[en] The present paper analyzed the fracture behavior of an RS/PM Al-Fe-V-Si alloy after high temperature exposure, in particular the effects of coarse Al13Fe4 particles formed during the exposure at 480 C. In situ SEM observations of crack opening processes found that brittle cleavage fracture occurred at these coarse Al13Fe4 particles, leading to the reduction in strength, fracture toughness, and ductility of the Al-Fe-V-Si alloy exposed to high temperatures. The results of fracture toughness were also interpreted using a simplified ductile fracture initiation model based on a basic assumption that crack extension starts to occur at a certain critical strain over a microstructurally significant critical distance. This model correlates microstructure to fracture toughness, confirming that the presence of coarse Al13Fe4 particles is the main metallurgical factor for the embrittlement phenomenon in the Al-Fe-V-Si alloy after high temperature exposure. 12 refs

[en] The correlation of microstructure and wear resistance of various molybdenum blend coatings applicable to automotive parts was investigated in this study. Five types of spray powders, one of which was pure molybdenum powder and the others were blends of brass, bronze, and aluminum alloy powders with molybdenum powder, were deposited on a low-carbon steel substrate by atmospheric plasma spraying (APS). Microstructural analysis of the coatings showed that they consisted of a curved lamellar structure formed by elongated splats, with hard phases that formed during spraying being homogeneously distributed in the molybdenum matrix. The wear test results revealed that the blend coatings showed better wear resistance than the pure molybdenum coating because they contained a number of hard phases. In particular, the molybdenum coating blended with bronze and aluminum alloy powders and the counterpart material showed an excellent wear resistance due to the presence of hard phases, such as CuAl₂ and Cu₉Al₄. In order to improve overall wear properties for the coating and the counterpart material, appropriate spray powders should be blended with molybdenum powders to form hard phases in the coatings

[en] TEM is used to investigate microstructural development in a rapidly solidified Al-Fe-V-Si alloy. The as-cast microstructure of a rapidly solidified Al-Fe-V-Si alloy was found to vary depending on casting conditions and also through the thickness of ribbon. For completely Zone A ribbon, intercellular phase consists of a microquasi-crystalline phase, while for the Zone A and Zone B mixed ribbon, it consists of a silicide phase. In either case, formation of globular particles of a cluster microquasi-crystalline phase is observed near the air side of the ribbon. Annealing study shows significant differences in the final microstructure depending on the initial status of the ribbon. Completely Zone A ribbon, whose microstructure is composed of a microquasi-crystalline phase, results in a very coarse microstructure after annealing as compared to the Zone A and Zone B mixed ribbon. This result has important implications for the development of high-performance elevated-temperature Al alloys. 12 refs

[en] An investigation was conducted into the effect of grain size on fracture toughness in the transition temperature region of Mn-Mo-Ni low-alloy steels used for nuclear pressure vessels. Three kinds of steels with different austenite grain sizes (AGS) were fabricated by varying the contents of Al and N, and their microstructures and mechanical properties were examined. Elastic-plastic cleavage fracture toughness, K_Jc, was determined by three-point bend tests of precracked Charpy V-notch (PCVN) specimens according to ASTM E1921 standard test method. When the AGS decreased, the total number of carbides increased, while the size and the aspect ratio of carbides decreased. Local fracture stresses, estimated from a theoretical stress distribution in front of a crack tip, were found to be mainly determined by the 92nd% size of carbides. Cross-sectional areas beneath fracture surfaces were observed to understand microstructural features to affect the cleavage crack propagation behavior. The results showed that measured cleavage fracture units were smaller than AGSs, indicating that packet boundaries as well as austenite grain boundaries played an important role in the cleavage crack propagation. Based on the electron back-scatter diffraction (EBSD) results, the cleavage fracture units could also be matched with the effective grain sizes determined by the misorientation tolerance angle of 25 deg.

[en] The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin α platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner α platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime

[en] The present study is concerned with the tempering effect in improving the hardness of a vanadium carbide (VC)/carbon steel surface-alloyed material fabricated by high-energy electron-beam irradiation. The mixture of VC powders and flux (50%MgO-50%CaO) was placed on a plain carbon steel substrate, and then electron beam was irradiated. The surface-alloyed layer of 1.8 mm in thickness was homogeneously formed without defects. The microstructural analysis indicated that coarse VC particles were formed along solidification cell boundaries, and the matrix inside cells was mostly composed of lath-type martensite and fine cuboidal VC particles. A large amount of these VC particles in the lath-type martensitic matrix provided hardness four times greater than that of the substrate. When the VC/steel surface-alloyed material was tempered, fine VC particles precipitated in the tempered martensitic matrix, thereby leading to additional hardness increase. In addition, reduction of residual stress and an increase in fracture toughness could be expected

[en] This study is concerned with the microstructural analysis and the improvement of hardness and wear resistance of SiC/Ti-6Al-4V surface composites fabricated by high-energy electron beam. The mixtures of SiC, SiC+TiC, or SiC+Ti powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then electron beam was irradiated on these mixtures. In the specimens processed with flux addition, the surface composite layers of 1.7-2.1 mm in thickness were homogeneously formed without defects, and contained a large amount (over 44 vol.%) of precipitates such as Ti₅Si₃ and TiC in the martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface composite layers could be explained from a Ti-Si-C ternary phase diagram, and greatly improved hardness and wear resistance 2 and 5-25 times higher than the Ti alloy substrate, respectively. These findings suggested that high-energy electron beam irradiation was useful for the development of Ti-base surface composites with improved hardness and wear properties

[en] The effects of volume fraction and stability of retained austenite on formability of a 0.1C-1.5Si-1.5Mn-0.5Cu (hereafter all in wt.%) TRIP-aided cold-rolled steel sheet was investigated after various heat treatments (intercritical annealing and isothermal treatment). Tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes of retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite parameters such as stability and initial volume fraction. The formability was improved with increasing volume fraction of retained austenite. However, when the volume fraction of retained austenite was same, the better formability was obtained in the specimens with the higher stability of retained austenite. This indicated that the strain-induced transformation of retained austenite to martensite could be stably progressed, thereby leading to the improvement of formability. Thus, the heat-treatment conditions should be established in consideration of the maximum volume fraction and high stability of retained austenite, and the optimal conditions were found to be intercritical annealing in the temperature range at which the austenite volume fraction was about 50%, followed by isothermal treatment at M_s temperature

[en] In the present study, the correlation of microstructural factors with yield ratio and uniform elongation was investigated in API X80 linepipe steels with different microstructures by varying thermomechanical control process conditions. The grain size and microstructure fraction of the API X80 linepipe steels were quantitatively measured by EBSD analysis and then tensile tests were carried out on them. Although all the steels showed complex microstructures of polygonal ferrite (PF), and bainitic microstructures such as acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), they had different grain sizes and microstructure fractions. The D steel with the finest grain size showed the highest yield strength due to grain refinement strengthening even though it had a relatively high fraction of PF with lower yield strength. The yield ratio usually increases with decreasing grain size because grain refinement typically produces an increase in yield strength and a decrease in work hardening due to the accumulation of dislocations at grain boundaries. On the other hand, uniform elongation increases with increasing PF fraction because an increase in the PF fraction eases slip and thus reduces plastic instability caused by accumulated dislocations at grain boundaries. Consequently, the microstructural factors affecting the yield ratio, uniform elongation, and work hardening exponent of the API X80 linepipe steels are different, and the work hardening exponent is considered to be a parameter affected by stress and strain simultaneously. Therefore, it was confirmed that there is no general correlation between yield ratios, uniform elongations, and work hardening exponents in the API X80 linepipe steels investigated in this study, and previously reported API linepipe steels.

[en] The aim of this article is to elucidate the influence of reduction ratio during roll bonding on the microstructural evolution, mechanical properties and room-temperature formability of Al-Cu 2-ply clad metal. The evolution of the interface microstructure was first characterized by a scanning electron microscope (SEM) and transmission electron microscope (TEM) attached with energy dispersive spectroscopy (EDS). The presence of an intermetallic compound as well as severe grain refinement was detected at the interface of the Al-Cu bimetal fabricated under the highest reduction ratio of 65% adopted in this study. Taking into account the difference of the microstructure with a reduction the ratio, mechanical properties and bonding strength were then evaluated by uniaxial tensile and peel tests. It was observed that the bonding strength, elongation and tensile strength for Al-Cu 2-ply sheets were incomparably reduced by decreasing the reduction ratio during the roll bonding process, which directly correlated with the microstructural evolution at the interface. Moreover, the higher reduction ratio during the roll bonding, the more room temperature formability could be achieved for Al-Cu 2-ply sheet by applying both three-point bending and Erichsen tests.