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High temperature low cycle fatigue tests at various strain amplitudes (±0.5%, ±0.6%, ±0.8%, ±1.0%) and strain rates (0.5%/s, 0.05%/s, 0.005%/s) were performed at 550 °C. The stress decomposition method and microstructure characterization were used to analyze cyclic hardening and softening behavior. Results showed that increased back stress and the combined action of back and friction stresses were responsible for the pronounced cyclic hardening at the early and late stages of initial hardening, respectively. The decrease in friction stress resulted in cyclic softening until final fracture. Moreover, the evolution behavior of dynamic strain aging (DSA) activity was discussed, which showed a clear dependence on loading cycles and directions. Specifically, the DSA activity was firstly attenuated and then gradually intensified from the late stage of cyclic hardening, and it was stronger in the compressive plastic deformation regime of hysteresis loops than in the tension part. • The increase in intergranular back stress and friction stress causes cyclic hardening. •The decrease in friction stress contributes to the cyclic stress softening. •The DSA activity attenuates firstly and then gets enhanced during cyclic loading. •The DSA activity under compression loading is stronger than that under tension part. [ABSTRACT FROM AUTHOR]

Abstract Stir tools with different pin eccentricities were applied to the friction stir processing of Al-5052 alloy. Results show that pin eccentricity enhances material flow and refines grains of stir zone. The stir zone produced by a stir tool with 0.4 mm pin eccentricity performs the highest hardness and yield strength, attributing to the enhanced grain-boundary strengthening. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the strain hardening behaviors of a low carbon Nb-microalloyed Si–Mn quenching–partitioning (Q–P) steel were investigated. The microstructures were analyzed by the scanning electron microscope (SEM) and transmission electron microscope (TEM). Mechanical tests were used to evaluate the room temperature tensile properties of the steel. The work hardening behaviors of the tested specimens were analyzed using the Hollomon approach. The results showed that a two-stage work hardening behavior was observed during deformation processes. In the first stage, for the quenched samples, martensite deforms plastically and the hardening exponent decreased. For the air-cooled samples, however, the carbide-free ferrite deforms preferentially, and then, the carbide-free ferrite and martensite co-deform. In the second stage, due to the effect of transformation induced plasticity of retained austenite, the hardening exponent decreased slowly and plateaus were observed in the plots of n i –ε t until fracture. Variations of the work hardening behaviors were related to the martensite and the volume fraction of retained austenite in Q–P steels and the microstructural evolution during partitioning and following cooling process. [Copyright &y& Elsevier]