The Effect of Microalloying B on the High Temperature Mechanical Properties of Ti 3 Al
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THE EFFECT OF MICROALLOYING B ON THE HIGH TEMPERATURE MECHANICAL PROPERTIES OF TI3 AL JOSEPH W. NEWKIRK AND GERALD B. FELDEWERTH Department of Metallurgical Engineering, University of Missouri-Rolla, Rolla, 140 65401
ABSTRACT The effect of adding boron to Ti3 Al on the microstructure and high temperature tensile properties has been studied. Boron caused a large grain refinement that dominated the tensile properties at all temperatures. Particles of Ti2 B were found in all of the boron containing alloys. TiB was found only at concentrations of 0.1% B or more. INTRODUCTION Titanium aluminides, such as Ti3 Al, are attractive materials for possible use in structural applications, particularly aerospace applications. The good qualities of this aluminide, such as high specific stiffness and high operating temperature, are offset by several drawbacks. Two drawbacks of interest to this investigation are poor room temperature ductility and a dramatic decrease in tensile strength above 600 0C. The removal or elimination of these drawbacks, hopefully without effecting the attractive properties, is a major step in the development of Ti3 Al based alloys. Low ductility at room temperature is a major problem, but fortunately has already been greatly improved by alloying with niobium. The addition of niobium to improve ductility is central to most development efforts. The loss of strength at high temperatures has received much less attention. Lipsett, et al. [1] attributed this effect to intergranular cracking. If true, strengthening the grain boundaries could improve the high temperature strength of the material. Boron is thought to strengthen grain boundaries [2,3] in Ni3 Al and FeAl and might be effective in Ti3 Al. Previous experiments [4] with B in Ti 3 A1 were aimed at forming borides for dispersion strengthening. Boron concentrations of 0.5 weight percent or more were used, along with other alloying elements. In this experiment, we chose to add smaller amounts of B, from 0.01% to 0.5%, to Ti 3 Al to study the solute effect of B on the microstructure and the properties, especially the high temperature properties. The microstructure was examined with optical and transmission electron microscopy and x-ray diffraction. Mechanical properties tests included room temperature hardness measurements and tensile tests performed at temperatures up to 900 0C. PROCEDURE One kg arc-melted buttons were produced by Titanium Metals, Inc. from high purity starting materials. Samples consisted of the base alloy and 0.01%, 0.05%, 0.1% and 0.5% B for a total of five alloys. Samples were examined in either the as-received condition or after annealing for ten hours at I000°C. Tensile samples were also annealed at 10000 C, but for just two hours. Samples were prepared for optical microscopy by mechanical grindMat. Res. Soc. Symp. Proc. Vol. 133. ' 1989 Materials Research Society
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ing followed by electrolytic polishing and etching using a non-acidic electrolyte [5]. TEN samples were produced by jet thinning using a perchloric acid, methanol,
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