Full Lamellar Structure Research Articles

Comparative study of the estimation of creep crack growth behaviour of TiAI by using a precrack and a notch CT specimens

The fatigue crack growth behaviour of Ti-48.2%A1 intermetallics (full lamellar structure) strongly depends on the microstructure, that is lamellar alignment, and it grows along the lamellar direction. Therefore, it is difficult to introduce a straight fatigue precrack in a CT specimen to provide a standardized specimen for the estimation of the crack growth rate. Concerning high temperature creep brittle materials, since notch opening displacement at the time of creep crack initiation is small, a sharp notched specimen can keep similar stress singularity as a fatigue precracked specimen. In this paper, comparative research on the characteristics of creep deformation and crack extension was performed by using a fatigue precracked and a sharp notched CT specimens. Based on this result, a mechanical model was constructed and finite element stress analyses on the creep deformation for both specimens were performed. By these analytical results, the different behaviour of the characteristics of creep deformation and crack extension between a precrack and a sharp notch specimens were verified mechanically. Furthermore, the validity of using a sharped notched CT specimen to estimate creep crack growth rate was assured for creep brittle materials.

Effect of extrusion temperature on the microstructure of a powder metallurgy TiAl-based alloy

The microstructure of a P/M Ti47Al2Cr1Nb1Ta (at.%) alloy powder-extruded at different temperatures has been studied using XRD and TEM. The as-prepared powder consists of mainly a phase and small amount of supercooled B/B2 phase. The microstructure of the alloy extruded at the (a + y) phase filed near the eutectoid temperature (~1130 °C) contains coarse B2 grains and a fine-grained (7 + Oh) duplex structure. Ordered (0 and plate-like 0.2 phases are found to exist within some B2 grains. The microstructure of the alloy extruded at mid of the (a + y) phase consists of coarse 0.2 grains and a fine-grained (y + 0.2) duplex structure. The observation of stacking faults coexisted with thin Y plates within the 0.2 grains reveals that stacking faults are intimately related to the formation of the 7 plates within the 02 phase. A nearly full-lamellar structure composed of alternating Y (TiAl) and 0.2 (TbAl) lamellae is developed for the alloy extruded in the a phase field near Ta (~1320 °C). Both lamellar grain size and lamellar interface spacing are finer than those m conventionally processed full-lamellar TiAl alloys.

ＴｉＡｌ金属間化合物のレーザ溶融加工における溶融凝固部の組織変化と機械的特性

Surface melting and fusion welding of a Ti-46 mol%Al-2 mol%Mo intermetallic compound were performed using a 2.5kW CO2 laser. Microstructures of the laser fusion zones were changed depending upon cooling rate. When the cooling rate at 1390K was above 3000K/s in the laser surface melting, α→γ transformation was suppressed and the fusion zone structure resulted in single phase of α2. In the cooling rates less than 3000K/s, massive γ phase was seen together with α2 phase. In the laser welding, whose cooling rates were approximately 80K/s to 1800K/s, the fusion zone microstructure consisted of massive α2, massive γ and lamellar (α2+γ) phases. Full lamellar structure was formed in the cooling rates less than 20K/s. The hardness of the fusion zones increased from 315Hv of the base metal hardness to more than 500Hv with increasing cooling rate. While all of the laser surface melted zones included cracking, in the laser welding, crack-free welds could be obtained at traverse speeds below 50.0mm/s and pre-heating temperatures above 673K. In the tensile test, the laser welded specimens without cracking were fractured in the base metal. However, the elongation of the weld metal itself was 1/10 times that of the base metal. The elongation of the weld metal was increased threefold by a heat treatment at 1173K for 0.6ks.

真空Ｐｓｅｕｄｏ‐ＨＩＰにより合成されたＴｉＡｌ系金属間化合物の組織と強度の温度依存性

A pseudo-HIP (PHIP) process has been applied for the reactive synthesis of near-net shape Ti-Al intermetalic compound materials. Ti-Al compound specimens of full density with composition range Ti-46 to 52at%Al were obtained after PHIP treatment at 1373K to 1623K. The high temperature bending strength of the Ti-48at%Al increased by increasing PHIP temperature. After PHIP treatment at 1623K the volume percent of lamellar structure increased by increasing the Ti content and a full lamellar structure of The Ti-46at%Al gave the highest strength. A structure of matrix gamma and small lamellar grains that was obtained in the range of Al rich composition had high yield strength at high temperature.