Haruyuki Inui

High-temperature structural intermetallics

In the last one and a half decades, a great deal of fundamental and developmental research has been made on high-temperature structural intermetallics aiming at the implementation of these intermetallics in aerospace, automotive and land-based applications. These intermetallics include aluminides formed with either titanium, nickel or iron and silicides formed with transition metals. Of these high-temperature intermetallics, TiAl-based alloys with great potential in both aerospace and automotive applications have been attracting particular attention. Recently TiAl turbocharger wheels have finally started being used for turbochargers for commercial passenger cars of a special type. The current status of the research and development of these high-temperature intermetallics is summarized and a perspective on what directions future research and development of high-temperature intermetallics should take is provided.

Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl

Abstract Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle o between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle o but its dependence on the angle o is not symmetrical with respect to o = 45°. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for o = 31°. The yield stress also strongly depends on the angle o but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.

Ordered domains in TiAl coexisting with Ti3Al in the lamellar structure of Ti-rich TiAl compounds

Abstract The morphology and distribution of ordered domains and their relative orientation in the TiAl phase coexisting with the Ti3Al phase in the lamellar structure of Ti-rich TiAl compounds have been studied by transmission electron microscopy. Six different types of ordered domain exist in the TiAl phase because of the orientation relationship between the TiAl and Ti3Al phases, (111)TiAl//(0001)Ti3Al and [110]TiAl//〈1120〉Ti3Al. The [110] direction in all the types of TiAl ordered domain is exactly parallel to the 〈110〉 direction of Ti3Al while 〈0] directions are rotated by 0·2-0·3° on the (111) lamellar planes with respect to the 〈110〉 direction of Ti3Al. The TiAl ordered domains exist not only across the lamellar boundaries but also within each of TiAl lamellae. The lamellar domain boundaries, that is the boundaries between the two neighbouring TiAl lamellae, are flat and parallel to both (111)TiAl and (0001)Ti3Al, while the domain boundaries existing within each of TiAl lamellae do not have any pref...

Temperature dependence of yield stress, deformation mode and deformation structure in single crystals of TiAl (Ti−56 at.% Al)

Abstract The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at.% Al has been studied in compression as a function of crystal orientation in the temperature range from −196 to 1100°C. The profile of yield stress-temperature curves for all orientations studied can be divided into three temperature regions; the yield stress rapidly decreases with increasing temperature at both low and high temperatures and an anomalous increase in yield stress is observed at intermediate temperatures, exhibiting a peak at 700–1000°C depending on crystal orientation. Both ordinary and [101] superlattice slip exhibit an anomalous increase in critical resolved shear stress (CRSS). However, the extent of the anomaly associated with the former slip is much smaller than that for the latter slip. The CRSS for [101] slip depends on crystal orientation. In the anomalous temperature region, the CRSS exhibits very small strain-rate sensitivity and thermal reversibility24Jun1996. Superlattice disloc...

Plastic deformation of MoSi2 single crystals

Abstract The deformation behaviour of MoSi2 single crystals has been studied as a function of crystal orientation in the temperature range from - 196 to 1500°C in compression. While [001]-oriented crystals can be plastically deformed only at high temperatures above 1300°C, plastic flow is possible from temperatures as low as room temperature for single crystals with orientations other than [001]. Five slip systems { 110)(111], {011)(100], {010)(100], {023)(100] and {013)(331] are identified to be operative, depending on crystal orientation. An anomalous increase in critical resolved shear stress (CRSS) is observed in the intermediate-temperature range for the former three slip systems. The anomalous temperature range as well as the magnitude of such an anomaly varies with slip system. The Schmid law is generally valid for the {110)(111], {011)(100] and (023)(100] slip systems. In particular, this is the case even in the temperature range of the anomaly in CRSS for the {110)(111j system. In contrast, the C...

Gamma Titanium Aluminide Alloys

Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, the understanding of their microstructural characteristics and property/microstructure relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.

Deformation structures in ti-rich tial polysynthetically twinned crystals

Abstract Room-temperature deformation structures in Ti-rich TiAl polysynthetically twinned crystals, which contain only a single grain with the two-phase TiAl/Ti3Al lamellar structure, have been studied for both the easy (shear parallel to the lamellar boundaries) and the hard (shear across the lamellar boundaries) types of deformation by transmission electron microscopy. For both types of deformation, numerous deformation twins of the {111}〈112] type and ordinary dislocations with b=½〈110] were observed. A few superlattice dislocations with b=½〈112] were also observed but those with b = 〈101] were hardly observed. Although these features are common to the two types of deformation, a clear difference between the two morphology of ordinary dislocations was observed. Ordinary dislocations introduced by the easy type of deformation do not align along any crystallographic directions and tend to cross-slip to form three-dimensional tangles while those introduced by the hard type of deformation tend to align al...

Directional solidification of TiAl-base alloys

Abstract Mechanical properties of the lamellar microstructure of TiAl-base alloys are extremely anisotropic with respect to the lamellar orientation. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of strength and ductility. Unfortunately, simple casting operations often lead to a solidification texture with the lamellar boundaries all perpendicular to the heat flow direction. This difficulty can be overcome by directionally solidifying TiAl-base alloys. We have been performing directional solidification experiments with and without using a seeding technique. The current status of directional solidification of TiAl-base alloys is reviewed.

Directional solidification and microstructural control of the TiAlTi3Al lamellar microstructure in TiAlSi alloys

Composite microstructures were grown from TiAlSi alloys with the gamma/alpha2 lamellar microstructure aligned parallel to the growth direction by directional solidification techniques using a seed material. Within the composition range of 40–50 at% Al, the addition of Si to TiAl shifts the primary alpha region towards a much lower Al content. At a composition of Ti43Al3Si, either alpha or alpha2 can be stable, from melting temperature to room temperature and the orientation of the lamellar microstructure can be controlled using a seed material. In addition to the gamma/alpha2 lamellae, large Ti5Si3 particles which formed from the liquid, and much smaller Ti5Si3 particles which formed from an eutectoid reaction were observed. The room temperature mechanical properties were determined by tensile and three-point bend tests. From bend specimens oriented with the notch parallel to the lamellar microstructure, the Ti43Al3Si alloy was found to have a greater fracture toughness than a TiAl-PST crystal of the same orientation.

Temperature dependence of thermal expansion and elastic constants of single crystals of ZrB2 and the suitability of ZrB2 as a substrate for GaN film

Coefficients of thermal expansion (CTE) and elastic constants of single crystals of ZrB2 have been determined in the temperature ranges from room temperature to 1073 K and from room temperature to 1373 K, respectively. The elastic constants of ZrB2 are best characterized by the large value of the Young modulus (as high as 500 GPa) and the small values of the Poisson ratio (0.13–0.15), indicating the high stiffness and hardness and the brittleness, respectively. The values of CTE along the a- and c-axis directions are 6.66×10−6 and 6.93×10−6 K−1, respectively, when averaged over the temperature range from room temperature to 1073 K. The CTE value along the a-axis direction of ZrB2 is only moderately larger than the corresponding value for GaN. This together with the small lattice mismatch along the a-axis direction between ZrB2 and GaN in the heteroepitaxial orientation relationship of (0001)GaN//(0001)ZrB2 and 〈1120〉GaN//〈1120〉ZrB2 indicate that only a small compressive stress develops in the GaN thin-f...