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Using molecular dynamics (MD) simulation, we study the temperature-induced $\alpha \leftrightarrow \gamma$ phase transformation of an Fe${}_{0.9}$Cr${}_{0.1}$ alloy. We find that the austenitic transition temperature is increased with respect to that of an Fe${}_{0.9}$Ni${}_{0.1}$ alloy containing the same concentration of impurity atoms. During the austenitic transformation, heterogeneous nucleation of close-packed (cp) nuclei leads to a polycrystalline structure. The microstructure formed closely resembles that found in pure Fe and in FeNi alloys.

The stress corrosion cracking (SCC) property of laser-MAG hybrid welded 304 stainless steel and Q345 steel was evaluated through cycle-immersion testing in 3.5 wt.% NaCl solution. The average SCC crack propagation rate of different zones under different initial stress intensity factors was calculated, and the SCC fracture and crack propagation path were observed. The microstructure and mechanical properties of the weld joint have also been examined. The result indicates that the fusion zone (FZ) is extremely prone to SCC. The average SCC crack propagation rate in FZ is $5.4489\times 10^{-6}$ mm/h, while no obvious SCC was found in the base metal (BM) and heat-affected zone (HAZ). The steel BM and HAZ may also suffer SCC, but not as fast as in FZ. Grooves caused by SCC were found on the fracture surface with a large amount of corrosion products accumulated close to the interface between the pre-crack section and SCC section. Crystallized-sugar-shaped pattern was found on the SCC zone of FZ. Crack jumping, deflection and crack closure occurred in the crack propagation path. Martensite on the FZ was considered to be the major reason that the FZ has a higher SCC propagation rate.

This paper’s aim is to describe the results of the phase transformation of BCuAl₉Fe₄ alloy after casting, quenching and aging. After casting, the microstructure of this alloy consists of $\alpha$ phase with grain size about 100 $\mu$m, mixture $(\alpha +\gamma )$ and the inter-metallic phase Fe₃Al. However, the proportion of the $\alpha$ phase in the casting alloy is coarse. The alloy was heated at $850^{\circ }$C for 2 h then quickly cooling in water. After quenching, the microstructure of alloy shows that the grain size reduced to about 40 $\mu$m. After quenching, the alloy was aged at $350^{\circ }$C for 2 h, the martensite phase of this alloy decomposed into order phase ($\alpha +{\gamma }_2)$ with fine grain size, dispersed in the matrix. The intermetallic phase was fine and evenly dispersed in the matrix. By TEM analysis, after heat treatment, the structure of martensite and the inter-metallic phase in this alloy which had small grain size were formed.

We study the approximation properties of piecewise constant functions with respect to triangular and rectangular finite elements in a metric defined on functions of bounded variation. We apply our results to a thin film model for martensitic crystals and to the approximation of deformations with microstructure.

We develop a computational model for the martensitic first-order structural phase transformation in a single crystal thin film, and we use this model to study the effect of spatial compositional fluctuation, spatial temporal noise, and the loss of stability of the metastable phase at temperatures sufficiently far from the transformation temperature.

In order to clarify the effect of annealing on the shape memory behavior of Ti-50.85at.%Ni alloy, the deformation and transformation behavior were investigated using tensile tests and differential scanning calorimeter (DSC). The martensitic transformation temperature increases with increasing annealing temperature until it reach as a maximum, and then decreases with further increasing annealing temperature. This can be rationalized by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure. The R-phase transformation temperature increases with increasing annealing temperature until reaching a maximum, and then decreases with a further increase of annealing temperature. This is attributed to the change of Ni content in the matrix caused by precipitation of Ti₃Ni₄. The critical stress for slip decreases rapidly with increasing annealing temperature, influenced by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure.

High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel–titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium–tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti–Ta and introduces a new concept providing for pronounced fatigue life extension.

Fe-Ni-Co-Al-Ta polycrystalline shape memory alloys have received extensive attention due to their excellent properties such as huge superelasticity, high strength and high damping. We herein report that Fe-Ni-Al-Ta polycrystalline alloys also have excellent superelasticity, which can exhibit a large superelastic strain of more than 17% in compression at room temperature. This superelasticity has slim hysteresis and is greatly affected by the content of Ta in the alloys. In this work, Fe-Ni-Al-Ta shape memory alloys reveal some characteristics that are different from the reported Fe-Ni-Co-Al-based shape memory alloys.

High Si bainitic steel has received much interest because of combined ultra high strength, good ductility along with high wear resistance. In this study, the microstructural evolution of dual phase bainitic ferrite-austenite steel after heavy compression was investigated. Compression tests were conducted at temperature of 298K on the rectangular billets at the strain rate of 0.001s_-1. The samples were deformed to 40% and 70% of their original thickness. The EBSD results show formation of nano grains with high angle grain boundaries through 70% compression, which confirms grain refinement. Additionally, 40% deformation resulted in enhancement of the dislocation density and formation of subgrains at ferrite unites. Also, it was found during 70% compression of the steel, the austenite transforms to the martensite, which is in agreement with thermodynamic calculations.

The changing of materials surface properties method always was taken into improving the fatigue strength. In this paper, an ultrasonic nanocrystal surface modification(UNSM) technique was used on the SUS 304 stainless steel to form a nanostructured surface layer with different static load(70N, 90N, 110N, 130N) and the vibration strike number was about 20,000times/mm². The untreated and different condition specimens fatigue strength was all tested by a dual-spindle rotating bending fatigue test machine. SPring-8(a large synchrotron radiation facility) was used to test the surface nanocrystallization components. The X-ray diffraction (XRD), the scanning electron microscopy (SEM), optical microscope and a micro-Vickers hardness tester (MVK-E3, Akashi) were separately used to get the surface residual stresses, fracture surface after fatigue testing, metallographic structure and the microhardness of the nanostructured surface layer. The result showed that martensite transformation took place on the surface of specimens, the surface residual stresses had only a small increase and some cracks occurred between the martensite layer and the austenite layer, but the fatigue strength of 90N improved 81%.

Experiments were carried out on the continuous cooling transformation (CCT) of 1500 MPa grade hot stamping steel by Formastor-F dilatometer. Microstructures and Vickers hardness were analyzed and a CCT diagram was drawn. The CCT diagram shows that when the cooling rate is more than 10°C/s, a full martensite is obtained and Vickers hardness is between 422 and 464 HV, which indicates that the test steel has a good hardenability. The test steel contains boron, manganese, and chromium which can all increase the hardenability of steel. The good hardenability is the key to achieve high strength for hot stamping steel. Bainite is obtained with the cooling rate below 10°C/s. Pearlite occurs when the cooling rate is less than 3°C/s and ferrite begins to form while the cooling rate is under 2°C/s. Therefore, the desired cooling rates are from 20 to 50°C/s to obtain the full martensite.

In order to enhance the hot-quenching steel's comprehensive performance, a C-Si-Mn-Ti-B hot forming steel is designed. By the means of experiment, WHF1300R/D steel's micro metallographic structure and mechanical performance in the procedures of hot rolling, annealing and hot stamping is studied and it performs good hardenability. Through the hot forming technic experiment, the most appropriate cooling velocity is found, which inhibits the formation of pearlite and bainite effectively. The microstructure of the hot-stamped WHF1300R/D steel sheet is composed of lath martensite. The strength and ductility of the hot-stamped WHF1300R/D steel sheet are matched well with a tensile strength of about 1300MPa and a total elongation up to 12%.

A dynamic tensile experiment was performed at various strain rates from 10^-4 to 10³ s^-1 on a dual phase steel, and the strain rate sensitivity on their tensile properties was studied. Results showed that the dual phase steel exhibited an obvious strain rate effect on strength. There were two stages in the effect of strain rate on strength. In the first stage, there was small sensitivity of strength to strain rate. In the second stage, there was obvious strain rate sensitivity, and the strength increased rapidly with the increase in strain rate. The strain rate sensitivity of the dual phase steel is considered to be originated from thermal activated mechanism. There is enriched thermal activated region in the first stage, and a lack thermal activated region in the second stage.

With the large-scale application of automotive light-weight concept in automobile industry, as the representative of the third generation of advanced high strength steels, Q&P steel is becoming the focus of international research, but its research reports about the carbon partition mechanism are rare. In this paper, establishing the theoretical model of carbon partition in the partitioning process of Q&P steel was discussed; the model can predict the relationship between the partition time and partition temperature of carbon better, so it can provides some theoretical basis for the actual production of Q&P steel.

Hot stamping parts can offer a low level of ductility after a strong crash due to the ultra-high strength and hardness. In this work, tailored properties of hot stamping process were developed with focus on the microstructures evolution and variable hardness of hard phase and soft phase. A low carbon steel sheet was carburized selectively at a temperature of 925°C for 1.5h in a fluidized bed carburizing furnace with an different proportion of atmosphere consisting of propane and air. Then the carburized specimens were austenitized and quenched in a oil bath to simulate a hot stamping process. The microstructure of the specimen after carburizing and hot stamping was observed by optical microscope (OM), the distribution of microhardness was analyzed by a vickers hardness tester, respectively. The results show that the higher micro-hardness can be obtained due to the hard martensite microstructure and achieved a lower hardness due to the presence of soft phase ferrite simultaneously. The interface characteristic of hard and soft sections shows microstructure gradients of martensite, pearlite and ferrite because of the repartitioning of carbon during hot stamping process. And tailored properties can be achieved for the specimens after hot stamping process by a local carburizing of a inexpensive low-carbon steel.

Microstructure engineering determines predominantly the properties and applications of steel. Martensite provides the highest strength compared to any other microstructural constitutes, thus press hardening steel with martensite microstructure has been widely used in vehicles’ body structure to enhance the passive safety in the event of a crash. However, for such applications there are also potential problems related to martensite microstructure, such as limited ductility (bendability), low impact toughness (low energy absorption and low fracture resistance) and low resistance to hydrogen induced cracking. Usually these properties dominate the fracture behaviour of martensitic microstructure and consequently the crash-resistance performance of press hardened components. Therefore, in order to achieve high performance, it is necessary to optimize the steel microstructure during entire metallurgical processes.

This paper introduces metallurgical solutions of microstructure engineering based on niobium metallurgy. The key point of microstructure engineering is to tailor and to fine-tune the microstructure details towards targeted applications with aims for high performance. The paper describes how to implement microstructure engineering in the press hardening steel by refining and homogenising the martensite matrix, strengthening and facilitating high angle grain boundaries in the martensite substructure, integrating a high density of homogenously distributed nano-sized NbC precipitates in the martensite matrix and finally creating a layer of soft microstructure near the surface area through controlled decarburization process during press hardening. The results and benefits of microstructure engineering to press hardening steel (22MnB5) are explained in detail.

Experiments were carried out upon the continuous cooling transformation (CCT) of 1700 MPa grade hot stamping steel by Formastor-F dilatometer. Microstructures and Vickers hardness were analyzed and a CCT diagram was drawn. The CCT diagram shows that when the cooling rate is more than 5°C /s, martensite is obtained and Vickers hardness is 529∼546 HV5, which displays that the test steel has a good hardenability. Bainite is gotten with the cooling rate below 5°C /s. Pearlite appears when the cooling rate is lower than 2°C /s and ferrite begins to form while the cooling rate is under 1°C /s. The CCT diagram of 1700 MPa grade hot stamping steel has been right shift compared with the 1500 MPa grade due to the addition of 0.03% carbon and 0.3% manganese which increase hardenability of steels and also have good effect to improve martensite hardening capacity and then increase the strength of steel. The 1700 MPa grade steel has the broader cooling rate range from 10°C /s 50°C /s to obtain a full martensite.

Two kinds of hot stamped steel 0.04%Nb and 0.04%Nb+0.04%V had been designed and developed on the basis of traditional 22MnB5 hot stamped steel. The hardenability test of three steels was carried out by end quenching method. The results show that the hardenability of 22MnB5 has been improved after adding trace elements, in which the hardenability of 0.04%Nb+0.04%V hot stamped steel is the strongest, and the hardenability of 0.04%Nb steel is not as good as 22MnB5. From the micro level analysis, it is considered that V has a better effect on hardenability than Nb. A comparative test on mechanical properties and bending properties of three kinds of steels had been carried out. The results show that the addition of Nb element alone has the best effect in improving bending properties. The strength and elongation of the newly designed two kinds of hot stamped steels have not been obviously improved. It is believed that it is related to the different influence of the hot stamping process and trace elements on the structure and properties of martensite.

Hot stamping has attracted significant attention to produce boron steel in automobile industry for its excellent shape accuracy and small springback. A qualified hot stamping component must have fully martensitic microstructure to ensure the strength reached 1500MPa. Then in the current paper, a methodology to identify the fully martensitic microstructure SEM photograph with Support Vector Machine is proposed. At first, different heat treatment experiments are implemented to obtain different microstructures, and the SEM photographs are collected. Then, the Gray-Level Co-occurrence Matrix of those SEM photographs are calculated, and the features of different SEM photographs are extracted to train the Support Vector Machine (SVM) classification. The experimental results show that GLCM based feature extraction achieves high accuracy, and an automated system has been developed to identify fully martensitic microstructure SEM photograph without human intervention.

Hot stamping parts have been widely used as re-enforcement for better crash safety and other attributes as well as in lightweight applications such as vehicle B-pillar, rocker and other structure parts. In this paper hot stamped torsion beam is studied as a function of product DVP and hot stamping process. Although hot-stamping process is fairly understood, the lightweight designed torsion beam’s property as a function of various parameters is not well studied. In order to study the temperature field, stress field, strain field, thinning rate and phase distribution in the hot forming process of torsion beam, the finite element software was used in this paper. The study found the maximum principal strained area appeared in the highest temperature region due to the different regions of part contact with the tool in the different order while the maximum principal stress area is located in the lower temperature region. In the process of hot forming, the slow cooling rate of the part is 77 °C/s, which is greater than critical cooling rate of martensitic transformation 30 °C/s; With the increase of the initial temperature of sheet metal, the maximum content of austenite and thinning rate of the parts increase, and the forming quality of the parts decreases; with the increase of stamping speed, the thinning rate of parts is violent fluctuation. When the stamping speed is too large, parts of the temperature of parts is greater than the martensite transformation temperature in some regions at the end of forming; with the increase of friction coefficient, the thinning rate and the minimum content of martensite firstly increase and then decrease; the maximum content of austenite decreases first and then increases. Research shows that: when the initial temperature of blank is 850, stamping speed is 75 mm/s, coefficient of friction is 0.45, the quality of the parts is the best and thinning rate is 5.804 %. At the end of forming, the minimum temperature of parts is 285 °C; minimum martensite content is 75.63 % and the maximum content of austenite is 24.36 %.