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CREEP DEFORMATION OF TITANIUM ALUMINIDES WITH TiB2

C. R. Feng and K. Sadananda Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375

ABSTRACT Creep deformation of as-forged and heat-treated titanium aluminides, Ti-48A1 with 5 and 10% volume fraction of TiB2 , has been studied under tension in the temperature range of 649"871"C to determine the rate-controlling mechanism. Microstructure of these alloys correspond to a mixture of equiaxed grains of y and lamellar regions containing eutectoid Y+a2 .

Data indicate that with increase in load, stress exponent increases from one to seven. The activation energy for creep varies from 340kJ/mol to 455kJ/mol in the temperature range investigated. Creep data of these alloys were analyzed using several theoretical models. INTRODUCTION Tensile creep behavior of gamma-based titanium aluminides and composites was studied by several groups (1-11]. Most of these studies were limited to a narrow range of temperature or stress. Reported stress exponents vary from 3 to 5 [1-6 and 8-11], with an activation energy of 320kJ/mol [2-3, 5-7 and 9]. Occasionally, higher values of activation energy and stress exponent were also reported [6, 8 and 9]. Although there has been no direct evidence, power-law creep involving dislocation climb has been considered as the rate-controlling mechanism [3]. In this study, the creep tests were conducted in a broader load range. Systematic data evaluation was made and data indicated that as the load was increased, the stress exponent increased from 1 to 7. The stress dependence of creep rates was found to be better represented by a sinh function rather than by a power-law function. The implication of this in terms of creep mechanisms are discussed. EXPERIMENTAL PROCEDURE The materials used in this investigation were produced by a proprietary 'XD' process by Martin Marietta Laboratory and were forged. The nominal compositions were Ti-48at%Al+5vol%TiB 2 (Ti48A1+5TiB 2 ) and Ti-48at%Al+I0vol%TiB2 (Ti-48AI+lOTiB 2 ). Specimens were tested in as-forged condition. Some specimens of Ti-48AI+5TiB 2 were heat-treated at 1250"C/16h+900"C/8h. The microstructures of all materials contain about 85voi% of equiaxed y-TiAl grains and 15vol% of grains with y-TiAl/a 2 -Ti 3 Al lamellar structure. The average grain sizes in as-forged Ti48AI+5TiB2 and Ti-48AI+lOTiB 2 were 2011m and 14gm, respectively. However, some large grains (-100Jm) with lamellar structure were occasionally observed. The size of the TiB2 particulate ranged from lilm to 31m. Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society

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The round button-head tensile specimens with gage length 25.4mm and gauge diameter of 3.8mm were machined from specimen blanks. The creep deformation behavior was studied in the temperature range of 649" - 871"C using a constant load lever arm creep-frame. The displacement as a function of time was measured using a LVDT with connecting rods fixed to the specimen shoulders. The specimens were thermally stabilized in the cr