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The fatigue crack growth (FCG) behavior of T6-aged Al-4.41Cu-0.69Mg-0.64Si-0.52Mn alloys forged at 350°C and 450°C were analyzed. FCG rate tests revealed that the alloy forged at 350°C exhibited a higher fatigue crack threshold (ΔKth) and lower FCG rate in the near-threshold region but a higher FCG rate in the Paris and instability regions than the alloy forged at 450°C. Examination of the crack paths suggested that intergranular propagations were predominant in the near-threshold region, and more crack deflections at grain boundaries contributed to the lower FCG rate of the recrystallized sample forged at 350°C. In the Paris and instability regions, transgranular propagations were predominant, and the lower FCG rate of the non-recrystallized sample forged at 450°C was attributed to the crack deflections between the dislocation walls and sub-grain boundaries. [ABSTRACT FROM AUTHOR]

Hot compression tests of as-homogenized Al-5.87Zn-2.07Mg-2.28Cu alloy were performed at the deformation temperature range of 350-450°C and a strain rate range of 0.001-1 s. Based on the dynamic materials model and Prasad's instability criterion, the processing map at the true strain of 0.69 was established to evaluate the workability of the alloy, and the related microstructures were investigated. The results show that the flow stress declines with increasing temperature and decreasing strain rate. The high-power dissipation efficiency of stable domains can be mainly attributed to the occurrence of dynamic recrystallization. Flow instability is prone to occur under the condition of low temperature and high strain rate due to the initiation and the propagation of micro-cracks. According to the processing map and corresponding microstructure characteristics, the optimum processing parameters are in the temperature range of 390-430°C and the strain rate range of 0.005-0.032 s. [ABSTRACT FROM AUTHOR]

The microstructure, high temperature mechanical properties and deformation mechanism of powder metallurgical (PM) Ti-45Al-7Nb-0.3W (at.%) alloy sheets were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tensile test results showed excellent high temperature mechanical properties with a yield stress (YS) of 670 MPa and an ultimate tensile stress (UTS) of 874 MPa at 700 °C and displayed an anomalous strengthening effect, which could be explained by the pinning mechanism of dislocation locks. In addition, the brittle-ductile transition temperature (BDTT) was found at between 800 °C and 850 °C. When deformed below BDTT, high density of dislocations and mechanical twins were observed in the microstructures. However, with increasing deformation temperature above BDTT, kinking of α 2 /γ lamellar colonies and dynamic recrystallization also occurred. It was noteworthy that the as-rolled sheets displayed superplastic behaviors at 950 °C with initial strain rates of 1 × 10 − 4 s − 1 and 5 × 10 − 5 s − 1 . At the same time, severe dynamic recrystallization took place and the grain boundary sliding was improved by β phase, which resulted in an elongation of 243% at the strain rate of 5 × 10 − 5 s − 1 . [ABSTRACT FROM AUTHOR]

TiAl-based alloys are known for their exceptional high-temperature properties but suffer from low hot workability. The emergence of additive manufacturing (AM) technology significantly decreases the workability requirement, while the microstructure heterogeneities in AMed TiAl are troublesome. This paper explores the hot deformation behavior and microstructure evolution of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting. The Arrhenius constitutive model with strain compensation was established, and hot processing maps were constructed. The microstructure showed that at the initial stage of deformation, a large number of dislocation slips occur preferentially in the soft coarse-grained layers. With the increase of strain, dislocation accumulation in heterogeneous interfaces, and dynamic recrystallization (DRX) occurs. Eventually, the coarse-grained region is constantly occupied by fine DRX grains. A high density of deformation twins at high strain rates further promotes the formation of tiny DRX grains. When the hot compression temperature reaches 1200 °C, the initial alternative fine-grained and coarse-grained hetero-structure transforms into duplex (DP) and near-lamellar (NL) microstructure, and microstructure heterogeneity is eliminated. The results enhance understanding of the hot deformation behavior of heterogeneous microstructure TiAl alloys. • Constitutive equation and processing map of heterogeneous TiAl alloy are established. • Thermal plastic deformation promoted the elimination of microstructure heterogeneity. • Coarse-grained and fine-grained layer exhibit different deformation mechanisms. [ABSTRACT FROM AUTHOR]

In this work, the effects of deformation on the microstructure and mechanical behaviors of TiAl alloy were investigated. Deformed microstructure observation was characterized by scanning electron microscopy, electron back scattered diffraction technique, transmission electron microscopy and DEFORM-3D software. Results indicated that the core area of the TiAl pancake was characterized by completely dynamically recrystallized microstructures, however some residual lamellar colonies can be observed near the edge area, which are primarily caused by a temperature drop and inhomogenous plastic flow. The main softening mechanism is dynamic recrystallization of &#947 grains. The as-forged alloy exhibited excellent mechanical properties at both room temperature and high temperature. Tensile test results showed that the ultimate tensile strength of the alloy increased from 832MPa at room temperature to 853MPa at 700°C, while the elongation increased from 2.7% to 17.8%. Even at the temperature of 850°C, the ultimate tensile strength maintained 404MPa, and the elongation increased to 75%. The as-forged alloy also exhibited remarkable low-temperature superplasticity at 850°C, with an elongation of 120%. [ABSTRACT FROM AUTHOR]

Abstract: Compression tests of 6061/B4CP composite have been performed in the compression temperature range from 300°C to 500°C and the strain rate range from 0.001s−1 to 1s−1. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300°C and 400°C with strain rate between 0.003s−1 and 0.18s−1 and the other was situated at the temperature between 425°C and 500°C with strain rate between 0.003s−1 and 0.18s−1. The estimated apparent activation energies of these two domains, were 129kJ/mol and 149kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350°C – 0.01s−1 and 500°C – 0.01s−1. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite. [Copyright &y& Elsevier]

Abstract: The hot deformation behaviors of Ag-containing 2519 aluminum alloy were studied by isothermal compression at 300–500°C with strain rates from 0.01s−1 to 10s−1. The microstructural evolution of the alloy was investigated using Polyvar-MET optical microscope and Tecnai G2 20 transmission electron microscope (TEM). It has been shown that the flow stress of the alloy increases with increasing the strain rate and decreasing the deformation temperature. When the strain rate is lower than 10s−1, the flow stress increases with increasing strain until the stress reached the peak value, after which the flow stress remains almost constant. This result indicates that dynamic recovery happens during deformation. When the strain rate is 10s−1 and the temperature is higher than 300°C, serrated flow behavior is generally observed with the stress decreasing with increasing strain, a typical phenomenon of dynamic recrystallization. [Copyright &y& Elsevier]