About the 1996 TMS Annual Meeting: Monday Afternoon Sessions (February 5)

ENVIRONMENTAL EFFECTS AND WEAR

February 5, 1996 Location: Anaheim Convention Center

Session Chairperson: TBA

2:00 pm

TENSILE PROPERTIES AND STRUCTURE OF SEVERAL SUPERALLOYS AFTER 5000 HOUR EXPOSURE TO LiF AND VACUUM AT 1173 K: J. Daniel Whittenberger, NASA Lewis Research Center, Cleveland, OH 44135

The solid- to- liquid phase transformation of LiF is under consideration for thermal energy storage in space- based solar dynamic system. Unfortunately, the melting point of this salt (1121 K) is beyond the commonly accepted upper use temperature of 1100 K for Cr- bearing superalloys in vacuum. The commercially available Ni- base superalloy Hastelloy^R B- 2 is Cr- free, but its high Mo content alloy can form phases which cause high temperature embrittlement. Hastelloy^R B- 2 and the Cr- containing Co- based Haynes Alloy^R 188 and Ni- based Haynes Alloy 230 were exposed to LiF and vacuum at 1173 K for 5000 h. Neither exposure had any significant effect on Hastelloy^R B- 2 in terms of microstructural surface damage or weight change. Nevertheless, measurement of the post exposure tensile properties of Hastelloy^RB- 2 revealed low tensile ductility at 1050 K. Little evidence of attack by LiF was seen in either of the Cr- containing superalloys; however considerable weight loss and near surface microstructural damage occurred in both alloys exposed to vacuum.

2:20 pm

PROTECTIVE ALUMINA SCALE OVER SUPERALLOYS: Vivek Saxena, Department of Metallurgical Engineering, Malaviya Regional Engineering College, Jaipur, India

The growing use of superalloys for high temperature applications needs oxidation resistance which can be effectively attained by forrning continuous, thin A1203 scale (4- 6 m) using selective oxidation technique. Spontaneous oxidation of Al and various other alloying elements takes place as the Gibbs free energy of oxide formation is large and negative. Preferential alumina scale development depends upon Al interdiffusion coefficient, solubility and diffusivity of oxygen in the alloy having critical weight of aluminiurn as depicted by oxide mapping. This paper is an attempt to review the principles of alloy oxidation along with factors affecting scale formation and its adherence to superalloy surface. The role of reactive elements, dispersoids and effect of sulphur has also been critically analysed.

2:40 pm

EVALUATION OF CORROSIVENESS OF FIRE- RETARDANTS ON ALUMINUM ALLOYS USED IN AIRCRAFT INTERIORS: J. E. Talia, J. Chaudhuri, S. N. Cheema, A. Arif, Dept. of Mech. Eng., Wichita State University, Wichita, KS 67208

This research evaluates the corrosive fire retardant products present in general aviation small aircraft interiors and eliminates those chemical compounds which are suspected of inducing corrosion. To this end, various existing fire retardant products were evaluated in conjunction with various aluminum alloys making up the aircraft interior skin. In order to supplement the literature search of the Service Difficulty Reports data:base, extensive experimentation was carried out with varying parameters, such as temperature and humidity in the sandwich test technique. This conclusively proved that the presence of halogen based salt particles may be the source of corrosion.

3:00 pm

EFFECTS OF He⁺ IRRADIATION ON THE MECHANICAL PROPERTIES OF POLYCRYSTALLINE NiAl: S. T. Eck, A. J. Ardell, Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095- 1595

Polycrystalline NiAl was irradiated with 2 MeV He⁺ ions at temperatures of 150 and 225 K to fluences ranging from 2.9 x 10¹⁵ to 5.8 x 10¹⁶ He⁺/cm². The peak doses under these conditions range from 0.1 to 2.0 dpa. The yield stress was found to increase with increasing fluence at a fixed temperature, but decreased with increasing temperature at a particular fluence. The yield stress increased significantly from 148 MPa for unirradiated specimens to 177 MPa for specimens irradiated to a fluence of 2.9 x 10¹⁵ He⁺/cm² at 225 K, but only increased gradually with further increases in the fluence. A similar effect was found at 150 K, with the exception that the yield stresses were slightly higher than those at 225 K at each particular fluence. We attribute the increase in yield stress to at least partial irradiation- induced disordering of the NiAl, but this awaits confirmation by transmission electron microscopy investigations, which are in progress. This work is supported by the Department of Energy.

3:20 pm BREAK

3:30 pm

MICROMECHANISM OF HYDROGEN- ASSISTED FATIGUE CRACK GROWTH IN Ti- 6Al- 4V TITANIUM ALLOY: P. S. Pao, C. R. Feng, R. A. Bayles, Naval Research Laboratory, Washington, D.C. 20375; G. R. Yoder, Office of Naval Research, Arlington, VA 22217

The micromechanism of hydrogen- assisted fatigue crack growth under high stress ratio in [[beta]]-annealed Ti- 6Al- 4V titanium alloy has been investigated. At stress ratio of 0.90, the threshold stress intensity of Ti- 6Al- 4V containing 1000 wppm hydrogen is significantly lower than that for alloy that contains only 60 ppm hydrogen. The fatigue crack growth rates of hydrogen charged material are also substantially higher than those for the as- received material. In the intermediate K region, an extensive secondary cracking along [[alpha]]/[[beta]] interfaces and the concomitant reduction in the power- law exponent are observed in the hydrogen charged Ti- 6Al- 4V. Subsequent TEM analyses of tensile deformed materials indicates the precipitation of titanium hydride along the [[alpha]]/[[beta]] interfaces and the presence of deformation twins in the hydrogen charged alloy. These deformation twins are terminated at the [[alpha]]/hydride interfaces and do not traverse across either hydrides or [[beta]] phase into the neighboring a grains. The observed secondary cracking along [[alpha]]/[[beta]] interfaces can be rationalized by the accumulation of stresses and the fracture at the [[alpha]]/hydride interfaces.

3:50 pm

OXIDATION KINETICS, MICROSTRUCTURE AND PROPERTIES OF A V-5Cr-5Ti ALLOY EXPOSED TO 300-600deg.C IN AIR: K. Natesan, W.K. Soppet, Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439; M. Uz, Chemical Engineering Department, Lafayette College, Easton, PA 18042

A study was carried out to assess the oxidation kinetics and the effects of air exposure on the microstructure and properties of a V-5Cr-5Ti alloy sheet. Samples were evaluated in the as-rolled and as-annealed (1050deg.C/1 h) conditions, and after subsequent exposure to air in the temperature range of 300-600deg.C for times ranging from 24 h to 2060 h. Tensile properties were determined following exposure at 500deg.C for up to 2060 h. Oxidation behavior at 300, 400, 500, 575, and 650deg.C was studied by a thermogravimetric testing method. Microstructural attributes (including phase, grain size, oxide thickness and intergranular fracture zone) were determined by various metallographic and X-ray diffraction techniques. The Vickers hardness profile along the thickness of each sample was measured to determine the depth of the hardened layer and to estimate oxygen diffusivity in the alloy. The results will be discussed, with the emphasis on oxidation kinetics and the possible correlation between processing, microstructure, and properties. Work supported by the U.S. Department of Energy, Office of Fusion Energy Research, under Contract W-31-109-Eng-38.

4:10 pm

ADSORPTION OF H2O AND O2 ON Ni3(Al,Ti) AND Ni3Fe SURFACES: CORRELATION OF SURFACE REACTIVITY WITH DUCTILITY, W. J. Chia, Y. W. Chung, Dept. of Mat. Sci. and Eng., Northwestern University, Evanston, IL 60208

Water adsorption on clean and oxygen- covered Ni3(Al,Ti) (100) and Ni3Fe has been investigated by x-ray photoelectron spectroscopy and thermal desorption. It will be shown that water adsorbs molecularly on Ni3(Al,Ti) (100) at 140K and dissociates at higher temperature. On the oxygen- precovered surface hydroxyls are produced upon water adsorption at 140K. Compared with water adsorption on the clean surface, more hydroxyls are produced. Some of the surface hydroxyls are produced by direct water dissociation, and some by adsorbed oxygen abstracting hydrogen atoms from H2O. Similarly, the reactivity of polycrystalline Ni3Fe towards water will be reported. The mechanical properties of Ni3(Al,Ti) and Ni3Fe such as ductility will be correlated with surface chemical properties.

4:30 pm

SURFACE HYDRIDES IN LOW AND HIGH PRESSURE HYDROGEN CHARGED Ti48AI- 2CR- 2NB: K. Li, U. Habel, T. M. Pollock, M. DeGraef, A. W. Thompson, MSE Dept. Carnegie Mellon University, Pittsburgh, PA 15213

The development of hydrides at the surfaces of duplex Ti- 48Al- 2Cr- 2Nb samples charged in gaseous hydrogen at pressures from 0.1 MPa to 13.8MPa have been studied. Under conditions where trace amounts of oxygen were available during charging experiments up to 60 hrs in duration, the surface microstructures were dramatically different from those of the bulk material. At higher pressures the bulk microstructure consisted of approximately 25 vol% hydrides including FCC hydrides and a recently identified 0 hydride. The 0 phase is body centered tetragonal with lattice parameters of a = 0.528nm and c = 2.845nm. The hydrides were embedded in a TiAl matrix with no traces of the [[alpha]]2 phase. The surface microstructure was composed of an outer mixed layer of TiO2 and A12O3 and an inner layer consisting of a continuous matrix of [[theta]] hydride with a small volume fraction of discontinuous alumina particles. At O.lMPa no changes in bulk microstructure were observed. However, near surface regions contained some titania and alumina and very small amounts of the [[theta]] phase within the [[alpha]]2 phase.

4:50 pm

EFFECTS OF SUBSTRATE CURVATURE ON RESIDUAL STRESSES IN OXIDE FILMS: A COMPARISON OF FEM SOLUTIONS AND OPTICAL FLORESCENCE EXPERIMENTS: R. Williamson, J. Wright, Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 83415- 2218; P. Hou, R. Cannon, Lawrence Berkeley Laboratory, Berkeley, CA, 94720; B. Veal, Argonne National Laboratory, 9700 S. Cass Ave., Argonne,IL 60439

Finite element analyses are utilized to investigate the effects of substrate curvature on the thermal residual stresses within an alumina film on an iron- aluminide substrate. For films that are normally compressive on flat substrates, large tensile stresses are observed as the substrate radius of curvature is decreased. Comparisons are made between FEM predictions and residual stress measurements using optical florescence techniques. The results are useful in designing components which utilize oxide coatings for surface protection.

5:10 pm

SLIDING WEAR AND FRICTION CHARACTERISTICS OF ACuZinc ALLOYS: M. David Hanna, Jon T. Carterand , M. S. Rashid, Metallurgy Department, General Motors Corporation, Research and Development Center, 30500 Mound Road, Box 9055 Warren, Ml 48090- 9055

ACuZinc, a GM- patented, high- performance zinc- copper- aluminum alloy, which is suitable for manufacturing net shape die castings, has demonstrated superior wear and friction properties compared to traditional zinc alloys. The wear and friction characteristics of ACuZinc, Zamak 3, ZA 27, and pure zinc were determined by pressing blocks of these materials against warm, oil-lubricated, rotating rings of AlSl 1144 steel. The mass wear rate of the alloys generally correlated directly with coefficient of friction and inversely with hardness.