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In this paper, an attempt has been made to explore the feasibility of employing friction stir welding (FSW) of 12-mm-thick reduced activation ferritic–martensitic (RAFM) steel plates using a polycrystalline cubic boron nitride (PCBN) tool at a rotation speed of 200 rpm. Deep penetration bead-on-plate welds were successfully produced. Microstructures evolved in stir zone (SZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ) and unaltered base metal zones, and their impact on microhardness was assessed. The SZ and TMAZ revealed high hardness, while HAZ became much softer than the base metal. Several grain boundaries in SZ and TMAZ were found to be free from carbides, and SZ contained martensite laths, Fe3C needles and very high dislocation density. HAZ displayed coarse and coagulated carbides. The inhomogeneous microstructure and hardness variation across the weld joint have been rectified by appropriate post-weld normalising and tempering (PWNT) treatment. Tensile tests were conducted at room and elevated temperatures (450 °C, 500 °C, 550 °C) on transverse samples profiled from as-welded and PWNT states; and the displayed stress–strain behaviour and mechanical properties were compared with those obtained concurrently on base metal at various temperatures. The observed tensile hardening/softening as a function of temperature has been explained based on the substructure evolution. It has been noticed that the variation of the fraction of low-angle grain boundaries and high-angle grain boundaries played a significant role in influencing the tensile deformation behaviour in the as-welded and PWNT states. The low ductility observed at 450 °C has been ascribed to dynamic strain ageing. Alloy displayed ductile fracture, in spite of variation in tensile properties depending upon the state of the material and tensile test temperature. This is the first ever investigation dealing with the tensile behaviour of friction stir welded RAFM steel under different states and conditions. [ABSTRACT FROM AUTHOR]

This study aims to discover the stress-state dependence of the dynamic strain aging (DSA) effect on the deformation and fracture behavior of high-strength dual-phase (DP) steel at different deformation temperatures (25–400°C) and reveal the damage mechanisms under these various configurations. To achieve different stress states, predesigned specimens with different geometric features were used. Scanning electron microscopy was applied to analyze the fracture modes (e.g., dimple or shear mode) and underlying damage mechanism of the investigated material. DSA is present in this DP steel, showing the Portevin-Le Chatelier (PLC) effect with serrated flow behavior, thermal hardening, and blue brittleness phenomena. Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness. Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness. Accordingly, the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect. [ABSTRACT FROM AUTHOR]

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]

The present investigation highlights the creep deformation and rupture behavior of indigenously produced as-cast Mg-10 wt.%Sn-3 wt.%Al-1 wt.%Zn (TAZ1031) and Mg-10 wt.%Sn (T10) alloys. Conventional creep tests were conducted at 423 K (150 °C) in the stress range of 30-45 MPa and at 45 MPa in the temperature range of 423-453 K (150-180 °C) for both the alloys. The nature of creep curves was found to be identical for both the alloys with short primary and secondary creep regime, followed by extended tertiary creep region. The creep resistance of the TAZ1031 alloy was better than the T10 alloy. The former exhibited reduced minimum creep rate and enhanced creep rupture life under identical creep test conditions with respect to the latter. The change of minimum creep rate (έmin) followed a power-law relationship with a change in applied stress (σ) for both the alloys with a significant change in stress exponent (n) value. Activation energy for creep deformation of TAZ1031 alloy (~ 117 kJ/mol) was found to be higher than T10 alloy (~ 105 kJ/mol). Fine secondary Mg2Sn precipitates within the α-Mg matrix and primary Mg2Sn, Mg17Al12 and MgZn2 precipitates along the grain boundaries improved the creep properties of TAZ1031 alloy. On the contrary, the threshold stress required for creep deformation was found to be significantly low in TAZ1031 alloy due to the presence of Mg17Al12. Both creep damage analysis and dimpled fracture surfaces revealed necking dominated creep deformation for the alloys. SEM studies revealed the absence of creep cavities along grain/dendritic boundaries for the investigated alloys, which substantiated the formation of dimpled fracture surfaces during creep deformation. [ABSTRACT FROM AUTHOR]

Deformation characteristic of centrifugal casting 25Mn steel was investigated by compression tests, and then processing maps were established. According to the deformation parameters identified from the established processing maps and hot ring rolling (HRR) process, the industrial test for the 25Mn ring blank was performed. Optical microscope (OM) and electron backscatter diffraction (EBSD) techniques were used for detecting grain boundary features and textures of deformation structures. The morphologies and mechanisms of tensile and impact fracture were revealed. The results show that softening effect plays a dominant role in higher temperatures of 1050-1150 °C and strain rates lower than 0.1 s. The average grain size of the rolled 25Mn ring is about 28 μm, but the grains are more coarse and inhomogeneous on the middle layer than that on rest of the areas. The texture on the outer layer is characterized by strong {110} 〈112〉 and weak {112} 〈111〉, followed by {001} 〈100〉 and {001} 〈110〉 on the inner layer and {110} 〈110〉 on the center layer, which is mainly associated with the shear deformation. The rolled ring with precise geometrical dimensions and sound mechanical properties is fabricated by HRR. Tensile fracture is composed of clear river-shaped pattern and a little dimple near the inner layer and outer layer, and the fracture mechanism is mainly quasi-cleavage fracture, accompanied by dimple fracture. The morphologies of impact fracture consist of tear ridge and cleavage platform. [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]

The purpose of this study is to evaluate the AE characteristics for the basemetal, PWHT (post-weld heat treatment) and weldment specimens of SA-516 steel during fracture testing. Four-point bending and AE tests were conducted simultaneously. AE signals were emitted in the process of plastic deformation. AE signal strength and amplitude of the weldment was the strongest, followed by PWHT specimen and basemetal. More AE signals were emitted from the weldment samples because of the oxides, and discontinuous mechanical properties. AE signal strength and amplitude for the basemetal or PWHT specimen decreased remarkably compared to the weldment because of lower strength. Pre-cracked specimens emitted even lower event counts than the corresponding blunt notched specimens. Dimple fracture from void coalescence mechanism is associated with low-level AE signal strength for the basemetal or PWHT. Tearing mode and dimple formation were shown on the fracture surfaces of the weldment, but only a small fraction produced detectable AE. [ABSTRACT FROM AUTHOR]

Although fortunately rare, fracture of implants causes significant problems for both clinicians and patients. The major cause of a fractured implant may be corrosion fatigue fracture. To investigate how to increase the fatigue life and corrosion resistance of dental implants, the surface morphology of six Steri-Oss fractured implants was analyzed. The period of implantation after loading in patient jaws varied between 23 months and 37 months. The topography and surface chemical composition were studied with electron probe micro-analysis (point mapping, energy dispersive x-ray spectroscopy) and field emission scanning electron microscopy. All samples were fractured at the screw root and the crest formed a keen-edged shape. The five samples were fractured at the first thread of the fixture and one sample at the third thread of fixture. The fatigue cracks were mainly nucleated and grown at scratches occurring for the screw root and crest formation and the cervix portion of the implant having a small curvature. The pits were nucleated in the vicinity of inclusions such as SiO2 and corrosion fatigue cracking was predominantly propagated. Corrosion products were found on the opposite side of the starting point of corrosion fatigue crack. From observations of fatigue striations, it is possible to predict the life time of fractured implants and estimate the cleavage fracture and dimple fracture of implants. In this study, analysis of fractured surfaces revealed the characteristics of the implant materials, problems of design, fatigue life, and manufacturing process. In order to protect against corrosion fatigue fractures and prolong the fatigue life of dental implants, we must consider the implant design, implant manufacturing, and surface treatment of the implant materials. [ABSTRACT FROM AUTHOR]

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]