Planar Growth Faults in Nb 3 Al
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PLANAR GROWTH FAULTS IN Nb 3 AI
L.S. SMITH, T-T. CHENG AND M. AINDOW IRC in Materials for High Performance Applications, and School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
ABSTRACT A transmission electron microscopy study of planar growth faults in the A 15 intermetallic phase Nb 3AI is presented. These faults are not observed in plasma melted material but a high density of these features is observed in arc-melted material which is cooled more slowly. The faults are found most frequently on planes ( 100), often changing from one plane to another along their length. It has not been possible to obtain a full diffraction contrast analysis from the faults but HREM images indicate that they are intrinsic with a displacement vector R=1/4. Since this vector does not lie in the plane, they cannot form by glide processes alone and therefore probably arise when vacancies coalesce into sheets on ( 100). Since this value of R is consistent with the removal of an Al-rich plane (004), it is argued that the formation of these faults may help to stabilise the A 15 phase to Nb-rich compositions.
INTRODUCTION Several intermetallic compounds of the general formula A3 B possess the cubic 3-W structure (A15). This class of compound is best known for their superconducting properties at liquid helium temperatures. Some of these compounds, notably Nb3 A1, Cr3Si and V3 Si, are also of interest for aerospace applications since they have high melting points, low densities, small lattice parameters and are tolerant to significant deviations from stoichiometry. The exploitation of these properties has been prevented by the limited ductility of A15 compounds, but the reasons for this are not well established. The purpose of the present research program is to investigate the defect microstructures and deformation mechanisms in Nb 3AI based alloys. In this paper we present a TEM study on the nature of extended stacking faults in as-cast specimens of Nb3AI-based alloys. Theoretical studies have suggested that faults in A15 alloys could have displacements R equal to 1/2 [1] and extended faults have been observed experimentally in Nb 3 AI [2,3]. In recent work [4,5], it has been shown that two different types of fault can occur in Nb 3 Al; narrow ribbons with R = 1/2 formed by the dissociation of perfect dislocations and large growth faults with R = 1/4. In this paper we consider these latter faults in detail: TEM images from such faults in binary Nb-Al alloys with =25% Al are presented and their origins and effects on deformation behaviour are discussed.
EXPERIMENTAL PROCEDURE Ingots with nominal compositions Nb-25at.% Al were prepared by two different routes, both using elemental starting materials. Small (=50g) buttons were prepared by arc melting under vacuum - the ingots were remelted twice to promote homogeneity. Larger (= 500g) ingots were produced by plasma melting under a positive overpressure of Ar plus He. The ingots were remelted four times, flipping between each remelt, to give
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