Role Of Martensitic Transformation Research Articles

Influence of loading direction on tensile deformation behavior of a lean duplex stainless steel sheet: The role of martensitic transformation

In this work, the tensile deformation behavior of a hot-rolled lean duplex stainless steel with metastable austenite (named as present TLDX) was studied at 20 °C along the loading directions of 0°, 45° and 90° to the rolling direction (corresponding to the L-oriented, D-oriented and T-oriented specimens, respectively). The initial microstructure, martensitic transformation characteristics, damage and fracture behavior were clarified through microstructure observations. The intrinsic mechanism of anisotropy in tensile deformation behavior was comprehensively discussed referring to the microstructure characteristics. The present TLDX exhibited remarkable mechanical anisotropy at 20 °C, characterized by the highest values of ultimate tensile strength (σu = 853 MPa) and plasticity (et = 70.6%, eu = 62.3%) for the L-oriented specimen and the lowest values (i.e., σu = 790 MPa, et = 53.3%, eu = 50.4%) for the T-oriented specimen, although the yield strength is comparable (σs∼524 MPa) for different loading directions. Strain-induced martensitic transformation (SIMT) with a sequence of γ→ε→α′ was detected for all specimens. The L-oriented specimen is most favorable for SIMT, followed by the D-oriented specimen, and finally the T-oriented specimen, which, caused by the different strain partitioning response, is the main factor contributing to the mechanical anisotropy at 20 °C. Different damage and fracture characteristics for different loading directions were also identified, which are closely associated with the initial microstructure.

Synergistic Size–Surface Effects on Martensitic Transformation of Shape Memory Alloy Nanorods for Micro/Nanoelectro-Mechanical Systems

Martensitic transformation (MT) of shape memory alloys (SMAs) deserves promising applications in various smart devices including micro/nanoelectro-mechanical systems owing to the material’s large reversible deformation from cyclic MT. However, such systems always suffer from critical fatigue problems under cyclic loadings with microdamage accumulations closely related to hysteresis energy and residual strain. Here, the synergistic effects of the sample size and surface composition on the MT of SMA nanorods are systematically investigated by combining global responses and atomic evolutions based on molecular dynamics. It is found that the surface energy plays a resisting role in MT and the resistance increases with decreasing sample size, which in turn increases the critical stress, hysteresis, and residual strain. Intriguingly, the nucleated austenite–martensite transition zone during MT can be influenced by surface composition, and an ultralow hysteresis and residual strain with little size-dependence can be achieved in the nanorod with the surface composed of alternate Ni and Ti atoms. The low energy density of the transition zone indicates better microscopic lattice compatibility and smaller dissipations during MT. This reveals that the surface modification is promising for modulating the size effect and improving the reversibility and the fatigue resistance of the superelastic SMA nanostructures.

Self-heating of metastable 304L austenitic stainless steel under cyclic loading: Influence of initial martensite volume fraction, testing temperature and pre-strain

In this study, special attention is paid to the thermal response of the austenitic stainless steel 304L under cyclic loading tests (i.e., self-heating tests) in different conditions. The cyclic behavior of the 304L austenitic stainless steel can be influenced by martensitic transformation. Hence, the role of martensitic transformation in the self-heating behavior of this material is studied. For this purpose, self-heating tests are performed on pre-strained specimens at a wide range of temperatures from 100 °C to −130 °C, which leads to different initial volume fractions of martensite ranging from 0% to 45%, respectively. Furthermore, the influence of the testing temperature and a pre-strain at room temperature on the self-heating curves is investigated. The results demonstrate that the decrease in the test temperature from room temperature to - 30 °C as well as the increase in the pre-strain values from 0% to 30% at room temperature, result in a shift of the self-heating curves to the right. It could mean that the fatigue limit of the material is improved.

Role of stacking faults in martensite transformation of FeMnSiCrNi shape memory alloy subjected to plastic deformation at high temperatures

FeMnSiCrNi shape memory alloy (SMA) undergoes plastic deformation at high temperatures, including 850, 900, 950 and 1000 °C. All the deformed FeMnSiCrNi samples consist of γ austenite and ε martensite, where the orientation relationship between γ austenite and ε martensite is [110]γ//[21¯1¯0]ε. Furthermore, ε martensite increases with increasing deformation temperature. Stacking faults play a predominant role in martensite transformation of FeMnSiCrNi SMA. The mechanisms of martensite transformation are revealed based on slip of Shockley partial dislocation as well as formation of intrinsic and extrinsic stacking faults. The critical transformation stress of FeMnSiCrNi SMA is further put forward based on stacking fault energy. Quenching stress plays an important role in martensitic transformation of FeMnSiCrNi SMA.

Local strains in 1.4301 austenitic stainless steel with internal hydrogen

304 austenitic stainless steel was precharged with hydrogen (∼ 0.3 at%) and subsequently slow strain rate tensile tested at room temperature. The motivation of this investigation was to further clarify the nature of crack initiation, and especially the role of martensitic transformation, in metastable austenitic stainless steel under the influence of hydrogen. However, it must be distinguished between martensite formed by pre-straining, i.e. γ-α′-transformation prior to the exposure to hydrogen and γ-α′-transformation during plastic deformation under the influence of hydrogen. In the first scenario martensite contents formed by pre-straining could not be correlated with HE effects in tensile tests. In the second scenario crack initiation could always be attributed to locations with local strain incompatibilities like twin-grain boundary triple junctions, γ-α′ phase boundaries, α′ martensite, δ-ferrite stinger and strain incompatibility sites within γ grains. The results clearly show that γ-α′ transformation only plays a role in two of the five failure modes indicating that γ-α′ transformation is not the sole root cause for the high susceptibility of 304 stainless steel to hydrogen embrittlement.

The Role of Martensitic Transformation in Thermomechanical Development of Microstructure and Plasticity of Two-Phase Ti-6Al-4V Titanium Alloy

Phase constituent morphology in microstructure of two-phase α+β titanium alloys is determined by conditions of thermomechanical processing consisting of sequential heat treatment and plastic deformation operations. Results of previous research indicate that particularly solution treatment preceding plastic deformation significantly changes α-phase morphology and determines hot plasticity of titanium alloys. In the paper thermomechanical processing composed of β solution treatment and following hot forging of Ti-6Al-4V titanium alloy was analysed. Development of martensite plates during heating up and hot deformation was evaluated. Microscopic examinations revealed that elongated and deformed α-phase grains were fragmented and transformed into globular ones. Significant influence of martensitic transformation on elongation coefficient of α-phase grains after plastic deformation was confirmed. Based on results of elevated temperature tensile tests it was established that α-phase morphology in examined two-phase α+β titanium alloy, developed in the thermomechanical processing, can enhance their hot plasticity – especially in the range of low strain rates.

The role of martensitic transformation on bimodal grain structure in ultrafine grained AISI 304L stainless steel

In the present study, metastable AISI 304L austenitic stainless steel samples were subjected to different cold rolling reductions from 70% to 93%, followed by annealing at 700°C for 300min to form ultrafine grained (UFG) austenite with different grain structures. Transmission electron microscopy (TEM) and nanoindentation were used to characterize the martensitic transformation, in order to relate it to the bimodal distribution of the austenite grain size after subsequent annealing. The results showed that the martensite morphology changed from lath type in the 60% rolled sample to a mixture of lath and dislocation-cell types in the higher rolling reductions. Calculation of the Gibbs free energy change during the reversion treatment showed that the reversion mechanism is shear controlled at the annealing temperature and so the morphology of the reverted austenite is completely dependent on the morphology of the deformation induced martensite. It was found that the austenite had a bimodal grain size distribution in the 80% rolled and annealed state and this is related to the existence of different types of martensite. Increasing the rolling reduction to 93% followed by annealing caused changing of the grain structure to a monomodal like structure, which was mostly covered with small grains of around 300nm. The existence of bimodal austenite grain size in the 80% rolled and annealed 304L stainless steel led to the improvement of ductility while maintaining a high tensile strength in comparison with the 93% rolled and annealed sample.

The role of martensitic transformation in corrosion fatigue failure of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) medical alloys

The effect of cyclic dynamic loading on the kinetics of changes in the electrode potentials of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) superelastic medical alloys in a model solution simulating the bone tissue environment in a living organism is studied with the use of electrochemical chronopotentiometry. An increase in the applied load from 450 to 780 MPa is found to affect dramatically the character of changes in the electrode potentials of the alloys during exposure. Ti-22Nb-6Ta alloy exhibits higher resistance to corrosion fatigue cracking than does Ti-22Nb-6Zr alloy. It is shown that the superelasticity of the alloys is responsible for the deceleration of the corrosion fatigue cracking due to the blocking effect of martensite crystals formed in the structure during the loading half-cycle, which is confirmed by the low rate of the potential decrease after the nucleation of corrosion fatigue cracks.

Correlation of martensitic transformation and surface mechanical behavior with cavitation erosion resistance for some iron-based alloys

Cavitation erosion resistance of four Fe–Mn–Si-based alloys and four Fe–Cr–Ni–Mn stainless steels has been investigated at a speed of 45 m/s using a rotating disc rig. Based on the research of surface mechanical effects and energy absorption of phase transformation, the role of martensitic transformation in cavitation erosion resistance of the tested alloys has been discussed. The results show that the effect of martensitic transformation on the resistance of the tested austenitic alloys is greatly influenced by cavitation-induced martensite structure. The resistance of the Fe–Mn–Si-based alloys is much better than that of the Fe–Cr–Ni–Mn stainless steels. The resistance of the tested alloys correlates with their localized surface elasticity H e and work-hardening capacity. The role of phase transformation in the resistance of the tested austenitic alloys is associated with the localized surface elasticity H e, work-hardening capacity and energy absorption of phase transformation, and it mainly depends on the elasticity H e and work-hardening capacity. As the values of H e of the tested martensitic and austenitic alloys are close, the resistance of the austenitic alloys can be obviously improved due to energy absorption of phase transformation. The excellent resistance of the Fe–Mn–Si-based alloys are mainly attributed to their excellent H e and excellent work-hardening capacity.

On the role of martensitic transformation on damage and cracking resistance in TRIP-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J R curves and CTOD measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite.

Role of martensite transformation in variation of physicomechanical properties of Nb-Ti (45 and 50 wt% Nb) alloys at low temperatures

The structural state of Nb-Ti (45 and 50 wt% Nb) alloys after thermomechanical treatment (TMT), and the influence of TMT on their mechanical properties, plastic deformation characteristics and electrical resistance are studied in the temperature range 295–4.2 K. It is shown that the direct and reversible β ⇌ α″ transformation, which proceeds orientationally, leads to: 1, anomalous temperature dependences of the relative elongation, proportionality limits and yield strength of hardened and drawn samples at 160-77 K, the degree of anomaly being dependent on the TMT; 2, pseudo-elasticity in the stress-strain curves σ( Δl), the temperature range of the pseudo-elasticity being dependent on the TMT; and 3, non-monotonous behaviour in the strain dependences of electrical resistance ρ( Δl) and stress σ( Δl) in all structural states.

Role of martensite transformation in strengthening steel with laser treatment and subsequent deformation

1. With mechanical testing in tension for specimens treated by laser irradiation there is transformation of residual austenite into martensite. 2. The amount of martensite formed during deformation is governed by the residual austenite content depending on the original condition of the hardened material structure. Prior quenching promotes greater hardening of the zone by laser irradiation treatment.

Role of martensite transformation in forming the mechanical properties of unstable austenitic steels

1. Ductility properties of the test unstable austenitic steels increase during martensite transformation if it is preceded by plastic deformation of austenite. 2. Maximum ductility of the test steels is obtained with strictly optimum kinetics for martensite transformation which depends on the level of prior deformation. After quenching maximum ductility is obtained with a low intensity of transformation; the ductility peak is observed close to the Md point. 3. After prior deformation maximum ductility corresponds to the greatest intensity of transformation. With an increase in the degree of prior deformation the ductility peak shifts in the direction of lower temperature, and the intensity of martensite transformation corresponding to maximum ductility increases. 4. Martensite transformation may have both a strengthening and a relaxation effect. After quenching steels it mainly develops as a strengthening effect, and after deformation as a relaxation effect.

18-8ステンレス鋼の−269°Cの荷重―伸び線図にみられるserrationおよびそれに及ぼすマルテンサイト変態の影響

A Load-elongation curve of 18-8 stainless steel was obtained at −269°C with an extensometer specifically made for cryogenic use. The role of martensitic transformation in the serrations was discussed from the results of strain and hardness distribution measurements, X-ray analysis and optical microscopic observation of surface relieves. The change in electrical resistivity with the increase of tensile strain was also measured at −269°C with the specimen of different shape prepared from the same material, and the cause of the overshoots observed in the electrical resistivity change was also discussed. The results are summarized as follows:(1) The serrations observed were large in general except for those at the very early stage of the deformation. The plastic deformation component, i.e. a slight deviation from a straight line, was observed at the upper part of the load increasing portions of the serrations.(2) The serrations in the yielding and uniform elongation regions occurred with localized plastic deformation of the specimen. The deformation was accompanied by martensitic transformation.(3) The over-shoots were observed in the electrical resistivity changes, which took place discontinuously with load drops at the serrations. The main cause for these overshoots was considered due to heat generation accompanied by martensitic transformation (γ→α′).