•A flow stress model is established and used to explore optimum properties.•The strength and ductility rely on dynamic recovery and storage rate of dislocation.•The alloy with both shearable/non-shearable precipitates obtains optimum properties.The mechanical performance of precipitation hardening alloy depends much on the nature of precipitates. For the sake of properties optimization, various precipitation states were designed to clarify the correlation between the precipitation and mechanical properties of a Cu–Be–Co–Ni alloy. The characters of each precipitation state were quantitatively characterized using TEM. Then both tensile and tension–compression (Bauschinger) tests were conducted to investigate the isotropic and kinematic contributions to the work hardening. Based on the quantitative analysis results of the precipitates, a flow stress model with the integration of yield stress, isotropic and kinematic work hardening behavior was established. The model indicates that the strength and ductility are determined by two contradictory factors: the dislocation dynamic recovery rate and dislocation storage rate. Optimization of strength and ductility relies on the balance of these two contradictory factors. Large non-shearable precipitates will increase both the dynamic recovery and the storage rate while fine shearable precipitates will make them decrease simultaneously. It is suggested that the alloy containing the mixture of above two kinds of precipitates will achieve the optimization.

A simple process so-called forced convection stirring casting (FCSC) was proposed to prepare large-diameter 7075 Al alloy ingots. The flow behavior, temperature, and composition fields of the melt in the FCSC process were simulated. The macromorphology, macrosegregation, microstructure, and mechanical properties of the ingots prepared by the FCSC were studied and compared with those prepared by normal casting (NC). The results showed that in the FCS device, the strong convection caused by the axial flow and circular flow rapidly promoted the uniformity of the temperature and composition fields of the melt. Microstructures of the FCSC ingots from the edge to the center were all nearly spherical grains, which were much finer and more uniform than that of the NC ingots. The rotation speed played an important role in the microstructure of the FCSC ingots, and the grains became finer and rounder as the speed increasing. The FCSC process effectively eliminated cracks, improved macrosegregation, and decreased the eutectic phase area fraction and the average grain boundary thickness of ingots. Mechanical properties of the ingots prepared by the FCSC are far better than that of the NC ingots.

AbstractA new and effective semisolid slurry preparation process with air-cooled stirring rod (ACSR) is reported, in which the compressed air is constantly injected into the inner cavity of a stirring rod to cool the melt. The slurry of a newly developed high thermal conductivity Al-8Si alloy was prepared, and thin-wall heat dissipation shells were produced by the ACSR process combined with a HPDC machine. The effects of the air flow on the morphology of α1-Al particles, mechanical properties and thermal conductivity of rheo-HPDC samples were studied. The results show that the excellent slurry of the alloy could be obtained with the air flow exceeding 3 L/s. Rheo-HPDC samples that were produced with the air flow of 5 L/s had the maximum UTS, YS, elongation, hardness and thermal conductivity of 261 MPa, 124 MPa, 4.9%, HV 99 and 153 W/(m·K), respectively. Rheo-HPDC samples show improved properties compared to those formed by HPDC, and the increasing rates of UTS, YS, elongation, hardness and thermal conductivity were 20%, 15%, 88%, 13% and 10%, respectively.

•Hall–Petch relationship is obeyed between yield strength and martensite microstructure size.•Packet boundaries and block boundaries hold similar ability to impede the propagation of crack.•Block width is the effective grain size for strength, toughness and fatigue crack propagation.The purpose of this study was to analyze the microstructure of lath martensite in 0.1C–1.1Si–1.7Mn (wt.%) steel and its effect on mechanical properties and fracture behavior. The microstructure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). Charpy V-notch impact samples and compact tension (CT) samples were used to investigate the Charpy impact properties and fatigue crack growth behavior of the steel, respectively. The propagation of cleavage crack and fatigue crack were analyzed to figure out the effective grain size. The results showed that the typical hierarchical lath martensite structure contained prior austenite grains, packets, blocks and laths; packet size and block width were positively correlated to prior austenite grain size, while lath width was maintained at about 0.29 μm. Yield strength was related to prior austenite grain size, packet size and block width, and obeyed Hall–Petch relationship. Grain refinement was effective in improving the resistance to cleavage fracture by introducing barriers to crack propagation; packet boundaries and block boundaries hold similar ability to impede the propagation of crack. Paris model can well describe the FCG behavior of the investigated steel. Block width governs the effective grain size for strength, toughness and fatigue crack propagation.Mechanical properties and fracture behavior of 0.1C–1.1Si–1.7Mn steel.

An advanced rheomolder, equipped with a thin-walled phone cover mold, was introduced and used to manufacture the AZ91D magnesium alloy. Effects of different processing parameters including pouring temperature, cylinder temperature and injection velocity on the microstructure and mechanical properties were investigated. The results showed that, with the decrease of pouring temperature or the increase of injection velocity, the size of α1-Mg decreases while the sphericity increases, and the porosity fraction reduces, which raise mechanical properties of rheomolded AZ91D. As the cylinder temperature decreases, the size of α1-Mg decreases and the sphericity increases initially and then remains relatively unchanged while the solid fraction increases continuously. Also, the cylinder temperature has a significant effect on mechanical properties of rheomolded AZ91D and the highest mechanical property is obtained at a cylinder temperature of 555 °C. The optimal rheomolded AZ91D with tensile strength of 273 MPa and elongation of 7.2 % were obtained at the pouring temperature of 600 °C, injection velocity of 2.1 m/s and cylinder temperature of 555 °C. Compared with high pressure die-casting, the tensile strength and elongation were increased by 22.4 and 213 %, respectively.

The cooling process following hot rolling has a significant effect on the shape quality of a hot-rolled strip. The temperature and stress fields in the cooling process for a 14 mm thick strip with yield strength of 500 MPa grade were analyzed by the finite element method and actual test data, and the relationship between residual stress and shape defects was described. Subsequently, the small-crown rolling process and the coil slow cooling process were investigated. The results indicate that these processes improved the shape quality of the final product significantly.

Two different kinds of experimental techniques were used to in-situ study the austenite formation during intercritical annealing in C-Mn dual phase steel. The microstructure evolution was observed by confocal laser scanning microscope, and the austenite isothermal and non-isothermal transformation kinetics were studied by dilatometry. The results indicate that banded structure is produced for the reason of composition segregation and the competition between recrystallization and phase transformation. Austenite prefers to nucleate not only at ferrite/ferrite grain boundaries, but also inside the grains of ferrite. Furthermore, the austenitizing process is accomplished mainly via migration of the existing austenite/ferrite interface rather than nucleation of new grains. The incubation process can be divided into two stages which are controlled by carbon and manganese diffusion, respectively. During the incubation process, the nucleation rate of austenite decreases, and austenite growth changes from two-dimensional to one-dimensional. The partitioning coefficient, defined as the ratio of manganese content in the austenite to that in the adjacent ferrite, increases with increasing soaking time.

The microstructure and mechanical properties of AZ91D alloy thin-wall parts produced by the rheomolding (RM) process were investigated and compared with the same alloy formed by conventional high-pressure die casting (HPDC). The results indicate that the RM process is able to get such AZ91D parts in which α1-Mg with average size of 27.36 μm are spherical and uniformly distributed in the matrix, and the matrix is a mixture of numerous fine α2-Mg and intermetallic β-Mg17Al12. High mechanical properties including ultimate tensile strength (UTS) of 270 MPa, yield strength (YS) of 169 MPa, elongation of 7.1%, and Vickers hardness of 102 are obtained in parts formed by RM due to the fine and uniform microstructure and less porosities. Compared with HPDC, the UTS, YS, elongation, and hardness of RM AZ91D are increased by 14.4, 9.7, 86.8, and 21.4%, respectively. The solidified grains in RM AZ91D alloy show a smaller aluminum gradient than that in HPDC. This indicates that the solidification of the RM AZ91D is closer to equilibrium.

In this paper, the evolutions of microstructure and mechanical properties of Cu-1.9Be-0.3Ni-0.15Co alloy were studied. The alloys in the condition of the solution treated (soft state) and 37% cold rolled (hard state) were aged at 320 °C for different time, respectively. The mechanical properties, electrical conductivity and microstructure of the alloy aged for different time were analyzed. Additionally, the precipitation kinetics of Cu-1.9Be-0.3Ni-0.15Co alloys was investigated. X-ray diffraction and transmission electron microscopy results reveal that both continuous precipitation and discontinuous precipitation existed in the hard-state Cu-1.9Be-0.3Ni-0.15Co alloy during the whole aging process; the sequence of continuous precipitation is G.P. zone → γ″ → γ′ → γ. Furthermore, the precipitation transformation mechanism of soft-state alloy is homogeneous nucleation, while hard-state alloy shows the heterogeneous nucleation (interface nucleation) with the nucleation rate of both states decaying rapidly to zero during aging at 320 °C.

The high-temperature mechanical properties and microstructure of forging billets of C-Si-Mn-Cr and C-Si-Mn-Cr-Mo ultra-high-strength cold-rolled steels (tensile strength≥1000 MPa, elongation≥10%) were studied. Through the comparison of reduction in area and hot deformation resistance at 600–1300°C, the Mo-containing steel was found to possess a higher strength and a better plasticity than the Mo-free one. The equilibrium phase diagram and atom fraction of Mo in different phases at different temperatures were calculated by Thermo-Calc software (TCW). The results analyzed by using transmission electron microscopy and TCW show that precipitates in the Mo-containing steel are primarily M23C6, which promote pearlite formation. The experimental data also show that a lower ductility point existing in the Mo-free steel at 850°C is eliminated in the Mo-containing one. This is mainly due to the segregation of Mo at grain boundaries investigated by electron probe microanalysis (EPMA), which improves the strength of grain boundaries.