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The features of crack formation near the fracture region and the fracture micromechanism are investigated during uniaxial tension of specimens of Ni–Cr stainless steels, Fe–19Cr–9Ni–0.1C and Fe–19Cr–9Ni–1.4Nb–0.1C, produced by the method of 3D wire-feed electron beam additive manufacturing (EBAM) as a function of the phase composition and phase distribution in the structure. It is found out that in the specimens with a twophase (austenite/δ-ferrite) microstructure the ferrite morphology and its volume content (up 25%) affect the plastic shear distribution in austenite and ferrite and exert a slight influence on the formation of strain localization micro- and macrobands in the pre-fracture stage, and the crack-formation mechanism is similar to that observed in cast stainless steels of similar compositions, which possess single-phase austenitic structure. In additively-manufactured steel containing niobium the formation of brittle niobium- and iron-based intermetallic phases favors the formation of pores and microcracks at the austenite/NbFeCrNi-phase or the δ- ferrite/NbFeCrNi-phase interfaces, while the processes of cracking in the strain localization bands (and their formation) turn out to be suppressed. Irrespective of the elemental and phase compositions of steel specimensformed by the method of 3D additive manufacturing, the principal micromechanism of their fracture, transgranular dimple fracture, is similar to that observed in the cast specimens of austenitic stainless steels. [ABSTRACT FROM AUTHOR]

In this research work, the fracture mode of dual phase medium carbon low alloy steels which was subjected to uniaxial tensile testing, was studied. The dual phase microstructure was developed by adopting intercritical heat treatments at an isothermal temperature of 770°C for holding times of 15, 30 and 60 min. After characterising the tensile properties with the aid of a universal tensile testing machine, a scanning electron microscope (SEM-EDS) was further employed to analyse the microstructure of the developed DPS. From the results, it was observed that despite high strength being induced in the steel, the strain-to-fracture remains near 25%. The presence of dimple fractures and pull-outs, which led to the realisation of the existence of intermetallic compounds, was also obtained from the fractographs. In general, sample B-2 was soaked for 30 min and exhibits the best combination of mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract: The ultrafine elongated grain (UFEG) structure with strong <110>//RD fiber deformation texture was produced by warm caliber-rolling at 500°C and final tempering at 550°C, namely tempforming in the 1100MPa medium carbon-low alloy steel with ultralow phosphorus and sulfur concentrations. Charpy impact tests were performed at temperature range of −196°C to 150°C on the UFEG structure along with a conventional quenched and tempered (QT) structure. A remarkable increase in upper shelf energy of 150J was obtained in the UFEG structure without delamination, while that of QT structure was 97J. The UFEG structures tempered at higher temperatures of 625 and 700°C showed remarkable increase in Charpy absorbed energy from 150J to 187J and 203J. Also, the QT structure absorbed almost same energy as UFEG structure at 700°C. The enhanced toughness was discussed with tensile ductility, void nucleation and growth, and their relations to microstructure including the <110>//RD fiber deformation texture. [Copyright &y& Elsevier]

Abstract: The microstructure and shear strength of a transition joint consisting of Sn–Ag–Cu solder alloy and Cu substrate were evaluated in reflowed condition. The results were compared with those obtained from a similar joint made with conventional eutectic Sn–Pb solder alloy. The reaction products at the interface are mainly Cu6Sn5 and Cu3Sn along with Cu6Sn5 for Sn–Pb and Sn–Ag–Cu to Cu joint, respectively. The shear strengths of the two systems are ∼55 and ∼68MPa, respectively. The fracture takes place through the Cu6Sn5 intermetallic phase as well as through the solder alloy and the strength depends on the thickness of the reaction layer. The width of the brittle reaction layer at the interface for Sn–Ag–Cu to Cu transition joint is thinner with respect to Sn–Pb to Cu joint; hence the shear strength of the former is superior to the latter. [Copyright &y& Elsevier]

Abstract: A failure investigation has been conducted on the turbo-disk used in a locomotive turbochanger, which is made from GH2132 high-temperature alloy. The fracture of three teeth and the crack of 11 teeth took place, which mainly occurred at the third root near the groove. Fractographic studies indicate that the crack originated at the root and propagated along the turning direction of the turbo-disk. In the initial crack propagation zone several different failure mechanisms performed successively, which are the intergranular corrosion crack, the corrosion-fatigue propagation and the dimple fracture. The greater bending stress on the tooth root and the presence of the corrosion medium in the service circumstance should be responsible for the failure of the turbo-disk. [Copyright &y& Elsevier]

Dimple fracture mechanisms are discussed for three kinds of aluminum alloys on the basis of an experimental approach and a finite element (FEM) analysis. The void growth and coalescence process was observed by an optical microscope and a scanning electron microscope. The fractographic observation for aluminum alloys 7075-T651 and 6061-T651 showed that several large voids called a dominant void are nucleated at inclusion sites or the second-phase particles ahead of the crack tip and followed by fine voids initiation leading coalescence of the dominant voids with the crack tip. On the other hand, in aluminum alloy 2017-T3, voids are nucleated very close to the crack tip and directly coalesce with the crack tip. FEM computation results suggested that the void nucleation and growth process is closely related to the triaxial stress state ahead of the crack tip. [Copyright &y& Elsevier]

In this study, it is aimed to investigate the damage mechanisms of a DP1000 steel with very fine-grain microstructure and further study the effect of the stress states and loading rates on the damage mechanism. To ensure clear and systematic conclusions are drawn about the effect of stress state on the damage mechanism, careful study is performed on achieving a constant stress state of the fracture initiation spot during the entire deformation history by designing a fracture specimen family for various stress states. The specimens feature an identical dog-bone specimen layout with only differences at the detailed notches and cuts to ease the experimental and analysis effort. After testing the optimized fracture specimens from quasi-static to intermediate loading rates, the scanning electron microscopy is employed to analyze the damage mechanism of the investigated material. Both dimple and shear fracture modes are observed in DP1000 and the underlying damage mechanisms are ferrite-martensite interphase debonding and martensite cracking. By systemic analysis, it is found that the stress state and loading rate have distinct influences on triggering the dominant fracture mode and the underlying damage mechanism. Furthermore, the damage mechanism of DP1000 is compared with DP600 steel and significantly different behavior is observed for two types of dual-phase steels. Unlabelled Image • A set of fracture specimens is proposed for various local stress states under global uniaxial tension. • Distinct plastic strain is found for the central and surface elements for samples under various stress states. • Both ferrite–martensite debonding and martensite cracking are found to be the main damage mechanisms. • A clear correlation of the damage mechanisms with stress state is concluded. • Higher loading rate promotes shear fracture and inhibits dimple fracture. [ABSTRACT FROM AUTHOR]