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Catastrophic brittle failure of ductile materials by liquid metal embrittlement (LME) is a widely documented phenomenon but the fundamentals of its initiation mechanism are poorly understood. The widespread use of Zn-coated advanced high strength steels in the automotive industry has been plagued by Zn-induced LME which is frequently observed in high-temperature forming and welding applications. In this study, numerical modeling and an atomic-scale experimental investigation are used in order to gain insight into the atomistic events that lead to the onset of LME cracking. The results showed that the formation of a stress-induced diffusion wedge (SIDW) at the exposed grain boundary (GB) due to the interdiffusion of Zn-embrittler atoms was the trigger for LME. The formation of the SIDW facilitated the diffusion of the Zn-embrittler atoms into the GBs, which compromised their mechanical integrity. The results show that LME initiation entails several steps: (i) solid-state GB diffusion, (ii) formation of the SIDW, (iii) eventual melting of the SIDW, and (iv) opening of the liquid wedge due to interdiffusion and the application of externally applied stresses. [ABSTRACT FROM AUTHOR]

A new tungsten matrix composite containing 30 vol% titanium carbide particles (W-TiC) produced by sintering under 20 MPa pressure at 2000°C in a vacuum has been developed in order to improve the elevated temperature strength of tungsten. Flexural strength tests of the W-TiC composite in the temperature range 20–1200°C showed that the strength was significantly increased by the presence of TiC particles. The flexural strength at 1000°C was 1155 MPa, which was much higher than that at 20°C (770 MPa). Microstructural observations showed that a interdiffusion zone was produced at the W matrix-TiC particle interface, and a strong bond was formed between TiC and W, which was very beneficial to the elevated temperature mechanical properties. The mechanisms of fracture at 20°C and 1000°C were investigated. The fracture at 20°C was brittle. There was a growth-coalescence process for the initial cracks during the fracture process of the W-TiC composite at 1000°C, and the W matrix exhibited ductile tearing. The excellent elevated temperature strength of W-TiC composite was attributed to the brittle-ductile transition in the W matrix, which allows more effective strengthening from TiC particles. [ABSTRACT FROM AUTHOR]

Abstract: The use of dual phase (DP) steels in the automobile industry unavoidably involves welding and dynamic loading. The aim of this investigation was to evaluate the microstructural change and mechanical properties of laser welded dissimilar DP600/DP980 steel joints. The dissimilar joints showed a significant microstructural change from nearly full martensite in the fusion zone (FZ) to the unchanged ferrite–martensite dual-phase microstructure in the base metal. The welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat-affected zone (HAZ). The dissimilar welded joints were observed to exhibit a distinctive unsymmetrical hardness profile, yield-point-like phenomenon, and single-stage work hardening characteristic, with yield strength and work hardening rate lying in-between those of DP600 and DP980 base metals, and ultimate tensile strength equivalent to that of DP600 base metal. Although the welded joints showed a lower fatigue limit than the base metals, the fatigue life of the welded joints at higher stress amplitudes was almost the same as that of the DP600 base metal. The welded joints failed in the soft zone at the DP600 side under tensile loading and fatigue loading at the higher stress amplitudes. Fatigue crack initiation occurred from the specimen surface and crack propagation was characterized by typical fatigue striation together with secondary cracks. [ABSTRACT FROM AUTHOR]

Cracks repairing of superalloy is very important to the aerospace industry. Most of the crack repairing studies have the same “crack width” which is quite different from the taper-shaped crack in reality. In this study tapered slots with maximum 200 μm width were artificially made by femtosecond laser in small plates of IN738LC superalloy to imitate service cracks. The “cracks” were repaired by diffusion brazing with a mixed filler alloy at 1100 °C for 24 h and followed by homogenization at temperatures of 1160 °C (HT-1), 1180 °C (HT-2) and 1190 °C (HT-3). With increasing homogenization temperature and time, the volume faction of borides in diffusion affected zone (DAZ) decreased. During the homogenization with HT-3, most of the borides in the DAZ of the joint dissolved, merely a small quantity of others coarsened by Ostwald ripening. In order of the processes as-bonded, HT-1, HT-2 and HT-3, the chemical composition of the isothermal solidification zones (ISZs) gradually came close to that of the base metal and the hardness of the ISZs increased, while the hardness of the DAZs decreased. A uniform bonded joint could be obtained with HT-3, and the shear strength of the joint reached up to approximately 95% of the base metal. The corresponding fracture path and fractography of the samples were observed to explain the mechanism of strength evolution. [ABSTRACT FROM AUTHOR]

Joining NiTi to Ti6Al4V is of great interest for applications in the biomedical and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallics to produce high strength joints. In this work, Niobium was used as an interlayer to prevent the formation of these brittle phases when joining NiTi to Ti6Al4V. The presence of this interlayer ensured that crack free welds were obtained and no brittle intermetallic compounds were observed. The Niobium interlayer was of a much higher melting temperature than the base materials so the bulk Niobium did not melt during the joining process, acting as a diffusion barrier between the NiTi and Ti6Al4V. The laser was focused on the Ti6Al4V side of the joint, which joined the Ti6Al4V and Niobium by fusion welding. At this interface a (Ti, Nb) region was formed due to dilution of the Niobium and mixing with the Ti6Al4V. At the NiTi–Nb interface a eutectic reaction was responsible for joining. Mechanical testing of the joints revealed that the minimum tensile strength matched the ultimate tensile strength of the weakest material, Niobium. These results highlight new possibilities for the use of high melting point filler materials when joining NiTi to dissimilar materials, so that the formation of undesired phases can be avoided. [ABSTRACT FROM AUTHOR]