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Combined with digital image correlation （DIC）， infrared thermal imaging （IRT）， electron backscatter diffraction （EBSD）， and microhardness distribution， the plastic instability behavior of medium-Mn steel processed by inter-critical quenching during uniaxial tension was analyzed. The results show that the strain-induced martensite transformation of experimental steel mainly occurs in the early stage of uniaxial tension. The Portevin-Le Chatelier（PLC）bands are obviously formed and propagated during this process， accompanied by thermal fluctuations. Moreover， the type of the PLC bands has transitioned from type A to type B and then to type A. The PLC bands are formed by the interaction between dislocation multiplication and interstitial solute atoms， i.e.， dynamic strain aging （DSA）. In the early stage of deformation， the dislocation will preferentially move to the low-density dislocation area， i.e.， propagate unidirectionally along the axis （the type A band）. Then， in the middle stage， the dislocation multiplication and initial interaction leads to the jumping （type B） PLC band formation. Finally， dislocation density and interactions increase in the later stage of deformation， leading to the formation of the type A PLC band again.

7A85-T74 forged aluminum alloy was chosen as the experimental material, and the microstructure, tensile properties and impact energy of the alloy were investigated after 5 h of thermal exposure at room temperature to 240℃. The mechanism of the influence of the microstructure on the mechanical properties of 7A85-T74 aluminum alloy was also analyzed by transmission electron microscopy. The results show that the grain size of 7A85-T74 aluminum alloy does not change much in the temperature range of 80-240℃, but the precipitation phase changes significantly with the increase of temperature. Below 120℃, the precipitate size, tensile properties and impact absorption energy do not change significantly with increasing thermal exposure temperature, and the precipitation strengthening mechanism is a mixture of dislocation cutting precipitates and dislocation bypassing precipitates. With the increase of the thermal exposure temperature from 120℃ to 240℃, the precipitate average radius increases from 3.8 nm at room temperature to 12.3 nm, and the precipitate changes from η' phase to η phase. The yield strength and tensile strength of the alloy decrease significantly by 45.7% and 33.5% respectively compared with that of room temperature and the elongation, reduction of area and impact energy of the alloy increase significantly. The precipitation strengthening mechanism changes to dislocation bypassing precipitates, and the fracture mode changes from mixed fracture consisting of intergranular fracture and dimple transgranular fracture to dimple transgranular fracture. The effect of precipitate size on the strength and impact energy of the alloy discusses based on the precipitation strengthening theory, and the results of the theoretical analysis are consistent with the experimental results.

The microstructures and mechanical properties of Fe–0.35C–3.2Al–5Mn (in wt%, same later) hot‐rolled medium‐manganese steels are systematically investigated. The results show that ultrafine‐grained duplex phases with bimodal morphologies of lath and equiaxed are obtained with intercritical annealing at 750 °C for 1 h, along with ultimate tensile strength of 950 MPa and total elongation of 71.2%, attributed to the dislocation slip and deformation of ferrite matrix, accompanied by the synergetic twinning of retained austenite plus transformation of retained austenite to martensite. The newly developed medium‐manganese steels, which exhibit an excellent product of ultimate tensile strength and total elongation of 67.6 GPa·%, demonstrate great practical applications. [ABSTRACT FROM AUTHOR]