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In this study, a corrugated constrained groove pressing (CCGP) technology based on a semicircle groove was proposed, improving the uniformity of strain accumulation in deformed metal sheets through staggered groove pressing. First, 6061-O aluminum alloy was processed at room temperature via conventional CCGP (Conv-CCGP) and 90° cross CCGP (Cross-CCGP). The effects of strain route and deformation pass on the microstructure evolution of the CCGPed alloy were investigated and correlated with mechanical properties. Changes in the mechanical properties of the CCGPed alloy at room temperature measured via tensile in different directions and hardness tests were studied. Results showed that the strength and hardness of the CCGPed sheet in both routes were considerably improved, but ductility was decreased. Cross-CCGP can effectively alleviate the anisotropy of the deformed sheet and achieve optimal strength in roll direction (RD) and transverse direction (TD) at two passes. Yield strength (YS) and ultimate tensile strength (UTS) along RD were increased by 190.2% and 15.5% relative to the initial alloy, reaching 119 MPa and 134 MPa, respectively. Along TD, YS and UTS were increased by 255.0% and 36.0%, reaching 142 MPa and 151 MPa, respectively. Characterization techniques, including electron backscatter diffraction and high-resolution transmission electron microscopy, were used to establish comprehensive models of dynamic recovery-based substructure evolution and dynamic recrystallization-dominated grain refinement. Meanwhile, the integrated microstructural changes and theoretical calculations identified dislocation strengthening and local grain refinement as the major contributors to the strengthening of the CCGPed alloy. In addition, tensile fracture surface analysis showed that the CCGPed alloy still underwent plastic fracture, but the fracture mode shifted from tension fracture into shear fracture. • A CCGP technology to uniform strain by staggered groove pressing was proposed. • CCGPed sheets show significant improvement in microhardness, YS and UTS. • Combined models of substructure evolution and grain refinement were developed. • Dislocations and local grain refinement on enhancement of strength were discussed. • The tensile fracture mechanisms of the CCGPed alloy were analyzed. [ABSTRACT FROM AUTHOR]

The microstructure of AerMet100 steel after aging was observed and analyzed. Based on the microstructure information of AerMet100 with different aging temperature, equivalent radius of M 2 C carbides was calculated and used in modified Wang's model to simulate the relation between yield strength and aging temperature. The simulation results showed that AerMet100 was mainly strengthened by M 2 C precipitate and solid solution. Solid solution strengthening decreased with the increasing of aging temperature because the growth of M 2 C and austenite gradually consumed Cr, Mo, Ni and C in the matrix. M 2 C strengthening was controlled by the opposite effect of growth rate and nucleation rate of M 2 C on strength. By adding the effect of austenite into the model and making simplification, the simulation results of the modified Wang's model were closer to the experiment results than the original Wang's model. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of ultrahigh strength steels (300M, Aermet100 and M54) were analyzed. The experiment results showed that the steels with austenite layers at the boundary of martensite laths had higher K IC than those without austenite layers. Finite element method (FEM) was used to establish models based on the actual microstructure of the steels. Extended finite element method (XFEM) was used to study the crack path and crack growth rate. Contour integral method (CIM) was used to study the best thickness of austenite layer, the effect of crack propagation direction and the value of K IC . According to the simulation results, 10–15 nm was the best thickness of austenite layer for the fracture toughness of the steels with martensite as matrix. The simulation results of normalized value of K IC were close to the experimental results. [ABSTRACT FROM AUTHOR]

Sag refers to the reduction in the free height of coil springs during actual usage, hence sag resistance is one of the crucial properties of spring steel and its improvement is essential for prolonging the service life of springs. However, limited research has been conducted on the effect of microstructure on sag resistance in recent years. This study aims to investigate the relationship between microstructure and sag resistance in 65SiCrV6 spring steel. The steel was quenched from various temperatures to obtain microstructures with varying sub-block size, and tempered at different temperatures to obtain microstructures with distinct tempered carbides morphology. The microstructures were characterized and the sag resistance was evaluated by hysteresis loop effect formed during cyclic tensile test. The results revealed that smaller sub-block sizes of martensite and higher proportions of high-angle grain boundary (HAGB) are beneficial for improving sag resistance. Incoherent granular cementite precipitated in the tempered microstructure improves sag resistance by enhancing interactions with dislocations, and coarsened cementite does not reduce the sag resistance due to the generation of back stress in heterogeneous microstructure. The hysteresis loop effect resulting from heterostructure as well as interactions between dislocations and tempered carbides increases with rising tempering temperature until it reaches a saturation state. This study highlights the importance of optimizing the microstructure with both small sub-block size and incoherent cementite precipitation through appropriate heat treatment process to enhance sag resistance, meanwhile provides valuable insights into understanding the micromechanism of sag resistance in spring steel. • The relationship between microstructure and sag resistance of spring steel was investigated in this work. • The smaller sub-block size and higher proportion of high-angle grain boundaries are conducive to sag resistance. • The incoherent granular cementite precipitated in the microstructure can enhance the sag resistance. [ABSTRACT FROM AUTHOR]

Abstract: The carbide precipitation behavior in austenite of a 0.04%C–0.10%Ti–0.21% Mo micro-alloyed steel was investigated by the stress relaxation method and transmission electron microscopy. Results showed that the precipitation-time-temperature diagram for carbide precipitation exhibited a typical “C” curve, the nose of which was located at about 925°C. The carbide was identified as a (Ti, Mo)C particle with a NaCl-type crystal structure that contains a certain amount of Mo. As compared with TiC particle in Ti steel, (Ti, Mo)C particle in Ti–Mo steel exhibits a superior coarsening resistance during the coarsening stage. The fraction of Mo in (Ti, Mo)C particle decreases with isothermal holding time or particle growing, indicating that the high level replacement of Ti by Mo in TiC lattice is in a “metastable” state with respect to the equilibrium precipitation of (Ti, Mo)C in austenite. [Copyright &y& Elsevier]

Abstract: The kinetics of strain induced precipitation in Ti-microalloyed steel with three levels of Mn was investigated using a two-stage interrupted compression (TSIC) test. The precipitation-time–temperature diagrams (PTT) were obtained by analyzing the softening kinetics curves of deformed austenite. Experimental results showed that the upper part of the PTT diagram for strain induced precipitation were shifted to lower temperatures and longer times as Mn concentration in steel was increased, but the retard effect of Mn on TiC precipitation weakened with decreasing testing temperature. Transmission electron microscopy (TEM) observation showed that the rate of TiC precipitation was decreased by more Mn addition at higher testing temperatures, which is in a good agreement with TSIC result. In addition, these results were discussed in terms of the effect of Mn on the solubility product of TiC in austenite, and the controlling factors of precipitation kinetics at different temperatures. [Copyright &y& Elsevier]

Abstract: The strain-induced precipitation kinetics of TiC in a 0.05% C–0.10% Ti HSLA steel was investigated by two-stage interrupted compression method. The precipitation–time–temperature (PTT) diagram for TiC precipitation was obtained by analyzing the softening kinetics curves of deformed austenite, which was confirmed to be of validity by employing transmission electron microscopy (TEM). Experimental results showed that the PTT diagram for TiC precipitation exhibited a typical “C” shaped and the nucleation of strain-induced TiC precipitation was a very rapid process in the temperature range 900–925°C. The relatively severe deformation applied on the steel was considered to be the main factor resulting in the fast kinetics of TiC precipitation. The TiC precipitates were heterogeneously distributed in either a chain-like or a cell like manner, implying that the precipitates nucleated on dislocations or on dislocation sub-structures, which were produced by deformation. The growth of TiC precipitates approximately followed a parabolic law. In addition, the coarsening of strain-induced TiC precipitates had already started before the completion of precipitation. [Copyright &y& Elsevier]

Abstract With the aim of achieving an enhanced high-temperature strength compared to the Co-Ni-Al-W-Ti-Ta alloy from our previous study, a new single crystalline CoNi-base superalloy has been developed based on thermodynamic calculations. The γ ′ solvus temperature and the γ ′ volume fraction were experimentally determined as 1247 °C and 85.2%. Thermodynamic calculations correctly predict the tendency of how the alloying elements influence the thermophysical properties of investigated alloys. In comparison with the original Co-Ni-Al-W-Ti-Ta alloy, the newly developed CoNi-base alloy exhibited stronger compressive strength ranging from the room temperature to 1100 °C. The deformation behaviors at different temperature stages were investigated in detail. Interestingly, superlattice intrinsic stacking faults (SISFs) appear in the specimen deformed at the anomalous peak temperature, whereas the formation of SISF in the original Co-Ni-Al-W-Ti-Ta alloy occurs as the compression temperature increases beyond the anomalous peak temperature. The relationship between the occurrence of planar defects and the alloying effects was discussed. [ABSTRACT FROM AUTHOR]

Abstract The thermal stability and high-temperature strength of single crystal CoNi-base superalloys, Co-30Ni-10Al-7W (at%) and Co-30Ni-10Al-7W-2Ta-3Ti (at%), containing γ′(L1 2) phase have been investigated. The investigated CoNi-base superalloy with alloying additions of Ti and Ta exhibits excellent γ′ solvus temperature, 1236 ℃, and high-temperature strength, 622 MPa at 900 ℃, compared to reported γ′-strengthening Co-base superalloys. Detailed analyses of the deformation mechanisms for the investigated alloys were carried out by using the transmission electron microscopy (TEM) characterization. Two different transitions of deformation mechanisms for the γ′ phase, from superpartial dislocation shearing to dislocation bypassing and to stacking fault shear, occurred in the investigated alloys as the deformation temperature increased above the peak temperatures. Meanwhile, the high-temperature strength of the investigated alloys largely decreased as the transitions of deformation mechanism took place. The influence of alloying effects on energies of faults and the relationship between high-temperature strength and deformation mechanisms were discussed. [ABSTRACT FROM AUTHOR]

The microstructure and yield strength of China low activation martensitic (CLAM) steel were observed and tested. The evolution of defects, including He bubbles and dislocation loops, and MC carbides were analyzed. According to the microstructure observation results, a two-step simulation model was established to analyze the relationship between microstructure and strength. For the first step, a rate theory model was used to simulate the evolution of defects (He bubbles and dislocation loops). The incubation, nucleation and growth stage of defects were shown by simulation results. For the second step, the simulation results of defect evolution were added into a superposition strengthening model, which combined different strengthening methods, to simulate the yield strength of CLAM steel in high temperature and irradiation condition. Based on both experimental and simulation results, the effect of high temperature and irradiation on microstructure and strength was discussed in CLAM steels. The simulation results of yield strength were basically consistent with experimental results. [ABSTRACT FROM AUTHOR]

In this study, we explore the potential roles of flash annealing in the microstructure and mechanical behavior of a cold-rolled "NANOHITEN" ferritic steel. The process produces an ultrafine-grained heterostructured matrix composed of recrystallized/transformed and non-recrystallized/transformed domains, leading to a much higher strength level (1 GPa) without a marked loss in ductility. [Display omitted] [ABSTRACT FROM AUTHOR]

The microstructure evolution on the rolling surface of GCr15 steel subjected to rolling-sliding and pure rolling contact loading was systematically investigated. Experimental results showed that the pearlite structure of the surface layer in the rolling-sliding sample transformed into nanocrystalline α-Fe–C alloy in which cementite underwent severe decomposition while the pearlite lamellae appeared unperturbed in the pure rolling sample. A white etching layer (WEL) was also detected in the surface of the rolling-sliding sample. The WEL formation was found to be due to cyclic shear plastic deformation instead of frictional heating. A surface layer of Fe 3 O 4 was detected in the pure rolling samples. Microhardness depth profiles of the rolling-sliding and pure rolling samples also showed different trends. [ABSTRACT FROM AUTHOR]

The mechanical properties and microstructure of reduced activation ferritic/martensitic (RAFM) steel after austenitizing at 930–1080 °C for 30 min and tempering at 760 °C for 90 min were investigated using field emission scanning electron microscopy, field emission transmission electron microscopy, electron back-scattered diffraction, X-ray diffraction, tensile, and impact tests. The results showed that the size of the M 23 C 6 particles decreased from 930 to 1030 °C and then increased from 1030 to 1080 °C, whereas their number densities showed the opposite trend with increasing austenitizing temperature. After austenitizing at 930 and 980 °C, the largest proportion consisted of the sub-grain structure, and when the austenitizing temperature was raised to 1030 and 1080 °C, the major component consisted of bundle martensite laths. The tensile strength increased gradually with increasing austenitizing temperature, and dislocation strengthening accounted for the largest proportion among the various strengthening mechanisms. Finally, fitting analysis showed that among the various hierarchical structural units in RAFM martensitic steel, the packets served as the effective "grain" that governed the increase in yield strength. [ABSTRACT FROM AUTHOR]

In order to deepen the understanding towards the creep rupture lifetime of G115 martensitic heat-resistant steel, a series of creep tests using 130–200MPa loading stresses at 923K were carried out to investigate the as-crept microstructural characteristics and cavity behaviors. The significant microstructure deformation and a dramatic increase in the areal density of high-angle grain boundaries occur when the stress loading is over 160MPa. Apart from the strain concentration at elongated grain boundaries and refined grains, the elevated stress also leads to the appearance of a strong α-fiber texture with {001}<110>//RD. The nucleation and growth of cavities can be effectively accelerated by the stress elevation, and ductile fracture is determined as the dominate mechanism within the test stresses. However, a transition towards brittle intergranular fracture is observed due to the rapid coarsening of Laves phase. Furthermore, a good prediction for creep lifetimes of G115 steel has been obtained based on the constrained diffusional cavity growth model. [ABSTRACT FROM AUTHOR]