To combine the advantage of density reduction and excellent performance, nanometer-sized B2 particles were introduced into the δ ferrite matrix of high-aluminium low-density steel by the addition of nickel (Fe–0.2C–11Mn–6Al–4/8Ni). Compared to Fe–0.2C–11Mn–6Al (0Ni) steel, the hardness and tensile strength of 4Ni and 8Ni steels are significantly improved. The improvement of tensile strength in 4Ni and 8Ni steels was primarily contributed by the precipitation strengthening or solution strengthening of B2 particles in δ ferrite. At the higher annealing temperature, the original dislocation density in δ ferrite is lower. However, dislocation multiplication during tensile deformation was more significant in the sample annealed at higher temperature, which was responsible for a higher work hardening rate.

AA6061 and AA7075 aluminum alloys were successfully joined by using single-pass and multiple-pass friction stir welding techniques after which the effects on the nugget zone evolution from a second overlapping pass and its welding direction, were investigated. In comparison to single-pass friction stir welding, the application of a second overlapping pass prolonged the dynamic recrystallization time, and the grains of the nugget zone became finer with increased high angle grain boundaries. Moreover, reversing the welding direction of the second overlapping pass enhanced the vertical flow of materials, increasing the strain of the friction stir welding in the nugget zone. As a result, the efficiency of the grain refinement and mixture of dissimilar materials during the second overlapping pass were significantly elevated. The tensile strength of the nugget zone was improved after the second overlapping pass due to both the grain refinement and mechanical interlocking of the AA6061/AA7075 alloys. The nugget zone, which was fabricated via the multiple-pass friction stir welding technique using an opposite welding direction, exhibited a 23% increase in yield strength as compared to the sample using the single-pass friction stir welding technique.

In this study, the hot rolled medium manganese steel containing titanium was solution treated at 1,000 °C and followed by aging treatment at 500, 550, and 600 °C. The influence of aging treatment on mechanical properties and wear resistance of medium manganese steel reinforced with Ti(C,N) particles was investigated. It was found that the matrix of medium manganese steel was austenite. The austenite grain size was refined, and Ti(C,N) particles were precipitated after aging treatment. Compared to that of the as-hot rolled sample, the initial hardness of 500 °C aged sample increased by 9.5% to 312.86 HV, whose impact energy was more than doubled to 148.5 J. As the aging temperature raised to 600 °C, the initial hardness changed slightly. However, the impact energy dropped significantly to 8 J due to the aggregation of Mn at the grain boundaries. In addition, the main wear mechanisms of the samples were fatigue wear and abrasive wear. It was worth noting that 500 °C aged sample exhibited the best wear resistance under a 300 N applied load, whose wear loss was just half of the as-hot rolled sample. The relationship between wear loss and mechanical properties indicated that the wear resistance of medium manganese steel was independent of the initial hardness. The large difference in the wear resistance was predominately due to the outstanding work hardening ability of 500 °C aged sample, whose strengthening mechanisms were contributed from transformation induced plasticity (TRIP) effect, dislocation strengthening, twinning induced plasticity (TWIP) effect, and precipitation strengthening.

We propose an effective heat treatment involving a combination of intercritical hardening and tempering to obtain high strength–high ductility in hot-rolled 0.2C–1.5Al–(6–8.5)Mn–Fe transformation-induced plasticity (TRIP) steels. An excellent combination of high ultimate tensile strength of 1045–1380 MPa and total elongation of 34–39% was obtained when the steels were subjected to intercritical hardening at 630–650 °C and tempered at 200 °C. Intercritical hardening impacted the co-existence of austenite, ferrite and martensite, such that the deformation behaviour varied with the Mn content. The excellent properties of the steels were attributed to cumulative contribution of enhanced TRIP effect of austenite and ferrite and martensite constituents. The discontinuous TRIP effect during tensile deformation involves stress relaxation and led to consequent enhancement of ductility.

Antibacterial austenitic stainless steel was nominated as a new biomaterial material. The copper rich phase precipitation behavior and comprehensive properties in copper-containing austenitic stainless steel after different aging treatment were systematically studied. It was found that a large number of dispersed copper rich phases developed at 750–800 °C. At 800 °C, copper rich phase precipitated quickly within 1 h, but grew and coarsened slowly in the later stage. The hardness of the material increased gradually with the precipitation of copper rich phase, until it reached the maximum after 6 h and tended to be stable. However, with the extension of aging time, high concentration of copper rich phase destroyed the passive film on the surface of stainless steel and accelerated the matrix corrosion, but promoted the release of copper ions and improved the antibacterial property. Therefore, the comprehensive experimental results indicated that 3.60% Cu-304L stainless steel treated by solid solution at 1050 °C for 5 min and aged at 800 °C for more than 6 h had good corrosion resistance and antibacterial property. These results could provide theoretical guidance for the optimal design of copper containing antibacterial stainless steel in production process.