Martensite Regions Research Articles

On the compressive deformation behavior of new beta titanium alloys designed by d-electron method

Three beta titanium alloys in the Ti-Al-Mo-Cr-V system have been designed using the d-electron method with the aim of activating different combinations of deformation mechanisms. In this regard Ti-4Al-7Mo-3V-3Cr (Ti-4733), Ti-3Al-5Mo-7V-3Cr (Ti-3573) and Ti-3Al-8Mo-7V-3Cr (Ti-3873) alloys have been designed and compared with a commercial Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy. To evaluate the accuracy of the d-electron theoretical predictions, uniaxial compression tests were performed at room temperature. Different characterization methods including X-ray diffraction, electron backscatter diffraction, optical and transmission electron microscopy were used to investigate the microstructural evolution and deformation mechanisms. As a result of the modified deformation mechanisms, all the designed alloys showed enhanced compressive properties in comparison to the Ti-5553 alloy in beta single phase state. It was found that with increasing stability of the beta phase, the deformation mechanism of the new Ti-alloys gradually changes from slip + stress-induced martensitic transformation to slip + stress-induced martensitic transformation + mechanical twinning and finally to slip + mechanical twinning. The results showed that in the case of twinning the prediction by the d-electron method is consistent with experimental observations whereas regarding the stress-induced martensitic transformation this method should be used with considering an expanded martensite region on the lower portion of the d-electron phase stability map.

Investigations on Deep Cryogenically Treated Low Alloy Steel Impregnated with WS2

The effect of ‘Deep Cryogenic Treatment’ (DCT) on the Powder Metallurgy processed Low Alloy (PMLA) steel impregnated with solid lubricant WS2 in four different weight proportions (0, 2.5, 5 and 10%) was investigated. Precipitation of Ni was observed after DCT in PMLA steel impregnated with 0% WS2. Finer needle-like martensitic regions, marginal precipitation of carbides and uniformly-distributed precipitation of Ni in the micro-structure and improved wear resistance were seen after DCT in PMLA steel with 2.5% WS2. Higher porosity was found to be detrimental to dry sliding in PMLA steel impregnated with 5 and 10% WS2.

Fine–scale structural features of intercritically aged HSLA-100 plate steel and their influence on yield strength and low-temperature impact toughness

Fine-scale structural features of an HSLA-100 steel intercritically aged at 950K (677°C) for 4.2ks (70min.) were investigated via transmission electron microscopy. The microstructure consisted of approximately 90vol.pct. of low‑carbon lath martensite. Average lath width was about 0.25μm, unchanged from the as-quenched condition, owing to a high number density of epsilon‑copper precipitates at these boundaries. About 10vol.pct. of medium‑carbon austenite existed as coarsened, broken-up islands between laths; martensite lath boundary coverage by austenite was notably reduced compared to the as-quenched condition. Greater continuity between adjacent low‑carbon martensite laths was accompanied by coarse copper precipitates within laths and an order of magnitude lower dislocation density than the as-quenched martensite. Collectively, these features produced a sub-grain coarsening effect. Austenite islands larger than about 0.5μm were unstable with respect to the formation of fresh, medium‑carbon martensite regions. Copper precipitation within austenite islands (owing to carbon and copper enrichment) might also have contributed to austenite transformation upon quenching. Intercritical aging was associated with a strong-and-undesirable deviation from strength-toughness behavior at sub-critical aging temperatures. Two key factors were identified as being responsible for this negative outcome: (i) a sub-grain coarsening effect associated with “delamellarization” of low‑carbon martensite and austenite and (ii) the formation of brittle martensite upon quenching. Quantitative estimates of the changes in strength and impact toughness are provided for microstructural differences between intercritically aged and subcritically aged material.

Effects of Overlapping Distances on Steel Microstructure and Properties After Multi-track Laser Quenching

In this study, multi-track laser quenching with different overlapping distances was used to assess SM45C microstructure and properties. Two overlapping distance types were evaluated, including overlapping distances in the transition and martensite regions, respectively. After multi-track quenching, hardness showed a threefold increase. With the overlapping distance in the transition region, surface hardness varied from 600 to 700 HV. There was a region of low hardness in each track. Hardness in the low hardness region was from 300 to 400 HV. The average hardness was basically over 500 HV with the overlapping distance in the martensite region, although there was a softening region. The increased hardness was mainly caused by the martensite. Microstructure in the overlapping region was self-tempered martensite, retained austenite and ferrite. Tempered sorbite and lowly tempered martensite were observed in heat-affected zones. Friction coefficients of quenched specimens varied between 0.275 and 0.325, with a 14% reduction relative to the base metal. The wear rate of the quenched specimen was only 40.8% of the base metal. With time, mass reduction in the quenched specimen was much smaller than that of the base metal. For specimens treated with laser quenching, slight fatigue wear was dominating.

Magnetic and magnetocaloric properties of Ni50−xCuxMn39Sn11 alloys

The effect of Cu substitution on the martensitic transition and magnetocaloric properties of arc-melted Ni50−xCuxMn39Sn11 (x = 6 and 7) Heusler alloys are investigated by differential scanning calorimetry, X-ray powder diffraction, scanning electron microscopy and magnetic measurements. According to XRD patterns, while Ni44Cu6Mn39Sn11 alloy exhibit orthorhombic structure, Ni43Cu7Mn39Sn11 alloy crystallize cubic L21 structure at room temperature. The substitution of Cu for Ni leads to decreasing on the martensitic transition temperatures to lower values. The magnetic entropy changes calculated from M(T) curves, for avoiding hysteresis losses in the vicinity of the martensitic transition region, by using Maxwell relation. The maximum magnetic entropy change, ΔSM is found 11.0Jkg−1K−1 for Ni43Cu7Mn39Sn11 alloy around the martensitic transition region, which is nearly twice as large as the Ni44Cu6Mn39Sn11 alloy under the magnetic field changing from 0 to 2T.

Microstructural evolution and mechanical properties of dual phase steel produced by strip casting

In the present study, an as-cast strip of dual phase steel (Fe-0.16C-0.3Si-0.81Mn-0.03Tiwt%) was successfully produced by twin-roll strip casting (TRSC) process. The as-cast strip was subjected to two different processing routes. The microstructure of the as-cast strip, cold-rolled sheet and annealed sheet was studied and related to mechanical properties of the annealed sheet. It was observed that the as-cast strip microstructure consisted of grain boundary ferrite, Widmanstatten ferrite, pearlite and intragranular ferrite, and the solidification structure was characterized by dendritic structure without the equiaxed center zone. The microstructure consisted of ferrite, martensite and non-recrystallization (N-R) region was obtained in the annealed sheet when the as-cast strip was directly cold rolled and annealed at 740–780°C for 3min. However, a typical dual phase microstructure of ferrite-martensite was obtained in the annealed sheet when the martensitic structure in the as-cast strip was subjected to cold rolling and annealing at 740–780°C for 3min. The dual phase microstructure in the annealed sheet having martensite fraction of ~ 51–77% was obtained, leading to tensile strength of ~ 851–914MPa and corresponding total elongation of ~ 16–11%. Tempering treatment reduced martensite hardness, leading to lower tensile strength (~ 702–518MPa) and corresponding higher total elongation (~ 19.8–30%) compared to non-tempered steel.

Correlative Microstructure Analysis and In Situ Corrosion Study of AISI 420 Martensitic Stainless Steel for Plastic Molding Applications

In this work, the corrosion behavior of tempered AISI 420 martensitic stainless steel (MSS) was studied by in-situ atomic force microscopy (AFM) in 0.1 M NaCl and correlated with the microstructure. Thermocalc simulation, dilatometry, and X-ray diffraction (XRD) were performed to investigate phase transformation which showed the formation of M3C, M7C3, and M23C6 type of carbides and also retained austenite. Optical microscopy, scanning electron microscopy (SEM), and AFM characterization revealed undissolved carbides and tempering carbides in the martensitic matrix. Volta potential mapping measured by scanning Kelvin probe force microscopy (SKPFM) indicated higher electrochemical (practical) nobility of the carbides with respect to the martensitic matrix whereas regions adjacent to carbides showed lower nobilities due to chromium depletion. Open circuit potential and cyclic potentiodynamic polarization measurements showed metastable corrosion activities associated with a weak passive behavior and a risk for localized corrosion along certain carbide boundaries. In-situ AFM measurements revealed selective dissolution of certain carbide interphases and martensitic inter-lath regions indicating higher propensity to localized corrosion.

Image Enhancement Algorithm for Optical Microstructural Characterization of Shape Memory TiNi Friction Stir Processed

A key topic regarding TiNi alloys concerns the possibility to attain junctions that preserve the shape memory properties of material. Experimental tests, previously performed on TiNi sheet friction stir processed, have highlighted the need to develop an appropriate image analysis method to quantify the various phases percentages present in the characteristics zones of Friction Stir Welding process. A proper Image Processing procedure has been performed in order to quantify the amount of the martensitic phase and to detect its morphology modification along to the processed region. Particularly each micrographic image, firstly, has been denoised using the 2D Wavelet transform technique and, successively, a texture segmentation procedure has allowed to evaluate the amount of the martensite and austenite phases and to classify the morphological changes of martensitic regions. In this research, two different wavelet de-noising techniques have been tested in order to select the better way to preserve the highest content of microstructural information in the image. The optical micrographic captures were subjected to a denoising procedure, using wavelet transform at five level of decomposition, that allowed to obtain on each image only two uniform regions for the austenite and martensite phases. The obtained results, in terms of preserving of accuracy of the microstructural characteristics, suggested the selection of stationary Haar wavelet transform method as de-noising method. The de-noised images were converted into grey scale images and enhanced by means of adaptive histogram equalization. Finally the texture segmentation procedures, followed by image binarization, allowed to distinguish and quantify the amount of each phase. The proposed image enhancement algorithm for optical microstructural characterization of Friction Stir Processed TiNi shape memory alloys constitutes a quick and cheap method to obtain a microstructural characterization of material strictly connected to the thermo-mechanical parameters of process.

Structure formation in grade 20 steel during equal-channel angular pressing and subsequent heating

The structure formation and the mechanical properties of quenched and tempered grade 20 steel after equal-channel angular pressing (ECAP) at various true strains and 400°C are studied. Electron microscopy analysis after ECAP shows a partially submicrocrystalline and partially subgrain structure with a structural element size of 340–375 nm. The structural element size depends on the region in which the elements are formed (polyhedral ferrite, needle-shaped ferrite, tempered martensite, and pearlite). Heating of the steel after ECAP at 400 and 450°C increases the fraction of high-angle boundaries and the structural ferrite element size to 360–450 nm. The fragmentation and spheroidization of cementite lamellae of pearlite and subgrain coalescence in the regions of needle-shaped ferrite and tempered martensite take place at a high ECAP true strain and heating temperature. Structural refinement ensures considerable strengthening, namely, UTS 742–871 MPa at EL 11–15.3%. The strength slightly increases, whereas the plasticity slightly decreases when the true strain increases during ECAP. After ECAP and heating, the strength and plastic properties of the grade 20 steel remain almost the same.

Microstructural and hardness investigations on a dissimilar metal weld between low alloy steel and Alloy 82 weld metal

The investigation on microstructure and hardness at the fusion boundary (FB) region of a dissimilar metal weld (DMW) between low alloy steel (LAS) A508-III and Alloy 82 weld metal (WM) was carried out. The results indicated that there were two kinds of FBs, martensite FB and sharp FB, with obvious different microstructures, alternately distributed in the same FB. The martensite FB region had a gradual change of elemental concentration across FB, columnar WM grains with high length/width ratios, a thick martensite layer and a wide heat affected zone (HAZ) with large prior austenite grains. By comparison, the sharp FB region had a relatively sharp change of elemental concentration across the FB, WM grains with low length/width ratios and a narrow HAZ with smaller prior austenite grains. The martensite possessed a K-S orientation relationship with WM grains, while no orientation relationship was found between the HAZ grains and WM grains at the sharp FB. Compared with sharp FB there were much more Σ3 boundaries in the HAZ beside martensite FB. The hardness maximum of the martensite FB was much higher than that of the sharp FB, which was attributed to the martensite layer at the martensite FB.

Effect of Carbon Distribution During the Microstructure Evolution of Dual-Phase Steels Studied Using Cellular Automata, Genetic Algorithms, and Experimental Strategies

The development of ferrite-martensite dual-phase microstructures by cold-rolling and intercritical annealing of 0.06 wt pct carbon steel was systematically studied using a dilatometer for two different heating rates (1 and 10 K/s). A step quenching treatment has been designed to develop dual-phase structures having a similar martensite fraction for two different heating rates. An increase in heating rate seemed to refine the ferrite grain size, but it increased the size and spacing of the martensitic regions. As a result, the strength of the steel increased with heating rate; however, the formability was affected. It has been concluded that the distribution of C during the annealing treatment of cold-rolled steel determines the size, distribution, and morphology of martensite, which ultimately influences the mechanical properties. Experimental detection of carbon distribution in austenite is difficult during annealing of the cold-rolled steel as the phase transformation occurs at a high temperature and C is an interstitial solute, which diffuses fast at that temperature. Therefore, a cellular automata (CA)-based phase transformation model is proposed in the present study for the prediction of C distribution in austenite during annealing of steel as the function of C content and heating rate. The CA model predicts that the carbon distribution in austenite becomes more inhomogeneous when the heating rate increases. In the CA model, the extent of carbon inhomogeneity is measured using a kernel averaging method for different orders of neighbors, which accounts for the different physical space during calculation. The obtained results reveal that the 10th order (covering 10-µm physical spaces around the cell of interest) is showing the maximum inhomogeneity of carbon and the same effect has been investigated and confirmed using auger electron spectroscopy (AES) for 0.06 wt pct carbon steel. Furthermore, the optimization of carbon homogeneity with respect to heating rate has been performed using a bi-objective genetic programming (BioGP) strategy for the steel composition varying from 0.06 to 0.12 wt pct carbon along with other parameters like average austenite grain size and time of heating as the input variables. The analysis of the results obtained from BioGP suggests that the homogeneity of carbon increases with the increasing carbon concentration of steel. This is corroborated by analyzing the AES results obtained for 0.28 wt pct carbon steel using the same technique as that used for 0.06 wt pct carbon steel.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) imaging of deuterium assisted cracking in a 2205 duplex stainless steel micro-structure

In the present work, the influence of deuterium on the microstructure of a duplex stainless steel type EN 1.4462 has been characterized by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) supported by scanning electron microscopy (SEM), focused ion beam (FIB), electron back scattered diffraction (EBSD) and energy dispersive x-ray (EDX) investigations. Characterization has been carried out before and after electrochemical charging with deuterium which has been used as a tracer, due to its similar behavior to hydrogen in the steel microstructure. In a first approach, the distribution of the deuterium occurring at temperatures above −58°C has been visualized. Further it turned out that sub-surface micro blisters are formed in the ferrite-austenite interface, followed by the formation of needle shaped plates and subsequent cracking at the ferrite surface. In the austenite phase, parallel cracking alongside twins and hexagonal close packed (martensitic) regions has been observed. In both phases and even in the apparent interface, cracking has been associated with high deuterium concentrations, as compared to the surrounding undamaged microstructure. Sub-surface blistering in the ferrite has to be attributed to the accumulation and recombination of deuterium at the ferrite-austenite interface underneath the respective ferrite grains and after fast diffusing through this phase. Generally, the present application of chemometric imaging and structural analyses allows characterization of hydrogen assisted degradation at a sub-micron lateral resolution.

Determining the up-down-up response through tension tests of a pre-twisted shape memory alloy tube

In this paper, the stress-induced phase transitions in a shape memory alloy (SMA) tube under torsion and pre-twisted tension are studied analytically. A constitutive model with the specific forms of Helmholtz free energy and mechanical dissipation rate is employed to formulate the governing system. Exact solution of the tube under pure torsion is first derived and the shear stress–shear strain response is determined, which reveals the hardening effect. For the pre-twisted tube under uniaxial tension, the one-dimensional asymptotic tensile stress–tensile strain relations for the austenite, the phase transition and the martensite regions are derived by using the asymptotic expansion method. By properly defining an elastic energy potential, the present system can be viewed as an elastic problem, which can be related to the problem of Ericksen's bar. The analytical formulas for the nucleation and propagation stresses in terms of the pre-shear strain (caused by the pre-twist) are obtained. Tension tests with fixed pre-twists on SMA thin-walled tube are conducted, with a focus on the stress–strain response. The measured values and the analytical formula for the propagation stress are used to determine the material parameters, which, in turn, yields the up-down-up response of a shape memory alloy tube under pure tension. The tendency and turning point of the phase transformation in the pre-twisted tube from localization to homogeneous deformation are also determined, which suggests a plausible way to avoid the instability in actuation applications.

A study on the multi-phase transformation kinetics of ultra-high-strength steel and application in thermal-mechanical-phase coupling simulation of hot stamping process

It is a significant issue to deeply understand the phase transformation kinetics and further predict the multi-phase compositions of hot stamping part with finite element method (FEM). As for BR1500HS ultra-high-strength steel, a time-temperature-transformation (TTT) test schedule from austenitizing temperature (1100°C) to different transforming temperatures (380–750°C) was conducted on Gleeble 3800 machine. On the basis of microstructure observations, ferrite+pearlite region (600–740°C), bainite region (420–600°C) and martensite region (lower than 420°C) were distinguished. According to the acquired dilatometric curves, the starting and ending TTT curves were fitted. Subsequently, Johnson-Mehl-Avrami type kinetics equation and Magee's equation were respectively solved to describe the diffusional and non-diffusional transformation kinetics as follows: the diffusional transformation efficiency increases to a maximum value followed by a gradual decrease till 100% with time extending; the non-diffusional transformation degree gradually increases with decreasing temperature without time influence. Eventually, the multi-phase transformation kinetics were applied to the construction of the thermal-mechanical-phase dynamic coupling finite element (FE) model of hot stamping process and analysis of the multi-phase evolution and distribution in a hot stamping part. Eventually, the hot stamping process experiment was conducted and proved that the simulation results were effective.

Microstructural Evolution and Recrystallization Kinetics of a Cold-Rolled, Ferrite-Martensite Structure During Intercritical Annealing

The recrystallization behavior of 80 pct, cold-rolled, low-carbon, dual-phase steel during intercritical annealing for different times was studied. The annealed microstructures showed that the recrystallization initially occurred in the deformed martensitic regions. The values of Avrami exponent for recrystallization varied from 3.8 to 4 with an activation energy of 46.9 kJ/mol. This study also introduced a novel method for the production of bimodal grain structures in low-carbon, ferrite-martensite steel.

Numerical analysis of 3D spot continual induction hardening on curved surface of AISI 1045 steel

In order to investigate and predict the material properties of curved surface AISI 1045 steel component during spot continual induction hardening (SCIH), a 3D model for curved surface workpieces which coupled electromagnetic, temperature and phase transformation fields was built by finite element software ANSYS. A small size inductor and magnetizer were used in this model, which can move along the top surface of workpiece flexibly. The effect of inductor moving velocity and workpiece radius on temperature field was analyzed and the heating delay phenomenon was found through comparing the simulated results. The temperature field results indicate that the heating delay phenomenon is more obvious under high inductor moving velocity condition. This trend becomes more obvious if the workpiece radius becomes larger. The predictions of microstructure and micro-hardness distribution were also carried out via this model. The predicted results show that the inductor moving velocity is the dominated factor for the distribution of 100% martensite region and phase transformation region. The influencing factor of workpiece radius on 100% martensite region and phase transformation region distribution is obvious under relatively high inductor moving velocity but inconspicuous under relatively low inductor moving velocity.

Effect of training on the temperature memory effect in Ti49.5Ni34.5Cu11.5Pd4.5 shape memory alloy with narrow hysteresis

The temperature memory effect (TME) has been investigated in Ti49.5Ni34.5Cu11.5Pd4.5 alloy with a narrow hysteresis. The TME can be enhanced through multiple partial thermal cycling (TME training). The TME obtained by training is repeatable and hard to wipe out, which can be detected even after 1000 following complete thermal cycles. A difference in the dislocation density between different martensite regions is introduced during training, which results in the TME. It increases with increasing training cycles, and graduates away slowly because of the rare introduced dislocations during the following complete thermal cycles, leading to the repeatable TME.

Mechanical characterisation of microstructural evolution in 304 stainless steel subjected to high-pressure torsion with and without hydrogen pre-charging

Micro-tensile tests were employed on a 304 metastable austenitic stainless steel to mechanically characterise the microstructures developed by processing through high-pressure torsion (HPT) with and without hydrogen pre-charging. The martensite formed by HPT processing of hydrogen-containing austenite exhibited low yield and tensile strengths but a high reduction of area compared to the one processed in the absence of hydrogen. This may be because dynamic martensite formed with hydrogen contains more retained austenite. Hydrogen charging into the austenite allowed the formation of ε–martensite, instead of deformation twinning, as an intermediate phase in the transformation to α′–martensite, which led to variation in the plastic behaviour. The inhomogeneity of the microstructure and the defects produced by deformation with hydrogen build a foundation but hardly play a crucial role in the hydrogen embrittlement (HE) of metastable austenitic steels. Excess hydrogen due to the dynamic martensitic transformation of hydrogen-containing austenite localises deformation in the retained austenite between the martensite regions formed, leading to the HE of metastable austenitic steels.

Transport and magnetic properties of Heusler alloy Ni–Mn–Sb prepared by solid state microwave heating

In this study, Heusler alloy [Formula: see text] has been prepared by solid state hybrid microwave heating within 40 min. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses reveal that the obtained samples are of single phase cubic structure and homogeneous. Temperature-dependence of electrical resistivity and magnetization measurements confirm the austenite to martensitic transition of the first-order nature with the absence of the exchange bias (EB), which is verified by the magnetic hysteresis loops. Compared with the samples prepared by conventional arc-melting method, the sample synthesized by fast microwave heating has lower martensitic transition temperature, smooth magnetization change in the martensitic region and disappeared EB, implying strongly suppressed antiferromagnetic (AFM) interaction. These results suggest that tuning the microstructure via different synthesis method can influence significantly the martensitic transition and magnetic interactions in the Ni–Mn–Sb system.

Numerical study of thermo-mechanical coupling effects on crack tip fields of mixed-mode fracture in pseudoelastic shape memory alloys

The thermo-mechanical coupling effects on mixed-mode crack tip parameters in pseudoelastic shape memory alloys are studied by a finite element solution. Different loading rates are applied on the K-dominant part of the small scale transformation region. Both plane strain and plane stress analyses are carried out to study how the coupling between thermal and mechanical regimes alters the near tip fields in the martensitic region. The crack tip energy release rate and the J-integral remain history-dependent. The stress/strain response, the angular distributions of the stress and the released temperature, the shape of the transformation zone and the released temperature are selected as the crack tip fields of interest. Special attention is given to study of how the aforementioned crack tip fields are affected by the increase of applied loading rate. A comparison is also drawn between the crack tip fields in plane strain and plane stress states to complete the discussion.