Interface Migration between Martensite and Austenite during Quenching and Partitioning (Q&P) Process

An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.

Mechanical and Transformation Behaviors of a C-Mn-Si-Al-Cr TRIP Steel under Stress

Transformation induced plasticity （TRIP） steels combine high strength and excellent ductility, making them suited for application in crash-relevant parts in the automotive industry. However, the high Si contents in the conventional TRIP steel will generate surface defects on the hot rolled strip, which is difficult to process in continuous galvanizing lines. In order to solve the above problem the TRIP steel with the addition of Al replacing majority of Si was designed. In the present paper, the volume fraction of various phases in a C-Mn-Si-Al-Cr TRIP steel was determined by metallographic examination and X-ray diffraction analysis, and the multi-phase microstructures were characterized using an atomic force microscope based on their height difference. Tensile tests were performed at different temperatures ranging from -40℃ to 90℃. The results show that transition temperature Ms^σ in the present TRIP steel cannot be determined due to its lower volume fraction of retained austenite, different from the conventional TRIP steel. While the yield stress and tensile strength at different temperatures are higher than those of the conventional TRIP steel, which is attributed to the addition of Cr. In order to evaluate the effect of martensitic transformation on the total elongation, the sample without retained austenite obtained by quenching in liquid nitrogen was carried out under tensile test. The results indicate that the elongation of the original sample containing 9% retained austenite is about 20% higher than that of the sample quenched in liquid nitrogen, which demonstrates that the retained austenite plays an important role in improving the elongation of the TRIP steel.

Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening

This work characterizes microstructural evolutions of electron beam melted(EBM) Ti-6 Al-4 V alloy modified via laser shock peening(LSP).The depth stress distribution and tensile properties of EBM Ti-6 Al-4 V alloy were measured before and after LSP.The results indicate that microstructure consists of β phase with 7.2%±0.4% vol.% and balance α lamellar in EBM sample,and the α lamella was refined into nano-equiaxed grains and submicro-equiaxed grains after LSP.The dominant refinement mechanism is revealed during LSP.Stacking faults were found in the LSP-treated sample,and their corresponding planes were determined as(0001) basal plane,(1010) prismatic plane,and(1011) pyramidal plane obtained by high resolution transmission electron microscopy.The subgrains and high-angle grains formed during dynamic recrystallization were identified by selected area electron diffraction pattern.The LSP treatment produces a significantly residual compressive stress approximately-380 MPa with the depth of compressive stress layer reaching 450 μm.Strength and elongation of the EBM sample were significantly increased after LSP.The strength and ductility enhancements are attributed to compre s sive stress,grain refinement and grain gradient distribution of α phase.

Deformation Temperature Dependence of Mechanical Properties and Microstructures for a Novel Quenching—Partitioning—Tempering Steel

The present investigation on a designed high strength Fe-0.25C-1.48Mn-1.20Si-1.51Ni-0.05Nb （wt%） steel treated by a novel quenching-partitioning-tempering （Q-P-T） process was focused on deformation temperature dependence of mechanical properties and microstructures. The results indicate that the Q-P-T steel deformed at various deformation temperatures from -70 to 300 ℃ exhibits superior mechanical properties due to excellent thermal stability of retained austenite. The microstructural characterization by transmission electron microscopy （TEM） reveals that the high strength of the Q-P-T steel results from dislocation-type martensite laths and fcc NbC carbides or/and hcp ε-carbides precipitated dispersively in martensite matrix, while excellent ductility is attributed to the significant transformation induced plasticity （TRIP） effect produced by considerable amount of retained austenite. The relationship between mechanical properties and microstructures at different deformation temperatures was clarified.

Dislocations across interphase enable plain steel with high strengthductility

Low cost,high strength and high elongation steel is always the major trends of steel development especially for automotive industry.However,raising strength usually brings about loss of ductility,which is known as the strength-ductility trade-off[1].Solving strength-ductility trade-off by increasing low cost carbon content is the century-long pursuit of researchers.Rashid[2]pointed out that investigation since 1900s indicates that the increase of carbon content leads to enhancing strength,but deteriorating ductility and other properties of steels.

Dynamic Compression Behavior and Microstructure of a Novel Low-Carbon Quenching-Partitioning-Tempering Steel

A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb （wt%） steel is treated respectively by novel quenching-partitioning-tempering （Q-P-T） process and traditional quenching and tempering （Q＆T） process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q＆T steel hardly has one. With the increase of com- pression strain rate from 7 × 10^2 to 5 × 10^3 s^-1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q＆T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 10^3 s^-1. However, relative small compressive s^xain of about 5% at 7× 10^2 s^-1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.

Orientation Relationships between Ferrite and Cementite by Edge-to-edge Matching Principle

The crystallographic features of pearlite were investigated by experiments and edge-to-edge matching principle. Two new orientation relationships between ferrite and cementite were determinated by selected area electron diffraction and then explained by our modified edge-to-edge matching method. The consistence of the experimental results with theoretical prediction confirms the practicability of the modified edge-to-edge matching model.