Stress-assisted Martensitic Transformation Research Articles

Pivotal role of retained austenite as a low temperature creep controlling mechanism in a martensitic spring steel

The rising demand for electric vehicles urges automotive suppliers to push their limits on the sag resistance of suspension coil springs. However, the mechanism contributing to sag loss in a coil spring or low temperature creep (LTC) in a spring steel is still unclear. Furthermore, the LTC rate-controlling mechanism remains elusive. In the current study, an attempt has been made to unfold the LTC controlling mechanism in a SAE 9254 steel grade. A combined analysis by means of (i) a mechanism-based exhaustion creep model (ECM) and (ii) advanced microstructural characterization prior to and post LTC suggests that dislocation glide, mainly localized in the metastable γ phase, is considered as one LTC contributing mechanism in martempered SAE 9254. Furthermore, stress-assisted martensitic transformation (SAMT) is considered as additional LTC contributing mechanism in SAE 9254 up to the temperature T≤323K. Beyond that temperature strain-induced martensitic transformation (SIMT) takes the place of SAMT. Our approach includes that the LTC rate-controlling mechanisms are stress-assisted recovery (SAR), SAMT, and SIMT especially at elevated temperature T.

Ultrafine-grained CrMnNi steels: Lueders phenomenon and texture inheritance

The mechanical behaviour as well as the microstructure and texture development were investigated for ultrafine-grained austenitic TRIP steels with two different austenite stabilities. The ultrafine-grained microstructure of low-carbon, high-alloy CrMnNi steels was obtained after thermo-mechanical processing (TMCP) consisting of cold rolling process up to 90% of rolling degree and subsequent reversion annealing treatment at different temperatures and holding times. The mechanical properties were evaluated during tensile tests at room temperature complemented by infrared thermography, digital image correlation, ferromagnetic phase fraction measurements as well as nanoindentation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed before and after tensile tests. In addition, the texture evolution was followed from cold rolling via reversion annealing up to tensile deformation. The results showed that an ultrafine-grained austenitic microstructure with a mean grain size between 0.7 and 1 μm was realized due to the applied TMCP. The ufg microstructure led to a significant increase in yield strength to more than 900 MPa for both steel variants. In addition, a Lueders band phenomenon was observed for steel with the lowest austenite stability, which was accompanied by dislocation-lack yield point and stress-assisted martensitic phase transformation at about 400 MPa. Texture inheritance from fcc cold rolling texture via the bcc cold rolling texture into fcc reversion annealing texture and again bcc texture after tensile deformation was observed. This texture inheritance is an indicator for the diffusionless shear transformation in this type of steels during reversion annealing.

Strain-Rate Dependence of the Martensitic Transformation Behavior in a 10 Pct Ni Multi-phase Steel Under Compression

The deformation-induced transformation of metastable austenite to martensite can contribute to improved performance of many steel alloys in a range of applications. For example, one class of Ni-containing steels that has undergone consecutive heat treatments of quenching (Q), lamellarization (L), and tempering (T) exhibits improved ballistic resistance and low-temperature impact toughness. To better understand the origin of this improvement, we tracked the volume fraction of austenite present in a QLT 10 wt pct Ni steel during compression at low and high strain rates ( $$\dot{\varepsilon }={0.001}\,{{\text{s}}^{-1}}$$ and $$\dot{\varepsilon }\simeq {2500}\,{{\text{s}}^{-1}}$$ , respectively) using ex situ vibrating sample magnetometry measurements and in situ time-resolved X-ray diffraction measurements. We observe that the austenite-to-martensite transformation occurs more readily during quasi-static loading than during dynamic loading, even at small values of applied strain, which is qualitatively different from the behavior of steels known to undergo a strain-induced martensitic transformation mechanism. We propose that the strain-rate dependence of transformation in the QLT 10 pct Ni steel is dominated by the transformation in small austenite particles, where stress-assisted martensitic transformation is likely to be the dominant mechanism. Indirect evidence for this hypothesis is provided by electron backscatter diffraction measurements of deformed specimens.

Deformation Behavior in Medium Mn Steel of Nanometer-Sized α′ + γ Lamellar Structure

Yielding and work-hardening phenomena in an Fe-10.62Mn-2.84Al-0.17C-0.5Mo steel, which is composed of nanometer-sized lamellae of α′ and γ, are described on the basis of the Hall–Petch relations. Unlike the general expectation, yielding in the steel, which consists of lamellae of α′ and mechanically stable γ, occurs through the propagation of pileup dislocations from α′ to γ. However, when γ is mechanically unstable, yielding occurs through the stress-assisted martensitic transformation (SAMT) within the unstable γ region, resulting in a low YS of about 500 MPa. The overall prominent work-hardening behavior of this steel after yielding is due to the active SAMT, which does not accompany the increase in mobile dislocation density and so causes the high elastic strain rate. The carbon partitioning treatment increases the SAMT starting strength to about 980 MPa, which is caused by the mechanical stabilization of γ. The overall low work-hardening behavior of this case is mainly attributed to the active propagation of pile-up dislocation from α′ to γ which causes the high plastic strain rate through the abrupt increase of mobile dislocation density.

Influence of Mg-PSZ Particle Size on the Fatigue Behaviour of a High Alloy Steel Matrix Composite

Metal Matrix Composites (MMC) based on a TRIP (TRansformation Induced Plasticity)- or TWIP (TWinning Induced Plasticity)-steel matrix reinforced with MgO-partially stabilized zirconia (Mg-PSZ) are an interesting research field as both components exhibit a deformation-induced or stress-assisted martensitic phase transformation and twinning, respectively. The present work deals with the fatigue characteristics of a reinforced CrMnNi-steel as a function of the ceramic particle size. Therefore, the particles were classified into three grades (grade 1: <10 μm; grade 2: 10-30 μm; grade 3: 30-50 μm) whereas the volume fraction concerning the composite material was kept constant at 10 vol.%. The composites were produced using the hot pressing technique. The tests were performed under total strain control in a range of 0.2% ≤ Δεt≤ 1.2%. The microstructure of fatigued specimens was examined using scanning electron microscopy.

The martensitic transformation of Ti–Ni shape memory thin films under proton irradiation

The martensitic transformation behavior of a Ti–Ni alloy irradiated by proton with different doses has been investigated. It is found that the samples irradiated by 150keV protons have a two-step phase transformation during heating and only one-step transformation during cooling. The exothermic peak at higher temperature disappears during the following thermal cycling. A model based on the stress-assisted martensitic transformation was established by the Transport of Ions in Matter (TRIM) calculations in order to explain the transformation behavior.

Orientation Dependence of the Martensitic Transformation in (Zr,Mg)O2 Embedded in a TRIP Steel Matrix

Metal matrix composites (MMCs) consisting of austenitic TRIP (TRansformation Induced Plasticity) steel reinforced by Mg-stabilized zirconia (Zr,Mg)O2 show martensitic phase transformations in both constituents during compressive deformation. The influence of the crystallographic orientation of individual (Zr,Mg)O2 particles on the capability of their stress-assisted martensitic transformation from tetragonal to monoclinic phase with respect to the external load was elucidated. The corresponding amount of the resulting axial compressive strain was calculated for all possible crystallographic orientations of the tetragonal phase, and correlated to the evolving fractions of m-(Zr,Mg)O2. In order to obtain the local crystallographic orientation and to identify the respective zirconia phase, the as-sintered material and a 25% compressed sample were examined by electron back-scatter diffraction (EBSD). After compressive deformation, up to 80% of the individual particle volume exhibited band-like structures containing m-(Zr,Mg)O2. The progress of the martensitic phase transformation was found to depend on the local crystallographic orientation of t-(Zr,Mg)O2 with respect to the load direction, although this effect is strongly influenced by the morphology of individual particles.

Stress-assisted martensitic transformations in steels: A 3-D phase-field study

A 3-D elastoplastic phase-field model is developed for modeling, using the finite-element method, the stress-assisted martensitic transformation by considering plastic deformation as well as the anisotropic elastic properties of steels. Phase-field simulations in 3-D are performed by considering different loading conditions on a single crystal of austenite in order to observe the microstructure evolution. The thermodynamic parameters corresponding to an Fe–0.3% C steel as well as the physical parameters corresponding to commercial steels, acquired from experimental results, are used as input data for the simulations. The simulation results clearly show the well-known Magee effect and the Greenwood–Johnson effect. The results also show that even though the applied stresses are below the yield limit of the material, plastic deformation initiates due to the martensitic transformation, i.e. the well-known transformation-induced plasticity (TRIP) phenomenon. It is concluded that the loading conditions, TRIP as well as autocatalysis play a major role in the stress-assisted martensitic microstructure evolution.

<i>In Situ</i> Tensile Deformation of TRIP Steel / Mg-PSZ Composites

Composite material on the basis of a TRIP (transformation induced plasticity) steel with zirconia particles as reinforcement was produced by powder metallurgical technology and conventional sinter process. The goal of such type of material is to obtain exceptional mechanical properties like high deformation energy absorption due to the combination of martensitic phase transformations both in steel and ceramic. The steel matrix was made of the commercial steel AISI 304, which shows a deformation-induced martensitic phase transformation from the austenitic phase (fcc) into the α’-martensite (bcc). The zirconia particles were partially stabilized with MgO and show a stress-assisted martensitic phase transformation from the tetragonal to the monocline phase. Flat specimens were tensile deformed in-situ in a scanning electron microscope in order to follow the damage behaviour of the material. Some zirconia particles were characterized before and after tensile testing both by backscattered electron contrast as well as by electron backscatter diffraction (EBSD) in combination with energy dispersive X-ray spectroscopy (EDS).

A method of discrimination between stress-assisted and strain-induced martensitic transformation using atomic force microscopy

A method of discrimination between stress-assisted and strain-induced martensitic transformation (MT) using atomic force microscopy is established. The shear angles of the three kinds of face-centered cubic to body-centered tetragonal MT in an Fe–0.1C–1.7Mn–1.2Si transformation-induced plasticity steel are calculated based on our developed method. Comparing the theoretical shear angle with the practical shear angle, the features of three kinds of MT, including stress-assisted, strain-induced and thermal-induced MTs, are revealed.

Internal friction of B2 → B19′ martensitic transformation of Ti 50Ni 50 shape memory alloy under isothermal conditions

The inherent internal friction (IF PT + IF I) B2→B19′ of cold-rolled and annealed Ti 50Ni 50 alloy that exhibits B2 → B19′ martensitic transformation is studied under isothermal conditions. The tan δ value of (IF PT + IF I) B2→B19′ is proportional to σ 0/ ν 1/2 and its damping mechanism is thus related to stress-assisted martensitic transformation and stress-assisted motion of twin boundaries. The tan δ value of (IF PT + IF I) B2→B19′ is smaller than those of (IF PT + IF I) B2→R and (IF PT + IF I) R→B19′ because the former shows no R-phase. The obliterated dislocations and defects after sufficient annealing can lower the tan δ value of the relaxation peak at about −60 °C.

Inherent internal friction of B2→R and R→B19′ martensitic transformations in equiatomic TiNi shape memory alloy

The inherent internal frictions IF PT B 2 → R + IF I and IF PT R → B 19 ′ + IF I of Ti50Ni50 alloy are studied under isothermal conditions. The tan δ values of IF PT B 2 → R + IF I and IF PT R → B 19 ′ + IF I are both proportional to σ0/υ1/2 and thus the damping mechanism of IF PT B 2 → R + IF I and IF PT R → B 19 ′ + IF I is related to the stress-assisted martensitic transformation and stress-assisted motions of twin boundary. The tan δ value of IF PT R → B 19 ′ + IF I is larger than that of IF PT B 2 → R + IF I because of the larger transformation strain and the greater twin boundaries associated with the R → B19′ transformation.

Effect of martensitic transformation in Ti–15 at %V -phase particles on lamellar boundary decohesion in -TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V β-phase particles dispersed in γ-TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the β-phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the β–γ two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The β-phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable β-phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the β–γ two-phase material. © 1998 Kluwer Academic Publishers

Dispersed-phase martensitic transformation controlled deformation behavior of two-phase metallic materials

A constitutive model which describes transformation plasticity accompanying stress-assisted martensitic transformation in two-phase material systems consisting of a stable matrix and a transforming dispersed phase is developed. The model consists of two parts: (a) a transformation thermodynamics/kinetics law describing the evolution of the transformed fraction of dispersed particles and (b) a constitutive law describing plastic flow resistance of the evolving three-phase (matrix phase, dispersed parent phase, dispersed product phase) composite material. The model is constructed in such a way that it can be readily implemented in a finite element program suitable for the analysis of boundary value problems. The model is used to analyze the uniaxial tensile behavior of the two-phase gamma Ti-A1 matrix/beta (Ti-A1-V-Fe) phase system in order to rationalize an experimentally observed nearly 100% increase in tensile ductility of the material due to the martensitic transformation in the beta phase. The model is also used to study the uniaxial tensile behavior of beta/alpha two phase Ti-10V-2Fe-3A1 (wt.%) alloy characterized by a sigmoidal stress-strain curve. A reasonably good agreement is obtained between the model predictions and the experimental data.

Transformation Toughening and Fracture Behavior of Molybdenum Disilicide Composites Reinforced with Partially Stabilized Zirconia

The effects of yttria stabilization (0–6 mol%) on the fracture toughness of molybdenum disilicide composites reinforced with 20‐vol%‐yttria‐stabilized zirconia particles are elucidated. Fracture toughness tests were conducted under three‐point bend loading using single‐edge‐notched specimens. The stress‐assisted martensitic phase transformation of zirconia associated with the fracture process was then studied using optical interference microscopy and laser Raman spectroscopy. Stress‐induced martensitic transformation from tetragonal to monoclinic phase was observed only in the plastic wake of the material stabilized with 2 mol % yttria. The degree of toughening in this composite was also predicted using micromechanical models that assess the combined effects of transformation toughening and crack deflection. However, the fracture toughness of the 2‐mol%‐yttria‐stabilized composite was in the same range as those of the other composites that did not exhibit evidence of transformation toughening. The toughening in the other composites is explained by considering the effects of crack deflection, residual stresses, and microcracking induced by residual stresses that occur as a result of the thermal expansion mismatch between the matrix and the reinforcements.

A micromechanical study on the coupling effect between microplastic deformation and martensitic transformation

Martensitic transformation is simulated in terms of computational micromechanics applying the finite-element method in order to study the effect of microstress and microstrain causing the unusual material behavior associated with stress-assisted martensitic transformation (MT). Both the preferred orientation of martensitic crystallographic variants (orientation effect) and microplastic deformation (accomodation effect) may lead to significant macroscopic deformations (transformation-induced plasticity, TRIP). The essential microstructural features of MT are described by stress-free transformation tensors ε T,J for martensitic variants emerging along habit plane variants j. Transformation-induced microstress and accomodation strain fields are analysed for inclusion-type, plate-shaped martensitic variants transforming within a representative volume element (RVE) under different levels of externally applied stress. The RVE is specified with respect to an average Schmid-factor for the transformation shear γ. The second homogenization step is carried out by a microfield approach coupling the RVEs with antisymmetric boundary conditions. In order to describe the thermodynamic state of the stressed phases we persue a thermodynamic field concept by introducing a thermomechanical potential applicable for martensitic transformation with plasticity effects. Transformation criteria for stress-assisted nucleation and stress-driven growth enable to clarify the role of plastic deformation arising from the progress of transformation. It will be shown that the hydrostatic microstress fields developing at the plate ends of transforming martensitic crystals alter the stress-assisted nucleation characteristics. Microplastic strain stabilizes the austenite thermodynamically. The applied modelling technique also allows to study the coupling between orientation- and accommodation effect. A incremental formulation ε TP for TRIP will be discussed compared to existing relations.

Micromechanical modelling of stress-assisted martensitic transformation

A computational micromechanics study of stress-assisted martensitic transformation using finite elements is carried out within a thermomechanical framework including the aspect of plastic deformation. The phase transformation is treated by a stress-free transformation tensor corresponding to a certain habit plane variant involving a shape change resulting from shear and dilatational deformations which are eigenstrains within the emerging martensitic phase. General plane strain and axisymmetric analyses are carried out introducing different micromechanical models with appropriate boundary conditions required for the microfield approach. The introduced models are compared with a micromechanical model developed by Tvergaard in the viewpoint of thermodynamic aspects concerning the formation of martensite. The representative volume elements are specified with respect to the Schmid factor for the transformation shear corresponding to an average crystallographic orientation of a grain. We examine elastic-plastic deformations and distribution of transformation-induced microstress fields in both the parent and the emerging product phase due to accommodation of the transformation shape change. Analytical results for the linear elastic case based upon the Eshelby approach will be compared with numerical results. The plastic behavior of austenitic and martensitic microconstituents is described in the context of J2-flow theory with isotropic hardening. The thermomechanical framework is based on an explicit form of a potential suggesting the form of Gibbs free energy, from which transformation conditions on the mesodomain-, interface- and nucleation-site level are derived. The arrangement of martensitic microregions transforming in a stress field is studied considering the thermomechanical coupling of orientation and accommodation effects.

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10−4 s−1 and 10−1 s−1 over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM s σ temperature. At temperatures much greater thanM s σ , austenite twinning will occur, while at temperatures close toM s σ , bcc martensite will form.

Pseudotwinning and pseudoelasticity in β Fe-Be alloys

Pseudoelasticity in β Fe-Be has been observed and rationalized in terms of pseudotwinning. a type of stress-assisted martensitic transformation which arises from the interaction of long-range order and twinning in this alloy. The ordered structure, which gives rise to a chemical contribution for the reversion of the pseudotwin upon release of the stress, is not homogeneous DO 3 as was previously reported, but a decomposed α + B2 microstructure. A crystallographic model predicts that the pseudotwinning process is describable by the stress-assisted martensitic phase transformation Pm3 m ( B2) → Cmmm. The electron diffraction evidence presented here to confirm this prediction is believed to be the first experimental proof of long-standing theories regarding the interaction of order and twinning. The macroscopic mechanical behavior of β Fe-Be has been investigated in compression at room temperature. Maximum pseudoelasticity is observed for specimens water-cooled and aged at 325–425°C. which gives rise to a modulated α + B2 microstructure. Aging at higher temperatures causes a sharp drop-off in pseudoelasticity and the disappearance of the modulated microstructure. In addition, pseudotwins in specimens exhibiting little pseudoelasticity always have accommodation dislocations associated with them. It is concluded that the modulated microstructure promotes pseudoelasticity by strengthening the parent matrix, thereby enabling it to accommodate the pseudotwins in an elastic manner.

On the origin of mechanical twinning—Deformation or transformation?

The hypothesis is presented that mechanical twinning is the result of a stress-assisted martensitic transformation, rather than being a deformation mechanism acting in response to a shear stress. The geometry of the proposed mechanism is shown to be in agreement with the behavior observed in the bcc lattice, and in fair agreement with that reported for the hcp lattice. The hypothesis presented can account for many similarities between mechanical twinning and martensitic transformation, as well as for a number of anomalous observations on various aspects of twinning.