1997 TMS Annual Meeting: Wednesday Abstracts

LIGHT WEIGHT ALLOYS FOR AEROSPACE APPLICATION IV: Session V: Ti Alloys

Session Chairperson: Soo Woo Nam, Dept. of Materials Science & Eng., Korea Adv. Inst. of Science and Technology, Taejon 305-701 Korea

8:30 am

AN EVALUATION OF THE DAMAGE TOLERANCE CHARACTERISTICS OF Ti6Al2Sn2Zr2Mo2Cr SHEET: T.D. Bayha, Lockheed Martin Aeronautical Systems, Marietta, GA 30063; D.L. Yaney, Lockheed Martin Missiles and Space Palo Alto, CA 94304

Ti-6Al-2Sn-2Zr-2Mo-2Cr (Ti-6-2-2-2-2) is an alpha-beta titanium alloy typically utilized in thick section, damage tolerance-driven aerospace applications. This alloy was developed for deep hardenability, with high strength, and moderate fracture toughness. Ti-62222, research efforts to date have been primarily focused on rolled plate and forged products; however, there is interest in developing thin sheet material for wing skin applications. A rolling practice to produce Ti-6-2-2-2-2 sheet materials for conventional and superplastically formed structure has been developed. The work described here characterizes the properties, microstructure, and fracture behavior or Ti-6­2-2-2-2 solution treated and aged sheet samples subjected to tensile and cyclic loadings. Tensile and fatigue crack growth tests were performed over a wide temperature range (-65°F to 500°F) to assess the suitability of the sheet in various applications. Crack growth testing was performed utilizing two Rratios at each temperature of interest. Test data indicates that the crack growth behavior of Ti-6-2222 is superior to Ti-6Al-4V sheet in the Paris regime under comparable test conditions. A complete metallurgical analysis utilizing both scanning electron and transmission microscopy was conducted to develop property/microstructure relationships, and selected test specimens were examined to provide insight regarding fracture mechanisms with respect to typical cyclic loading conditions.

8:50 am

EFFECT OF / VOLUME FRACTION ON THE SUPERPLASTIC DEFORMATION BEHAVIOR OF Ti6Al4V ALLOY: J.S. Kim, D.M. Li, C.S. Lee, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790784, Korea

Present investigation has been made to study the superplastic deformation behavior of Ti-6AI4V alloy with respect to the variation of / volume fraction. Load relaxation tests were employed at 600 and 800°C, to obtain stressstrain rate curves for the microstructures of 3 and 16µm grain sizes. Superplastic deformation test was also carried out to confirm the results of load relaxation test. The experimental results were analyzed by the theory of inelastic deformation which consists of two mechanisms, i.e., one is the grain matrix deformation and the other is the phase/grain boundary sliding. Grain matrix deformation is dominant at 600°C and well described by the state equation when permeability parameter, p, is 0.15. Phase boundary sliding is dominant at 800°C and also consistent with the viscous flow equation with Mg=2.

9:10 am

CHARACTERIZATION OF INTERMETALLIC PRECIPITATES IN A / TITANIUM ALLOY: X.D. Zhang*, J.M.K. Wiezorek*, D.J. Evans**, L. Fraser*; *The Ohio State University, Department of Materials Science and Engineering, 2041 College Rd, Columbus, OH 43210; **Materials Directorate, Wright Laboratory, WL/MLIM, Wright Patterson AFB, Dayton, OH 45433

A two phase - titanium alloy, Ti6Al2Mo2Cr2Sn2Zr0.2Si (Ti-6-22-22S), has recently been reconsidered as a structural material for aircraft applications. This alloy exhibits specific strength and fracture toughness superior to that of Ti6A14V. However, similar to other alpha-beta titanium alloys, microstructural stability is one of the major concerns regarding industrial application of Ti62222S. For instance, precipitation of additional phases during long term high temperature exposure is predicted to affect significantly the performance of components in service. Three different precipitates have been observed in Ti-6-2222S alloys, namely silicides, and 2 phases. The presence of intermetallic precipitates, such as 2, in the parent a matrix has been reported to result in brittle behaviour of the / alloys due to the formation of intense planar slip on {101U} prismatic planes. The present paper presents results of the characterization of intermetallic 2 precipitates in the phase of Ti62222S by methods of scanning ant transmission electron microscopy (SISM and TEM respectively). The precipitation of 2 precipitates has been studied as a function of heat treatment conditions in order to examine the microstructural stability and growth behaviour. The observed dispersion of small, brittle intermetalllic precipitates is expected to have a detrimental effect on the fracture performance of Ti-6-22-22S. Hence, it is important to ascertain the origin of this precipitation phenomenon. The 2 precipitates have been characterized chemically and structurally by high spatial resolution microanalysis and high resolution electron microscopy.

9:30 am

CRACK PROPAGATION IN GRADIENT MICROSTRUCTURES IN TITANIUM ALLOYS: A. Berg, J. Kiese, L. Wagner, Technical University of Brandenburg at Cottbus, 03013 Cottbus, Germany

Fatigue crack nucleation, microcrack growth and long crack growth are often oppositely affected by microstructural changes. For example, refining the microstructural unit size, e.g., grain size, improves the material's resistance to crack nucleation and microcrack growth, while long crack characteristics often deteriorate. To obtain optimum resistance of a component to various stages of fatigue life, microstructural gradients were developed in the near- titanium alloy TIMETAL 1100 and the metastable alloy Ti3Al8V6Cr4Mo4Zr (Beta C) by thermomechanical treatments. In TIMETAL 1100, the prior grain size of fully lamellar microstructures was varied from about 100 µm at the surface of the component to about 500 µm in the bulk, while for Beta C, the degree of age-hardening was varied over the cross section resulting in hardness values ranging from 700 HV 0.1 at the surface to 300 HV 0.1 in the bulk. Fatigue tests were performed in 3point bending using a servohydraulic testing machine. Optical microscopy was used to study fatigue crack nucleation and crack growth. The results are compared with nongradient references.

9:50 am

MECHANICAL AND THERMOMECHANICAL SURFACE TREATMENTS TO ENHANCE BCF STRENGTEI IN HIGHSTRENGTH- TITANIUM ALLOYS: J. Kiese, W. Walz, L. Wagner, Technical University of Brandenburg at Cottbus, 03013 Cottbus, Germany

Shot peening and deep rolling with and without subsequent heat treatments were performed on various microstructures of the highstrength metastable titanium alloys Ti3Al8V6Cr4Mo4Zr and TilOV2Fe3Al to improve the fatigue behavior. To evaluate optimum process parameters, a wide range of Almen intensities, rolling forces and annealing temperatures was chosen. The change in surface layer properties was evaluated by TEM, X-ray diffraction, optical microscopy and microhardness measurements. Smooth and notched (K+ = 3.0) specimens were tested in rotating beam loading and the fatigue behavior was compared to the electrolytically polished reference. The results are explained in terms of the resistance to fatigue crack nucleation and microcrack growth as affected by the changes in near surface dislocation structure, residual compressive stresses and extensive precipitation hardening.

10:10 am

INVESTIGATION OF THE CREEP DEFORMATION MECHANISM OF THE LAMELLAR TiAl ALLOY MADE BY HOT EXTRUSION OF A BLENDED ELEMENTAL POWDER MIXTURE: Soo Woo Nam, Han Seo Cho, SunKeun Hwang*, and Nack I. Kim**, Dept. of Mat. Sc. & Eng., Korea Advanced Institute of Sci. & Tech. Taejon 305-701, *Inha University, and **Pohang Univ. of Sci. & Tech., Korea

Creep properties of the lamellar structured Ti46.6AI1.4Mn2Mo (at.°/O) alloy, which is made by hot extrusion of a blended elemental powder mixture, are investigated in air environment over the temperature range from 750°C to 900°C at constant stress levels ranging from 100MPa to 250MPa. Average grain size of 150µm is measured and the grain boundary phase is distributed inhomogeneously before the creep tests. The dislocation climb controlled creep deformation is suggested on the basis of the measured average activation energy for creep of 388kJ/mol and normalized stress exponent of 4.3 within the temperature range from 775°C to 900°C at stress level ranging from 150MPa to 250MPa. However, activation energy of 90kJ/mol within the temperature range of 750°C to 775°C and stress exponent of I.3 at 800°C within the stress range of 100150MPa are measured. Microstructural studies conducted on the creep fractured specimens showed the secondary cracks along the lamellar grain boundaries and these secondary cracks are assumed to be formed by pore nucleation, growth and coalescence during the tertiary stage.

10:30 am

MECHANICAL BEHAVIOR OF BINARY AND TERNARYALLOYED Al2Ti: J.C. Ma, J.E. Benci, Department of Materials Science and Engineering, Wayne State University, Detroit, MI 48202

Polycrystalline binary Al2Ti was produced via casting or powder metallurgy and then further processed yielding material in six conditions. The yield strength as a function of test temperature and the room temperature fracture toughness were measured for these six material conditions. The fracture toughness values were determined from the critical load necessary to initiate cracks with a Vickers indenter. Both properties show a strong dependence on processing condition. Off-stoichiometric al2±Ti1± and ternary-alloyed Al2Ti + X were also produced, and samples in the as-cast and cast & annealed conditions were prepared. The composition of the off-stoichiometric alloys was varied between 63 and 71 at% aluminum in 2% increments. Ternary-alloyed Al2Ti contains approximately 2 at% of either Si, V, Cr, Mn, Fe, Ni, Cu, Nb, Mo or W. The room temperature hardness and fracture toughness and the room and elevated temperature yield strength were measured for each composition in both the as-cast and cast & annealed material conditions. The effect of thermal mechanical processing, such as hot forging, on the room temperature properties was also investigated.

10:50 am

MICROSTRUCTURAL EVOLUTION DURING HIGH TEMPERATURE DEFORMATION OF -TiAI INTERMETALLIC COMPOUNDS: Hee Y. Kim, Woong H. Sohn, Soon H. Hong, Dept. of Mat. Sci. & Eng., Korea Advanced Institute of Science and Technology, Taejon, 305701, Korea

The microstructural changes during high temperature deformation behavior of Ti-(46,48)Al2W and Ti47Al2Cr4Nb intermetallic compounds have been investigated by isothermal compression tests at temperatures ranged from 1000°C to 1200°C with strain rates ranged 10-3/10-1/s-1. The stressstrain curve exhibited a peak stress then the flow stress decreased gradually into a steady state with increasing strain. The stressstrain curves showed a flow softening which is attributed to the dynamic recrystallization. The dependences of flow stress on temperature and strain rate were formulated using Zener-Hollomon parameter. The activation energies were measured as 437kJ/mol, 374kJ/mol and 300kJ/mol and the stress exponents were measured as 5.8, 5.3 and 4.9 for Ti46AI2W, Ti48AI2W and Ti47AI2Cr4Nb, respectively. The dynamically recrystallized microstructures were investigated and the relationships between recrystallized grain size and temperature compensated strain rate was discussed. The texture evolution during high temperature deformation was analyzed using the orientation distribution function (ODF). The controlling mechanisms during high temperature deformation of TiAl intermetallic compounds with varying temperature, strain and strain rate were discussed.

11:10 am

COMPARISON OF FATIGUE CRACK GROWTH RESISTANCE OF GAMMA ALUMINIDES AND Ti-BASED ALLOYS: S. Lesterlin, C. Mabru, C. Sarrazin, G. Henaff, J. Petit, Laboratoire de Mechanique et de Physique des Materlaux, URA CRNS 863 ENSMA, BP 109, 86960 Futuroscope, France

Gamma aluminides present attractive properties for high temperature structural applications and could be adopted against traditional titanium alloys. The resistance to fatigue crack growth is critical against traditional titanium alloys. The resistance to fatigue crack growth is critical for aeronautical applications. In this paper, the fatigue behavior of long cracks in Titanium Aluminides (lamellar and fully ) is compared, based on microstructure, temperature and environment criteria, with five conventional Titanium alloys in different thermomechanical conditions. At room temperature, higher threshold (related to enhanced near-threshold contribution of crack closure) and lower KIC contributes to much more steep propagation curves on TiAl. At high temperature in air, the crack growth regime identifies on Ti alloys as a corrosion-fatigue mechanism assisted by water vapor is shown to be also oporative for TiAl alloys. In all the cases, the propagation of fatigue cracks in both types of materials seems to be governed by the same mechanisms and the TiAl alloys appears to be more sensitive to moisture than Ti based alloys.

11:30 am

ENVIRONMENTALLY ASSISTED FATIGUE CRACK PROPAGATION IN Ti 6246 at 500°C: J. Petit, C. Sarrazin-Baudoux, S. Lesterlin, Y. Chabanne, Laboratoire de Mechanique et de Physique des Materlaux, UFA CNRS 863, ENSMA, BP 109, 86960 Fuuturoscope, France

The fatigue and crack propagation behavior of Ti 6246 alloys elaborated with a fine Windmanstatten microstructure, has been investigated at 500°C. Tests were performed in air, high vacuum (10-4 Pa), low vacuum (1Pa) and humidified Argon (100 Pa and atmospheric pressure). Partial pressures of water vapour and oxygen were controlled by mean of hygro-metals and mass spectrometer. Crack closure was detected using a capacitive gauge mounted at the mouth of the notch of CT specimens. The enhancement of the crack growth rates in the mean-threshold area and the mid-rate range observed in the different environments in comparison to high vacuum, is clearly related to the presence of water vapor even at very low partial pressure. Critical conditions (partial pressure, frequency, closure, load ratio) for the occurrence of water vapor assisted corrosion-fatigue, creep-fatigue and stress-corrosion are explored, and the specific role of oxygen and water vapor is discussed on the basis of microfractographic observations, fracture surface analysis (R.B.S. technique) and closure measurements.