Properties of TiAl sheet pack-rolled at temperatures below 1000°C

TiAl alloy specimens with microcrystalline (MC, grain size is 5 μm) and submicrocrystalline (SMC, grain size is 0.4 μm) structures were successfully pack-rolled to sheet with a thickness down to 0.4 mm in the temperature ranges of 800°C to 1000°C and 900°C to 1000°C, respectively. An 18/10 stainless steel was used as a rather inexpensive can material for pack-rolling. Unidirectional rolling and bidirectional cross-rolling were used. Because of a wider temperature range for pack rolling and a lower cost for production of the alloy preforms, the microcrystalline structure was found to be a better microstructural condition for the TiAl sheet rolling. The sheet produced by unidirectional rolling had an anisotropy of mechanical properties, i.e., strength was smaller and elongation larger in the rolling direction than in a transverse direction. The anisotropy decreased when the rolling temperature increased. The bidirectional rolling led to in-plane isotropic properties of the sheet. The produced sheet showed elongation of about 3 pct at room temperature, brittle-to-ductile transition in the temperature range of 750°C to 850°C, and superplastic behavior in the temperature range 900°C to 1000°C. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Influence of foreign particles on fatigue behavior of Ti-6Al-4V prealloyed powder compacts

The effect of three types of contaminants on the fatigue life of prealloyed Plasma Rotating Electrode Process (PREP) Ti-6A1-4V hot isostatically pressed (HIP’d) powder compacts was studied. Ultraclean powder was seeded separately with SiO₂, A1₂O₃, and 316 stainless steel (SS) contaminants of 50 μm, 150 μ, and 350 μm nominal size, a total of nine conditions. Seeded compacts were fatigue tested at room temperature and the results were compared to those of an unseeded baseline compact. It was found that a substantial loss in fatigue life occurred even at the smallest seeded contaminants used in this work. Angular nonmetallic SiO₂ and A1₂O₃ contaminants were found to be more detrimental to fatigue strength than spherical metallic 316 SS contaminants of the same size range also indicating a shape effect. The loss of fatigue life suggests that at the high stress levels there is almost no crack initiation period and fatigue lives are controlled mainly by crack growth.     

Interrelations between fracture toughness and other mechanical properties in titanium alloys

Critical stress intensity factors and general tensile properties were measured for a class of metastable beta and alpha-beta titanium alloys with a limited number of compositions but processed to have a wide variety of strength levels and microstructures. These data were used to test thirteen theoretical relations between the fracture toughness and tensile parameters. Many of the theoretical relations had been proposed previously, but several new forms were derived in the present work. The correlation showed that for alloys exhibiting a limited range of microstructures, the simpler correlations gave the better fits withK _Ic α γ⁻¹specifically being the best one. For correlations of alloys with a wide range of microstructures, more complex correlations which included microstructural parameters were found to be superior.     

Synthesis of Alloy Ti ― 6Al ― 4V with Low Residual Porosity by a Powder Metallurgy Method

The possibility of producing titanium alloy Ti 6Al 4V with minimal residual porosity from mixtures of elemental powders by the method of pressing and sintering without hot deformation during or after sintering was investigated. Various powder mixtures based on titanium and titanium hydride with alloying additions of either elemental powders having different particle sizes, or master alloys, were studied. It was shown that the synthesis of Ti 6Al 4V from mixtures of titanium hydride and master alloys is optimal with respect to the attainment of high relative density. In this case the sintered material has density up to 99%, homogeneous microstructure with relatively small (100-120 m) -phase grains, and a low concentration of impurities, in particular oxygen, which provide a high level of mechanical properties (_ten = 970 MPa, = 6%).     

Recrystallization and grain growth in metastable beta III titanium alloy

The progress of recrystallization and subsequent grain growth has been systematically investigated in a metastable beta titanium alloy (Ti-11.5 Mo-6 Zr-4.5 Sn). Quantitative evaluation of the kinetics of these processes over a wide range of temperature, deformation, and initial grain sizes has been performed. For a given deformation, the average grain boundary velocity, decreasing with the reciprocal of annealing time, suggests the occurrences of recovery with second order kinetics concurrent with the recrystallization. The amount of deformation, varying from 20 to 80 pct cold reduction and proportional to the stored energy of deformation in the alloy, increases the average grain boundary migration rate during recrystallization by three orders of magnitude. The temperature dependence of the recrystallization rate, however, remains unaffected by the amount of deformation at 83 kcal/mole (347 kJ/mole). The isothermal grain growth kinetics follow the power law such that the time exponent of the process remains at a value of 0.35 at most annealing temperatures. The excellent agreement between the driving force exponent of recrystallization and the time exponent of grain growth based on a model which relates the driving force dependence of the rates of both processes, clearly suggests that the kinetics of these processes are controlled by a single mechanism,i.e. impurity dependent boundary migration. This paper is based on a presentation made at a symposium on “Recovery, Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.