Hiroyuki Kokawa

Optimization of grain boundary character distribution for intergranular corrosion resistant 304 stainless steel by twin-induced grain boundary engineering

Abstract The effects of process parameters, pre-strain, annealing temperature, time, etc. on grain boundary character distribution (GBCD) and intergranular corrosion in thermomechanical treatment were examined during grain boundary engineering of type 304 austenitic stainless steel. Slight pre-strain annealing at a relatively low temperature resulted in excellent intergranular corrosion resistance due to optimized GBCD, i.e. the uniform distribution of a high frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network in the material. The optimum distribution can be formed by introduction of low energy segments on migrating random boundaries during twin emission and boundary–boundary reactions in the grain growth without generation of new random boundaries.

Effect of micro-texture on fracture location in friction stir weld of Mg alloy AZ61 during tensile test

Abstract Microstructural evolution of magnesium alloy AZ61 during friction stir welding and details of the relationship between the microstructure and mechanical properties are discussed. Tensile properties of the weld of Mg alloy AZ61 were strongly influenced by crystallographic orientation distribution as well as by grain size and dislocation density.

Hall-Petch relationship in friction stir welds of equal channel angular-pressed aluminium alloys

Abstract The effect of grain size on hardness in the stir zones of friction stir (FS) welds of equal channel angular (ECA)-pressed Al alloys 1050 and 5083 was examined. The hardness was found to be essentially related to grain size through the Hall–Petch relationship in the stir zone of Al alloy 1050. The k H slope of the Hall–Petch equation for the stir zone of Al alloy 1050 was different from the previously reported ones, which was attributed to dynamic recrystallisation during friction stir welding (FSW). On the other hand, the relationship between hardness and grain size in the stir zone of Al alloy 5083 was expressed by the Hall–Petch equation with a change in slope. The change in slope was attributed to the homogeneous distribution of many fine particles.

Sliding behaviour and dislocation structures in aluminium grain boundaries

Abstract Dislocation structures of grain boundaries whose sliding behaviour had already been studied, have been observed by transmission electron microscopy of the slid grain boundaries. The ordered (coincidence) and random boundaries exhibited quite different sliding behaviour and dislocation structures. In the case of ordered boundaries, sliding was difficult and remarkable slide-hardening was shown. Transmission electron microscopy evidenced the presence of a number of EGBDs in these grain boundaries. In contrast, sliding of random boundaries easily took place showing slight slide-hardening. EGBDs were hardly observed in random boundaries. A systematic change in grain boundary misorientation during sliding was observed. An off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations. These results can be qualitatively explained by a dislocation model of sliding in which the absorption of lattice dislocations into the grain boundary is considered t...

Microstructural evolution and its effect on Hall-Petch relationship in friction stir welding of thixomolded mg alloy AZ91D

Microstructural evolution of a thixomolded magnesium (Mg) alloy AZ91D during friction stir welding was investigated. Friction stir welding resulted in a homogeneous microstructure consisting of fine recrystallised α-Mg grains in the thixomolded material. The microstructural homogenisation and refinement was attributed to dynamic recrystallisation accompanied by the dissolution of the eutectic structure during the welding. The grain refinement in the stir zone was effective in increasing the hardness, as predicted by the Hall-Petch equation. The effect of grain size on hardness was smaller than that in conventional and rapidly solidified AZ91. This phenomenon may be explained as being due to the microstructure of the stir zone which consisted of fine equiaxed grains with a high density of dislocations.

Preferential precipitation site of sigma phase in duplex stainless steel weld metal

Duplex stainless steels are characterized by favorable combination of mechanical and corrosion properties, consisting roughly of equal parts of austenite ({gamma}) and ferrite ({alpha}). But exposure to elevated temperatures brings partial decomposition of ferrite to austenite and sigma phase, which deteriorates their properties. Sigma phase forms often at ferrite/austenite ({alpha}/{gamma}) interfaces through nucleation process. The heterogeneous nucleation of sigma phase at an {alpha}/{gamma} interface depends on the chemical driving force and the interfacial energy. Many studies have examined the effect of chemical driving force on sigma phase formation in duplex and austenitic stainless steel weld metals with different chemical compositions, but no detailed report has described the influence of {alpha}/{gamma} interfacial energy on sigma phase nucleation. The Kurdjumov-Sachs (K-S) orientation relationship is accepted to bring a coherent and low energy {alpha}/{gamma} interface in duplex stainless steels. The coherency of {alpha}/{gamma} interface can affect the sigma phase formation. The present study has examined the effect of crystallographic orientation relationship at {alpha}/{gamma} interface on sigma phase formation in a duplex stainless steel weld metal where the chemical element distribution is relatively uniform because of rapid cooling during weld thermal cycle.

Suppression of chromium depletion by grain boundary structural change during twin-induced grain boundary engineering of 304 stainless steel

Abstract An analytical transmission electron microscopic study of a twin-induced grain boundary engineered 304 austenitic stainless steel demonstrated that the chromium depletion at a low-energy boundary segment introduced by twin-emission into a random boundary was smaller than that of the original random boundary after sensitization.

Microstructural characterisation of stir zone containing residual ferrite in friction stir welded 304 austenitic stainless steel

Abstract Friction stir welding was applied to a 2 mm thick 304 austenitic stainless steel plate. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively, which are typically observed in friction stir welds in aluminium alloys. The hardness of the SZ was higher than that of the base material and the maximum hardness was observed at the TMAZ. The higher hardness at the TMAZ was attributed to high densities of dislocations and subboundaries. Microstructural observations revealed that the ferrite was formed along grain boundaries of the austenite matrix in the advancing side of the SZ. It is suggested that the frictional heat due to stirring resulted in the phase transformation of austenite to ferrite and that upon rapid cooling the ferrite was retained in the SZ.

Effect of interfacial microstructure on lap shear strength of friction stir spot weld of aluminium alloy to magnesium alloy

Abstract In the present study, an attempt was made to join two dissimilar light metal alloys which are becoming increasingly familiar in the automotive industry, i.e. AA5083 aluminium alloy and AZ31 magnesium alloy, by the friction stir spot welding process. Lap welds were produced with various welding parameters, and interfacial microstructures and lap shear strengths of these welds were examined. Friction stir spot welding produced defect-free welds, even though a thick interfacial layer composed mainly of intermetallic compounds was present. The thickness of the interfacial layer did not appear to affect the lap shear strength of the weld, whereas the distribution of intermetallic compounds in the interfacial layer did so. Microstructural factors of the interface governing the lap shear strength of the weld were examined.

Effect of grain boundaries on isothermal solidification during transient liquid phase brazing

The influence of liquid penetration at grain boundary regions on the rate of advance of the solid-liquid interface during isothermal solidification of transient liquid phase (TLP) brazed nickel joints has been examined. The test samples used in this study were Ohno-cast nickel with a grain size of >4 mm and a fine-grained nickel with a grain size of around 40 μm. Both Ni-base materials had the same chemical composition. The rate of isothermal solidification was greater when fine-grained nickel was employed during TLP brazing using Ni-11 wt pct P filler metal at 1200 °C. Liquid penetration at grain boundaries accelerates the isothermal solidification process by increasing the effective solid-liquid interfacial area and increasing the rate of solute diffusion into the base material. An analysis of electron channeling patterns has confirmed that random high-angle boundaries have a greater influence on the rate of isothermal solidification than ordered boundaries including small-angle or twin boundaries.