NiTi-based and Cu-based alloys as examples. Transformation behaviour and mechanical properties of TM-treated alloys are compared with undeformed ones. The following effects can be observed for thermal transformation cycles: (a1) lowering or raising of transformation temperatures, (a2) widening or decreasing of hysteresis, (a3) induction of multiple step transformation mechanisms, (a4) reduction of martensitic retransformability (e.g. plastic deformation of martensite = marforming). The mechanical properties are also highly affected: (b1) increase in pseudo yield stress, (b2) increase or decrease in shape memory strain, (b3) increase in conventional strength, (b4) increase in elongation at fracture (by hot rolling of austenite = ausforming). It is shown how tailor-made microstructures are obtained, which in turn provide the best bulk properties for specified applications of SMA in engineering.

Tensile properties and inhomogeneous deformation of ferrite-martensite dual-phase steels

The tensile properties and inhomogeneous deformation of coarse ferrite-martensite dual-phase steels containing 17–50% martensite were analysed. The stress of dual-phase steels at equal strain increased with increasing volume fraction of martensite, f, but the rate of increase was reduced after f=0.3. The strain hardening rate was dependent on f at small strains (ɛ ⩽ 0.03), however, it became independent of f at larger strains. It was found that the deformation of the dual-phase steels divided into three different stages when f was less than about 0.3. The concurrent in situ stress-strain states of ferrite, martensite and their composite, and the stress ratios and strain ratios between ferrite and martensite were evaluated by means of a new stress and strain partition theory. The martensite phase deformed plastically after the uniform strain for f 0.25. The theoretical analyses for inhomogeneous deformation implied that the volume-fraction dependence of the stress and the characteristics of the strain-hardening rate were influenced by the plastic deformation of martensite. Further, the in situ stress-strain curves of ferrite and martensite and the internal stresses at respective phases were calculated from the partitioned stresses and strains.