Materials Science Forum Vol. 1016

Abstract: The paper presents microstructural and mechanical results of medium manganese steel deformed under high strain rates. The rotary hammer tests at strain rates of 250, 500 and 1000 s^-1 were applied. Mechanical properties under dynamic tensile loads were determined. According to the obtained results, when strain rate increased the yield point of the steel increased. An opposite trend was present regarding total elongation. In case of tensile strength, its level is similar for all analyzed deformation rates. The microstructure of the steel after the dynamic tensile test is composed of bainite, martensite and martensitic-austenitic islands. The strain-induced martensitic transformation was identified in microscopic investigations.

Abstract: Severe plastic deformation (SPD) processing of Al alloys could obtain high strength by grain refinement mechanism. The minimum grain size of Al alloy, obtained at higher strain rate at low temperature, is determined the stacking fault energy of the alloy. SPD-processed pure Al metal, has high stacking fault energy, has relatively large grain size. During SPD processing, large strain is introduced, and the dislocation is rearranged in the specimen. The re-arrangement of dislocation in SPD-processed Al alloy with intermediate stacking fault energy significantly delayed, thus the strain remains in the grain interior. The extra-hardening, a kind of strain hardening, results from an incomplete of dynamic recrystallization during SPD processing. Al-Mg solid solution alloy has intermediate stacking fault energy and the minimum grain size of this alloy approaches about 200 nm after SPD. The mechanical property of this alloy is remarkably higher than the predictable strength by Hall-Petch relationship due to the extra-hardening. In addition, the increase in strength by the extra-hardening varies with the Mg content of Al-Mg alloy. In this study, the effect of Mg content, i.e. the stacking fault energy of the alloy, on the degree of the extra-hardening of SPD-processed Al-Mg alloy was investigated in terms of the dislocation density and low-angle grain boundary of the alloy.

Abstract: Since 2018, the institute of metal forming has been studying the novel twin-roll casting (TRC) of magnesium wire at the pilot research plant set up specifically for this purpose. Light microscopic and scanning electronic investigations were carried out within this work and show the unique microstructure of twin-roll cast AZ31 magnesium alloy with grain sizes of about 10 μm ± 4 μm in centre and 39 μm ± 26 μm near the surface of the sample. By means of a short heat treatment (460 °C/15 min), segregations can be dissolved and grain size changes in centre to 19 μm ± 12 μm (increase) and near the surface to 12 μm ± 7 μm (decrease). Further, the mechanical properties of the twin-roll cast and heat-treated wire were analysed by tensile testing at room temperature. By heat treatment, the total elongation could be increased by a third whereas the strength decreases slightly. In heat-treated state, no preferred orientation is evident. In addition to the twin-roll cast and the heat-treated condition, the rolled state was analysed. For this purpose, the twin-roll cast wire was hot rolled using an oval-square calibration. After hot rolling, a dynamic recrystallization and grain refinement of the twin-roll cast wire could be achieved. It can be seen, that an increase in strength as well as in total elongation occur after wire rolling. Beside this, a rolling texture is evident.

Abstract: Beta titanium alloys have several attractive features; this has resulted in this group of alloys receiving much attention since 1980’s. Among the attributes which distinguish them for their superiority over other structural materials are (i) high strength to which they can be heat treated, resulting in high strength to weight ratio (ii) high degree of hardenability which enables heat treatment in large section sizes to high strength levels (iii) excellent hot and cold workability, making them as competitive sheet materials etc. The standard heat treatment consists of solution treatment in beta or alpha plus beta phase field followed by aging. However, certain aging treatments can render the materials in a state of little or no ductility; the designer has to be aware of this behaviour and has to keep away from such treatments while working with the materials. Such unfavourable aging treatments may adversely affect not only the static properties such as reduction in area and elongation in a tensile test, but also dynamic properties such as impact toughness. Results of fractographic studies are in line with those of mechanical testing. The authors would present the foregoing analysis, based primarily on the wide-ranging researches they carried out on beta titanium alloy Ti15-3 and to some extent data published by researchers on other grades of beta titanium alloys. An attempt is made to explain the mechanisms underlying the embrittlement reactions that take place in beta titanium alloys under non-optimal aging treatments.

Abstract: Nickel aluminum bronze (NAB) castings possess favourable combinations of strength and resistance to corrosion, biofouling and cavitation/erosion, and so have long been used in naval applications. Nonetheless, in seawater environments NAB castings are susceptible to selective phase corrosion and so such components periodically require either replacement, which is very costly, or repair. However, repairs involving traditional, high heat input welding operations can lead to distortion and microstructural changes that unacceptably degrade NAB corrosion performance, and so repairs are not commonly performed. In the present work, cold spray is explored as an alternative for NAB (alloy CuAl9Fe5Ni5) repair without excessive distortion or base metal degradation, and preliminary results of its performance reported. Suitable cold spray parameters have been determined using an iterative approach by analyzing deposits in terms of microstructure, porosity and adhesion to the substrate. It is intended that these parameters will later be used to create simulated repairs which can be more thoroughly characterized for strength, toughness and corrosion performance.

Abstract: Our group prepared an ReB₂-based ceramic with a composition of Re-74.5at% B to investigate its microstructure, high-temperature microvickers hardness, and high-temperature tribological properties in air. The microvickers hardness of the ReB₂-based ceramic was higher than 2600 at temperatures below 1073 K. The friction coefficients of ReB₂-based ceramic/Si₃N₄ sliding pairs were stable and low (≃ 0.15) at 1073 K. We concluded that the low friction coefficients of the sliding pairs resulted from the formation of low-friction hexagonal BN and B₂O₃ films. The friction coefficients of the ReB₂-based ceramic/Si₃N₄ sliding pairs were also low at 298 K (≃ 0.3 to 0.4) and 1273 K (≃ 0.1), but were unstable and high ( 0.6) at 673 K.

Abstract: It was investigated that the effects of retained austenite (γR) conditions on ductility of advanced high strength steels for automotives. 0.4mass% C steels were heattreated in various austemper conditions to control the retained austenite conditions. In the result of the evaluation of mechanical properties of these steels, it was confirmed there were steels which indicated different elongation even if they had almost same volume fraction and carbon content of retained austenite. In order to clarify the reason, the conditions of retained austenite and work hardening behavior were investigated in detail. It was indicated that the existence of high carbon content region in a part of retained austenite promoted the deformation induced martensitic transformation in the high strain range and improved the elongation of AHSS.

Abstract: Fast electron irradiation can induce the solid-state amorphization (SSA) of many intermetallic compounds. The occurrence of SSA stimulated by fast electron irradiation was found in the Al_0.5TiZrPdCuNi high-entropy alloy (HEA). The relationship between the occurrence of SSA in intermetallic compounds under fast electron irradiation and the empirical alloy parameters for predicting the solid-solution-formation tendency in HEAs was discussed. The occurrence of SSA in intermetallic compounds was hardly predicted, only by the alloy parameters of δ or ΔH_mix, which have been widely used for predicting solid-solution formation in HEAs. All intermetallic compounds with ΔH_mix ≤ -35 kJ/mol and those with δ ≥ 12.5 exhibit the occurrence of SSA. This implies that the intermetallic compounds with a largely negative ΔH_mix value and a largely positive δ parameter are favorable for the occurrence of SSA.

Abstract: To clarify the effect of the acid solution type on corrosion resistance, the corrosion behavior of stainless steel brazed joints in HCl aqueous solution was evaluated through electrochemical measurements. Anodic polarization curves of a ferritic stainless-steel base metal, Ni-based brazing filler metals, and a brazed joint were recorded. In addition, in situ observations were conducted to observe the corrosion behavior of each structure of the brazed joint. Corrosion potentials of the brazing filler metal were lower than that of the base metal. In situ observations of the brazed joint revealed the order of corrosion in aqueous hydrochloric acid. According to the electrochemical measurements, under an actual corrosive environment, the brazing filler metal can function as an anode and selectively corrode. In addition, the anodic polarization curve of the brazed joint showed values between those of the polarization curves of the brazing filler metal and the base metal, indicating that the corrosion resistance could be electrochemically evaluated in HCl aqueous solution.

Abstract: Fatigue tests were conducted under several stress ratios, including negative maximum stress to elucidate the fatigue crack initiation mechanism of a magnesium alloy, AZ31. The specimen surface near the crack initiation site was analyzed by EBSD. On the basis of the results of EBSD analysis, it is concluded for an alternating cyclic stress condition (fully reversed cyclic stress) that fatigue cracks formed from grains where both the grain size and Schmid factor of the basal slip system are large, and that the crack initiation mechanism is based on irreversible slipping and unrelated to twinning. Under compression-compression fatigue test (R=10), cracks were formed along boundary of grains with large Schmid factor and misfit of both side grain are large. At the tip of the initiated crack, twin bands were observed.