Crystallographic texture in mechanically alloyed oxide dispersion-strengthened MA956 and MA957 steels
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I.
INTRODUCTION
M E C H A N I C A L mixtures o f metallic powders o r intermetallic compounds can be forced to form a m e t a stable solid solution by means o f intense deformation, a process known as "mechanical alloying. "(]] At the same time, dispersoids, such as oxide particles, can also be incorporated into the final powder. The mechanically alloyed powder can then be processed in the solid state (for example, by extrusion) to produce bulk samples o f the desired chemical composition. Many mechanically alloyed metals are now commercially available, but steels produced using this method show particular promise in a variety o f applications. F o r example, MA956 is a chromium-rich ferritic stainless steel containing some 8 at. pct aluminum forced into solution as an aid to oxidation resistance, (2,3] together with a dispersion o f yttrium oxide particles for creep resistance. An alternative ferritic stainless steel variant (MA957) contains titanium rather than aluminum and is designed for applications in the nuclear industry.~4] The steels powders, after mechanical alloying and processing into bulk form, have an ultrafine microstructure containing submicron grains o f ferrite. The hardness in this condition is unacceptably high, so the steels are usually recrystallized into a coarse columnar grain structure akin to a directionally solidified microstructure. A columnar (or directionally recrystallized) grain structure is i d e a l for elevated temperature applications where creep resistance is important. However, the crystallographic orientation o f the columnar grains can be o f importance in many applications. The factors controlling the recrystallization texture are not understood for mechanically alloyed steels. The purpose o f the present work was to undertake an investigation o f the crystallographic textures o f MA956 and MA957 both before and after the recrystallization heat treatments. Both o f the alloys are T.S. CHOU, Research Student, and H . K . D . H . BHADESHIA, Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted February 3, 1992. METALLURGICAL TRANSACTIONS A
intended to be ferritic at all temperatures in the solid state, so it should, in principle, be possible to relate textural changes during recrystallization to extensive data which already exist on the development o f texture in rolled lowalloy steels. (5-8]
II.
EXPERIMENTAL PROCEDURE
The alloys, whose chemical compositions are given in Table I, were supplied by INCO Alloys (Hereford, United Kingdom) in an unrecrystallized state. They were fabricated by charging three primary powders (elemental iron, prealloyed metallic alloys, and yttria) into a water-cooled vertical attritor f o r the mechanical alloying. The consolidation o f the resultant powder was achieved by extrusion at 1000 °C while packed in a mild steel can. This was followed by rolling at 1000 °C with a reduction in diameter from 54 to 25 mm for MA956 and from 54 to 9.5 mm for M
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