The effect of different heterogeneous microstructures on the tensile properties of high-strength wind power steel Q500MD, a 500 MPa grade wind power steel was assessed. Hence, the as received steel is subjected to an intercritical annealing combined with quenching or tempering one, namely the following three heterogeneous heat treatments: intercritical annealing, quenching + intercritical annealing and quenching + intercritical annealing + tempering. Then the microstructure and tensile property of the acquired three type steels with different heterogeneous microstructures were examined viauniversal testing machine, scanning electron microscopy and electron backscatter diffraction. The results indicate that the as received steel presents typical thermo-mechanical control process (TMCP) heat treatment microstructure composed of a large amount of acicular ferrite and a small amount of martensite. After heterogeneous heat treatment, their microstructures transform into a heterogeneous microstructure of soft phase intercritical ferrite and hard phase martensite, and the martensite varies with different post heat treatment processes. Specifically, the steel microstructure turns into intercritical ferrite, granular martensite, and fibrous martensite after quenching and intercritical annealing. Due to the existence of heterogeneous nucleation in the intercritical annealing process, new microstructure can be generated and grain refinement can be achieved. The heterogeneous microstructure leads to a low yield ratio and high plasticity of high-strength wind power steels. Compared with the as received steel, the presence of the soft-phase intercritical ferrite alters the initial yield threshold and subsequent work hardening behavior of the steel during tensile deformation, resulting in low yield ratio. Based on the analysis of microstructure evolution and tensile properties of the steels prepared by different heterogeneous heat treatment, it can be concluded that the presence of intercritical ferrite and martensite in the microstructure after intercritical annealing can enhance the strength and plasticity of high-strength wind power steels. Furthermore, incorporating quenching prior to intercritical annealing yields a finer grain structure, thereby further reducing the yield ratio of high-strength wind power steels.

Three Mg-8Zn-xCu (x = 0, 0.5 and 1, mass fraction, %) alloys were prepared by friction stir processing (FSP). Meanwhile, the influence of Cu content on the formability and mechanical properties of FSP Mg-8Zn alloys at high rotation rate was investigated by differential thermal analysis (DTA) and backscattered electron (BSE) etc. It was found that with the increasing Cu content, the eutectic in the Mg-8Zn-xCu alloys was transformed from a low-melting-point binary eutectic (346 °C) to a high-melting-point ternary eutectic (434 °C) during solidification process. The predominant eutectic phase was evolved from the Mg₇Zn₃ to the MgZnCu phase. The elevated eutectic temperature effectively suppressed the liquation (i.e., liquid formation) and liquation-induced cracking during FSP. By a Cu content of 0.5%, most of the secondary phases were dissolved into the Mg matrix via the thermo-mechanical coupling effect of FSP. This resulted in a super high Zn solid solution content of 6.62% (mass fraction) in Mg matrix, approximately 4 times of the room-temperature equilibrium solubility. Compared with the FSP Mg-8Zn alloy, the FSP Mg-8Zn-0.5Cu alloy exhibits a 50 MPa increase in tensile strength, achieving 300 MPa, and the elongation to fracture significantly increases from 13.4% to 30.2%. It is the improvement of microstructural uniformity that leads to a simultaneous enhancement of strength and ductility, supplemented by the synergistic action of high-content Zn solid solution strengthening, grain refinement, and MgZn₂ dynamic precipitation strengthening. As the Cu content was increased to 1%, stripe-like particle clusters formed within the processing zone due to the undissolved secondary phases, leading to a significant degradation in tensile properties.

The oxidation behavior of the T6-treated Al-5Zn-2Mg (in mass fraction) Al-alloy in air at 400 ^oC and 550 ^oC for 0 h to 384 h is comparatively studied via intermittent weighing, SEM+EDS and XRD etc. The results show that the alloy is mainly composed of Al-matrix with η' phase, η phase and Fe-containing phase. Among them, the η' phase is dispersed within grains, the η phase is preferentially distributed along grain boundaries, and the Fe-containing phase is mainly banded along the extrusion direction (ED). During oxidation at 400 ^oC, the alloy exhibits good resistance to high temperature oxidation. Within the range of 0 h to 192 h, it shows a slight increase in mass. From 192 h to 384 h, the mass increase does not change significantly with time. After oxidation for 384 h, the mass increase is only 0.07 mg/cm². Upon oxidation at 550 ^oC, the mass of the alloy increases significantly with the extension of time. After oxidation for 384 h, its mass increased by 0.33 mg/cm². Besides, failure analysis shows that the surface morphology of the alloy remained basically unchanged after oxidation at 400 ^oC and no O enrichment was observed on the surface. However, after oxidation at 550 ^oC, the surface of the alloy turned gray-black, implying the occurrence of significant oxidation in the Fe-containing adjacent regions and its vicinity, resulting in the enrichment of Mg and O there.

The effect of isothermal quenching temperatures below M_s point on the microstructure and mechanical properties of bainite/martensite multi-phase 42CrMo steel was studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), thermal expansion analyzer and mechanical property tester. The results show that the microstructure of 42CrMo steel is composed of bainite and martensite phase after isothermal quenching at 250 ^oC, 280 ^oC and 310 ^oC, correspondingly, the quenched 42CrMo steels were named as 250#, 280# and 310# steel respectively. With the increasing isothermal quenching temperature, the phase transformation rate of bainite is enhanced, and bainite volume fraction increases from 18% to 63%. The mode of impact fracture transforms from brittle cleavage fracture to mixed fracture, while the impact toughness is significantly improved. The bainite/martensite multi-phase microstructure of 250# and 280# steels is fine. However, bainite and martensite laths of 310# coarsens, and large-sized martensite block appears, so that its tensile strength decreases. The total grain boundary length of 280# is 1.65 times that of 250#, and 2.43 times that of 310# respectively. The significantly increased grain boundary density makes 280# possess the highest yield strength. After being isothermally hold at 280 ^oC for 1.5 h then quenched, the yield strength and tensile strength of 42CrMo steel reach 1399 MPa and 1708 MPa, respectively, and the elongation and impact absorption energy are 12.3% and 51.6 J, respectively. Compared with 310# and 250#, the 280# steel has better overall mechanical properties.

In order to satisfy the need of practical engineering application, 690 MPa HSLA steel butt joints with 27 mm in thickness were welded by independently developed electrodes with 4.0 mm in diameter. The effects of non-preheating (-1 ^oC) and low preheating temperature (66 ^oC) welding on microstructures and mechanical properties of butt joints were studied and the mechanisms of strength-toughness were revealed. The results showed that preheating temperature had significant effects on microstructures and mechanical properties of butt joints. By comparison of non-preheating welding, the strength of weld metal slightly decreased while that of butt joints changed within a small range, the impact toughness at -50 ^oC (KV₂) of weld center and fusion line changed obviously while that of the heat affected zone (HAZ) changed slightly. Cooling rate of the butt joint was lower in low preheating temperature welding compared with that without preheating, which promoted the formation of acicular ferrite with excellent plasticity and the degeneration of lath bainite in weld metal and fusion zone. Meanwhile, the M-A with larger size and higher quantity were formed. Comparing with the non-preheating welding, the higher impact toughness of weld metal for low-preheating temperature welding was correlated with the improvements of plasticity of acicular ferrite and interface bonding strength of M-A and acicular ferrite. Meanwhile, the lower impact toughness of the fusion line was correlated with the abnormal growth of acicular ferrite and dense distribution of M-A with larger size. Meanwhile, the improvement of M-A and acicular ferrite interface bonding strength was correlated with ultra-low carbon design of weld metal. Butt joint with excellent strength-toughness properties was obtained in the conditions of low-preheating temperature welding. Mean value of tensile strength of butt joint was 827 MPa, the average -50 ^oC impact toughness of weld center, fusion line, and HAZ (2 mm outside of fusion line) were 99, 98,and 260 J, respectively.

A low-density high-Mn test steel Fe-30Mn-9Al-0.96Mo-0.9C-0.49Si for cryogenic application was designed, and the effect of grain sizes on the microstructure and mechanical properties of the steel was investigated by tensile test at -196 ^oC, in terms of the microstructure evolution. The grain orientations of this steel are randomly distributed, and after tensile deformation at -196 ^oC the majority of the grain orientations tends to be <001>// the tensile direction or <111>// the tensile direction. Regardless of room temperature and -196 ^oC, the yield strength can be increased by approximately 160 MPa by refining its grain size from (16.5 ± 11.6) μm to (3.4 ± 2.2) μm. Accordingly, the steel after grain refinement presents yield strength, tensile strength and total elongation as 1304 MPa, 1664 MPa and 31.9% at -196 ^oC, respectively. Although the stacking fault energy can be reduced by lowering deformation temperature, the ordered structure still makes the plastic deformation of the steel to still be controlled by softening of slip planes. Thus, the plastic deformation mechanism of the steel is still governed by the planar slip. However, the dynamic slip band refinement and interaction between the slip bands can be affected by grain size.

Selective laser melting (SLM) offers a novel pathway for fabricating medium-entropy alloys (MEAs), yet the process inherently introduces defects such as porosity and microcracks, which are challenging to eliminate solely through subsequent heat treatment. Herein, the influence of various post-processing treatments on the microstructure and hardness of an Al₈(CoCrNi)₉₂ MEA was assessed. It was found that the as-printed alloy exhibits a typical columnar grain structure with cellular dislocation structure. While subsequent heat treatment alone induced a certain degree of recrystallization, its effect on mitigating macroscopic defects was minimal. In contrast, thermomechanical processing significantly refined the microstructure, nearly eliminated entirely the pores and microcracks, and induced the formation of a heterogeneous multiphase structure, consequently leading to a substantial enhancement in hardness. This research demonstrates that thermomechanical processing, as an effective post-processing route, can optimize the microstructure of SLM-fabricated MEAs, alleviate inherent defects related with the additive manufacturing technique, and markedly improve their mechanical properties.