On Microstrucitural Evolution in Gas Atomized Ti-50 at.% Al-2 at.% Nb Powder
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ON MICROSTRUCITURAL EVOLUTION IN GAS ATOMIZED Ti-50 at.% A1-2 at.% Nb POWDER DONALD S. SHIH, GARY K. SCARR and JAMES C. CHESNUTIr Engineering Materials Technology Laboratories, GE Aircraft Engines Mail Drop M-89, 1 Neumann Way, Cincinnati, Ohio 45215-6301, USA ABSTRACT Microstructural development in helium gas atomized Ti-50A1-2Nb (at.%) powder has been characterized using x-ray diffractometry and transmission electron microscopy. The structure of the as-atomized powder was a mixture of two phases: an ordered fct (L0) ,yTiAl phase and an hcp a Ti phase containing fine (= 10-20 nm) D0 19 (12 domains. Large powder (= 250 jim) contained 14 volume % hcp phase, while fine powder (= 37 jim) contained = 72 volume % hcp phase. However, this a+a 2 constituent was metastable and transformed to the y phase during annealing. INTRODUCTION TiAl based alloys have attracted an enormous amount of research effort partly because of the large potential for elevated temperature applications. The ordered y phase, TiAl, has an fct L10 structure with a c/a ratio of 1.02 [1]. Conventional melt processing is often inadequate for TiAl based material due to severe solute segregation during solidification. Rapid solidification processing (e.g. melt-spining, atomization), however, has proved to be an efficient alternative for these intermetallics because refined microstructural features and reduced compositional segregation can be achieved [2]. Several papers [3-5] have examined the pathways for microstructure development in rapidly solidified binary y alloys. In this study, microstructural evolution in the helium gas atomized Ti-50AI-2Nb powder and response of microstructure to heat treatment have been investigated using x-ray diffractometry (XRD) and transmission electron microscopy (TEM). EXPERIMENTAL PROCEDURES Powders of Ti-50A1-2Nb were atomized in a helium atmosphere at Crucible Research. The chemical composition of the atomized powder, Table I, shows that the oxygen concentration increased significantly with reduced powder size; the finest powder contained 0.285 wt.% oxygen. Analyses of the starting ingot indicated most of the tungsten was picked up during atomization. Microstructural development of this powder was studied in two size fractions, 0 = 37 gtm and * 250 gim, both before and after heat treatment in a Ix10-3 Pa vacuum at 10000 C for three hours. Step-scanning XRD experiments were carried out using a Philips diffractometer incorporating a curved graphite diffracted beam monochromator. Powder TEM samples, prepared as described elsewhere [6,7], were examined in a Philips 420T microscope. Table I. Chemical Composition (at.%)a of Gas Atomized Ti-50A1-2Nb Powder Ti Bal
Al 50
Nb 2.0
Fe 0.1
W 0.3
C .07
S .006
_Qb.140
N .0046
H .0017
a S, 0, N and H concentrations are in wt.%. b average concentration for all sizes of powder; 0.285 wt.% 0 for = 37 jim powder, 0.120 wt.% 0 for =150 iLtmpowder and 0.105 wt.% 0 for = 250 jim powder.
Mat. Res. Soc. Symp. Proc. Vol. 133.
1989 Materials Research Society
168
RESULTS AND DISCUSS
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