ABSTRACT Hot working die steels require high hardness, strength and toughness, and there is a strong industrial drive to improve mechanical property combinations through microstructure design and optimization. A study was carried out on a commercial hot working die steel plate (Uddeholm Dievar) using austempering, isothermal holding at 340°C followed by cooling, to understand LB/M microstructure and hardness development, considering spatial alloying element distributions. Local substitutional alloying element compositions between solute-rich and solute-depleted regions led to an around 30°C difference in the local undercooling for isothermal bainite formation. Inhomogeneous distributions of LB/M microstructures were evident when the lower bainite phase fraction was less than 30 vol.%. As the isothermal holding time extended, carbon partitioning resulted in a progressive refinement of bainitic plates and martensite laths. Needle-like bainitic cementite grew to plate-like morphology under the paraequilibrium condition with carbon diffusion. Carbon-enriched sandwiched LB/M interface microstructures, consisting of coplanar thin-plate martensite and adjacent ultrafine martensite laths, formed at the onset of final cooling to room temperature. The decrease of micro-hardness for LB/M microstructures followed the rule of mixture, with micro-hardness remaining relatively unaffected for lower bainite phase fraction up to ∼30 vol.%. Narrow nano-hardness distributions for LB/M microstructures with ≤ 10vol.% lower bainite changed to broad ones for LB/M microstructures with ≥30 vol.% lower bainite. This work has shown that substitutional alloying element micro-segregation and carbon redistribution on isothermal holding contribute to spatial heterogeneous LB/M phase distributions and need to be carefully dealt with for the LB/M microstructure design in hot working die steels.

ABSTRACT Application of titanium alloys is restricted due to their low hardness and poor wear resistance. Duplex surface treatment consisting of depositing of Al coating by arc ion plating (AIP) plus micro-arc oxidation (MAO) was carried out on Ti6Al4V alloy to improve its tribological performance. Ceramic coating mainly composed of α-Al2O3, γ-Al2O3 and mullite was fabricated on the aluminum-coated titanium alloy after the MAO treatment, and its thickness, compactness and hardness increased with the MAO time before the titanium alloy was oxidized. After the substrate titanium alloy was locally oxidized due to oxygen anions inward migration, a thin transition layer containing titanium oxides and aluminum titanate formed and incorporation of Ti in the alumina coating occurred. Occurrence of the strong eruptible discharges at these positions was facilitated probably due to the large difference in permittivity of the alumina and the titanium oxides, which led to the formation of penetrating cracks in the MAO coating. The hardness and compactness of the ceramic coating decreased. Tensile test results showed bonding strength of MAO ceramic coating/aluminum layer/titanium alloy substrate system was more than 40 MPa, and the maximum value could reach around 68 MPa. The bonding strength of the ceramic coating/titanium alloy substrate system decreased to around 34 MPa due to the weak bonding between the alumina ceramic coating and the titanium alloy substrate. Tribological test results show wear resistance of titanium alloy was greatly improved due to the fabrication of hard alumina ceramic coating on titanium alloy.

ABSTRACT Aluminum alloys are commonly used in the aerospace industry because of their good specific strength and corrosion resistance. However, reducing residual stresses and machining deformation of aluminum alloy components are the focus of researches. In this paper, a material model of 2A14 aluminum alloy during solution treatment and aging process is established with thermal expansion, high temperature tensile and stress relaxation experiments. The stress relaxation during aging is described with the Zener-Wert-Avrami model based on the experimental data. Quenching and aging experiments of 2A14 aluminum alloy C-ring samples were carried out. Temperature changes of C-rings were measured during quenching, residual stresses in the samples after solution, and aging are measured by XRD, respectively. Based on measured temperature variations, surface heat transfer coefficients of a C-ring during quenching were calculated by inverse heat transfer calculation and a novel optimization method. The changes of temperature, stresses, and notch opening distance of a C-ring during solution and aging process were calculated, and calculated results were in good agreement with the measured results.

ABSTRACT Nanoeutectic oxide layers have been obtained by laser surface nanocrystallization on oxide ceramics with eutectic composition. The eutectic layer can effectively improve the surface performance of oxide materials. This paper provides a comprehensive review of this promising technique. It includes the numerical simulation of temperature field in the melt pool associated with laser surface melting, formation mechanisms and growth kinetics of the coupled nanoeutectic structure, and the influence of processing parameters on structure and properties of eutectic oxide layer. In combination with other preparation techniques, such as directional solidification and additive manufacturing methods, the formation mechanism and prevention of bubbles and cracks are clarified. The relationship between microstructure and performance with gradient feature is also indicated. This review attempts to inspire imaginations and insights into material design and process optimization for laser surface nanocrystallization of oxide ceramics with eutectic composition.

ABSTRACT Fe–20Mn–0.7C twinning-induced plasticity (TWIP) steel with the grain sizes in the range 1.7–10.2 μm was produced by cold rolling and annealing. The effects of grain sizes on the mechanical properties of the TWIP steel were studied by tensile test at room temperature (RT) and –180°C. The yield strength increased with decreasing grain size at RT and –180°C, and the corresponding Hall–Petch relationships were established at both temperatures and are as follows (i) σ 0.2 = 205 + 453 d –1/2 (at RT) and (ii) σ 0.2 = 317 + 755 d –1/2 (at –180°C). When the temperature decreased from RT to –180°C, the stacking fault energy decreased from 24to 17.9 mJ/m2, approximately 25%, and the deformation mode is transformed from deformation twins to martensite transformation.

ABSTRACTThe corrosion behaviors of CP Ti, Ti-Ru, Ti-6Al-4V-Ru (Ti-64-Ru) and Ti-5Al-5Mo-5V-1Cr-1Fe-Ru (Ti-55511-Ru) in HCl solution were investigated using open circuit potential, polarization curves, mass loss measurement and X-ray photoelectron spectroscopy. The corrosion current density of Ti-64-Ru was 5 times higher than that of Ti-55511-Ru. While the passive current density of Ti-55511-Ru was higher than that of Ti-64-Ru and a higher corrosion rate was observed in Ti-55511-Ru. Iron oxide was not detected on the Ti-55511-Ru surface. Ru increased the thickness of the passivation film of Ti from ∼49 to ∼208 nm and made it more stable. The effective role of Ru on improving the corrosion resistance of Ti alloys was revealed through direct experimental evidence.

ABSTRACTA novel isothermal bainitic transformation combined with quenching and partitioning (i.e. ‘disturbed’ bainitic austempering) process was employed in an Mn–Si–Cr–C low-alloyed steel to reduce the fraction and size of the blocky martensite/austenite (M/A) islands; the M/A islands that are inevitably formed during the conventional bainitic austempering process are coarse and undesirable for property optimization. Owing to a novel heat treatment, an enhanced combination of strength and ductility was achieved (e.g. ultimate strength, ∼1445 MPa; total elongation, ∼28%). A pronounced TRIP effect of the refined retained austenite contributes a lot to the overall improvement.

ABSTRACTA Fe–0.20C–1.49Mn1.52Si–0.58Cr–0.05Nb low-carbon steel was treated by a novel quenching-partitioning-tempering (Q-P-T) process and the traditional quenching and tempering (Q&T) process for comparison, respectively. X-ray diffraction results indicate that there is 10.8% volume fraction of retained austenite (VRA) in the Q-P-T martensitic steel, while no diffraction peak of retained austenite was detected in Q&T martensitic steel. The Q-P-T steel and the Q&T steel were subjected to a dynamic tensile test at a strain rate of 500 s−1 and a quasi-static tensile test at a strain rate of 5.6 × 10−4 s−1, respectively. The results indicate that the high strain rate in the dynamic tension raises the strength of the Q-P-T and Q&T steels compared with their quasi-static tensions owing to strain rate hardening effect, however, the elongation of Q-P-T steel decreases slightly, while the elongation of Q&T steel evidently increases. This difference is attributed that the existence of the dislocation absorption by the retained austenite (DARA) effect in quasi-static tension is suppressed under dynamic tensile loading so that the enhancement influence of DARA effect on ductility cannot be effectively compensated by the adiabatic softening of the martensitic matrix, which leads to a slight decrease in the elongation of the Q-P-T steel in dynamic tension compared with that in the quasi-static tension. While Q&T steel has no DARA effect in quasi-static tension due to no retained austenite, and thus the adiabatic softening of the martensitic matrix increases the ductility of Q&T steel in dynamic tension.