Showing papers by "Kouichi Maruyama published in 2004"

Effects of zinc on creep strength and deformation substructures in Mg–Y alloy

Abstract: Compressive Creep behavior of a hot-rolled Mg–Y binary alloy and Mg–Y–Zn ternary alloys was investigated at 650 K. Creep strength of Mg–Y alloys was significantly improved by the addition of zinc. In Mg–Y–Zn alloys, many “planar faults” were formed on (0 0 0 1) matrix planes at high temperatures even with small addition of 0.02 mol% zinc. The stacking fault energy decreased by the simultaneous addition of yttrium (Y) and zinc and many a -dislocations on basal planes were extended. The separation width of the extended dislocations is stabilized by the segregation of yttrium and zinc and the separation width significantly increases under the interaction of partial dislocations on different basal planes. The excellent creep strength in Mg–Y–Zn was explained by the decrease of the mobility of these widely extended dislocations.

Microstructural Characteristics and Creep Behavior of 45XD TiAl Alloys

Abstract: A near lamellar microstructure and two fine-grained fully lamellar (FGFL) microstructures in Ti-45Al-2Nb-2Mn+0.8 vol%TiB 2 alloy were prepared by selected heat treatments, and the fully lamellar microstructures were aged for stabilizing the lamellar plates. Microstructural examination and tensile creep tests at 760°C showed that the near lamellar microstructure possessed inferior creep resistance due to its coarse lamellar spacing and its larger amount of equiaxed y grains at colony boundaries. The fine lamellar spacing as well as the fine lamellar colony size gave a major contribution to make the minimum creep rates smaller in the fully lamellar TiAl alloys. Since aging treatments stabilized the lamellar microstructures and delayed the degradation process during creep deformation, the aged samples exhibited lower minimum creep rate and longer creep life than the corresponding samples without the aging treatment. These results suggest that a fine as well as stabilized fully lamellar structure is a critical factor to improve the creep resistance of TiAl alloys in terms of short and long-term creep.

Grain Boundary Morphology and Its Effect on Creep of TiAl Alloys

Abstract: Three kinds of microstructures with different grain boundary morphologies and their creep properties of a Ti-47Al-2Nb-2Mn+08 vol%TiB 2 alloy are investigated Tensile creep tests and microstructural examinations indicatethat a stabilized fine-grained fully lamellar (FGFL) microstructure with relatively smooth grain boundaries shows inferior creep resistance A stabilized fully lamellar (FL) microstructure with well-interlocked grain boundaries and wider lamellar spacing yields reduced minimum strain rate and extended creep rupture life Furthermore, a nearly lamellar microstructure (NL) with well-interlocked grain boundaries exhibits better creep resistance than the stabilized FGFL microstructure though it has four times wider lamellar spacing and 15 vol% equiaxed y grains at the grain boundaries, but worse creep resistance than the stabilized FL microstructure Examinations to the deformed microstructures show that grain boundary instability involving spheroidization of the lamellae is a major microstructural degradation process, resulting in fine globular regions at the grain boundaries Voids develop along the grain boundaries, particularly in the fine globular regions, leading to intergranual fracture It is suggested that grain boundary sliding (GBS) is operating in the stabilized FGFL microstructure, and promotes mutually with the grain boundary instability during subsequent creep deformation, resulting in increased minimum strain rate and shortened tertiary stage The well-interlocked grain boundary inhibits the onset of GBS and enhances the grain boundary stability effectively These results demonstrated that the grain boundary stability has a great effect on creep behavior of TiAl Alloys

Creep of lamellar TiAl alloys: degradation, stabilization and design of lamellar boundaries

Abstract: Hard oriented PST crystals of a Ti–48 mol% Al alloy, in which the stress axis is parallel or normal to lamellar boundaries, were creep tested. The lamellar structures contain γ/α 2 boundaries and three types of γ/γ boundaries. Changes of lamellar structures in the PST crystals were examined during high-temperature exposure with (creep) or without applied stress (annealing). Coarsening and spheroidization of the lamellar structures are typical degradation processes observed experimentally. γ/γ lamellar boundaries migrate and coalesce with other γ/γ boundaries and α 2 lamellae dissolve during the high-temperature exposure. These events result in the coarsening and the spheroidization. Thermal stability of each lamellar boundary depends on its boundary type. A higher density of γ/α 2 lamellar boundaries is recommended for retarding the coarsening and the spheroidization, and thereby making a thermally stable lamellar structure. The high density of γ/α 2 boundaries is confirmed experimentally to provide good creep resistance.

Texture dependence of elongation anisotropy in an AZ61 magnesium alloy sheet

Abstract: Extruded and subsequently rolled sheets of AZ61 (Mg-6Al-1Zn in mass%) were tensile tested at room temperature at an initial strain rate of 1×10-3s-1. The effects of extrusion ratio and tensile direction on fracture elongation were investigated. Extrusion ratios were 3.4/100 and 1.0/100 in area reduction. Tensile direction was chosen to be 0, 45 and 90° with respect to the rolling direction. Texture change with strain was also investigated in order to understand a major dislocation slip system.We found that the magnitude of basal-plane tilt with respect to the tensile axis was a controlling factor for a major slip system and for fracture elongation. When the basal planes were tilted by more than approximately 16° from the normal direction towards the tensile direction, a major slip system was basal a dislocation slip and poor ductility was obtained. In contrast, with less tilting than 16°, a major slip system changed to non-basal a dislocation slip, which leads to better ductility. The present results indicated an importance of texture control in tensile ductility of Mg alloys.

Increase in γ/α2 Lamellar Boundary Density and its Effect on Creep Resistance of TiAl Alloy

Abstract: Several lamellar microstructures of a Ti-48 % Al alloy were made by changing heating rate in the α+ dual phase field, and their creep properties were investigated at 1150 K. Average spacing and average length of α2 lamellae decrease with increasing heating rate. The decrease of α2 lamellar spacing is most effective at lower heating rate, and minimum creep rate decreases with increasing the heating rate, since a high density of γ/α2 boundaries stabilizes lamellar microstructure during creep. On the other hand creep rate increases at high heating rate, since α2 lamellar length becomes shorter with increasing heating rate. A reduction of creep rate by one order of magnitude is achieved at the optimum heating rate providing the best combination of narrow spacing and sufficiently long length of α2 lamellae.

The Creep Behaviors of Two Fine-grained XD TiAl Alloys Produced by Similar Heat Treatments

Abstract: The microstructural characteristics and creep behavior of two fine-grained XD TiAl alloys, Ti-45Al and 47Al–2Nb–2Mn+0.8vol%TiB2 (at%), were investigated. A nearly lamellar structure (NL) and two kinds of fully lamellar (FL) structures in both alloys were prepared by selected heat treatments. The results of microstructural examination and tensile creep tests indicate that the 45XD alloy with a NL structure possesses an inferior creep resistance due to its coarse lamellar spacing and larger amount of equiaxed γ grains at the grain boundaries, whereas the same alloy in a FL condition with fine lamellar spacing lowers the minimum creep rates. Contrary to 45XD, the 47XD alloy with a NL structure exhibits the best creep resistance. However, 47XD with a FL structure with finer lamellar spacing shows inferior creep resistance. On the basis of microstructural deformation characteristics, it is suggested that the well-interlocked grain boundary and relatively coarse colony size in FL and NL 47XD inhibit sliding and microstructural degradation at the grain boundaries during creep deformation, resulting in better creep resistance. Therefore, good microstructural stability is essential for improving the creep resistance of these alloys.