Masao Takeyama

A newly developed hot worked TiAl alloy for blades and structural components

Abstract A new TiAl alloy has been developed, with a composition of Ti–42Al–5Mn (at.%). The manufacturing process employs a conventional hot working, in which the cast ingot is heated to the β+α region, subsequently forged or rolled during cooling. Manufacturing of TiAl blades and structural components by this process is demonstrated.

Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

Phase equilibria and microstructural control of gamma TiAl based alloys

Abstract Phase equilibria among the β (bcc or B2), α (hcp) and γ (L10) phases in TiAlM ternary systems at elevated temperatures have been studied, where the M is β stabilizing element for pure titanium. The β(B2) + α + γ three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the α ↔ β allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the γ phase can be in equilibrium with the β(B2) phase at relatively low temperatures in the ternary systems and create a reaction pathway of α → β(B2) + γ. Upon cooling along the same pathway of α → γ as in the binary alloy, the addition of third element affects the transformation rate, and the massive γ structure is found to be formed even under slow cooling rate.

Phase equilibria in TiAl alloys containing 10 and 20 at% Nb at 1473 K

Alloys with compositions around Ti-24Al-11Nb in the Ti-Al-Nb ternary system have been studied quite extensively because they constitute the base compositions of the super [alpha]2 titanium aluminides. In contrast to the above alloys, the [gamma] titanium aluminides containing niobium at the same concentration level have attracted less attention until quite recently. An earlier study on phase equilibria suggested the existence of an isolated ternary T[sub 2] phase field at a composition region around 45Ti-45Al-10Nb at 1,473 K. A recent investigation casts doubt on the existence of the T[sub 2] phase as an equilibrium phase. In this study, the phase equilibria of alloys with compositions around Ti-45Al-10Nb and Ti-40Al-20Nb have been examined to establish a [gamma] TiAl part of the isothermal section of the Ti-Al-Nb ternary system at 1,473 K.

Eutectoid transformations accompanied by ordering

Abstract Eutectoid transformations accompanied by ordering, unlike ordinary ones, proceed through non-pearlitic modes of transformations. Eutectoid invariants are classified into two categories in binary systems. The eutectoid invariant of A3(α)→D019(α2) + L10(γ) in the Ti-Al binary system belongs to the first category, in which one product phase has an ordered structure of a parent phase. Its transformation product exhibits a γ α 2 lamellar structure consisting of nearly perfectly aligned alternate lamellae of γ and α2, which is formed by precipitation of γ plates in either α or α2 matrix with the Blackburn orientation relationship. The eutectoid invariant of A1(γ)→D022(γ″) + L12(γ′) in the Ni3V-Ni3Al pseudo-binary system is an example of the second category, in which both product phases have different ordered structures of a parent phase. The transformation of a 75Ni-18V-7Al alloy results in a ‘checkerboard’ pattern consisting of a periodic array of columns of γ′ and two γ″ orientation variants, which are formed by phase separation simultaneous with ordering.

Microstructural evolution of massively transformed γ-TiAl during isothermal aging

{gamma}-TiAl base alloys have been expected as high-temperature structural materials because of their superior properties such as high specific elastic modulus and strength, high melting temperature and reasonable oxidation resistance. Recently, the authors reported that the microstructure changes from a massively transformed {gamma} grain structure to a equiaxed {gamma} grain structure involving fine {alpha}{sub 2} particles both in the {gamma} grain interiors and at the {gamma} grain boundaries, when a Ti-48at.%Al alloy quenched from an {alpha} phase field was isothermally aged in an {alpha}{sub 2} + {gamma} two phase field. More recently, Wen et al. suggested that this reaction occurs through the following two stages: (1) precipitation of thin {alpha}{sub 2} plates and small {alpha}{sub 2} particles, and (2) spheroidization of the fine {alpha}{sub 2} plates and growth of the {alpha}{sub 2} particles. However, the detailed formation mechanism of the {alpha}{sub 2} particles, especially the spheroidization mechanism of the fine {alpha}{sub 2} plates, is not clarified yet. The purpose of this study is to investigate the microstructural development of the massively transformed {gamma} structure during isothermal aging, and to discuss the {gamma} {yields} {alpha}({alpha}{sub 2}) phase transformation.

Strengthening of nickel -base superalloys for nuclear heat exchanger application

Strengthening mechanisms of nickel-base superalloys have been discussed with the background of the Japanese research and development activities in this field. As candidates for materials of intermediate heat exchangers which will be used for a future programme of nuclear steelmaking systems, two kinds of alloys have successfully been developed in Japan. The designs of these alloys have been reviewed from metallurgical aspects including their composition and creep properties. In addition to the conventional methods to strengthen these alloys, such as solid solution hardening or particle precipitation hardening, a grain-boundary precipitation strengthening due to tungsten-rich α2 phase in the Ni-Cr-W system, would be expected as a further advanced method.

Novel Concept of Austenitic Heat Resistant Steels Strengthened by Intermetallics

Alloy design concept for the development of a new class of austenitic heat resistant steels strengthened by Fe2M Laves phases (M: transition metals) has been proposed. The phase diagram studies on Fe-Ni-M ternary systems demonstrate that Fe2Nb with C14 structure is the most promising, because more than 40at% Ni can dissolve into the Fe sublattice sites and large γ+Fe2Nb two-phase region exists along the equi-niobium concentration direction. The control of the c/a ratio of the Laves phase using the composition homogeneity region by alloying makes it possible to disperse the Laves phase finely in the γ matrix. Based on the knowledge, a model alloy Fe-20Cr- 30Ni-2Nb (at%) was proposed and the TTP diagram of the Laves phase was constructed. The Laves phase homogeneously nucleates in the matrix and its fine morphology remains almost unchanged even after long-term aging at 1073K.

Transmission electron microscopy on the phase equilibria among β, α and α2 phases in Ti–Al binary system

Abstract The phase relationships among β-Ti, α-Ti and α2-Ti3Al phases in Ti–Al binary system at temperatures around 1473 K have been examined by transmission electron microscopy, particularly paying attention to the size of anti-phase domains (APDs), using Ti–(25–35) at.%Al alloys quenched from various high-temperature phase regions. Regardless of the phases present at high temperatures, all alloys become α2 single phase in the quenched state. The region of α2 phase prior to quench exhibits no APDs in the quenched state. The prior β phase region shows finer APDs with an average size of 40 nm, whereas the prior α phase region exhibits large APDs of 150 nm in size. Based on the difference in APD size as a criterion, together with hardness and transformed morphology, the phase regions at given temperatures can be determined. The results suggest that the α2 phase is congruently in equilibrium with α phase at 1465 K (congruent type), rather than in equilibrium with β phase straight with two peritectoid reactions (peritectoid type). From thermodynamic consideration, oxygen content in alloys is not a factor to change the phase equilibria from congruent type to peritectoid type among these phases.

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Abstract Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1 50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.