The Synthesis and Evaluation of Nb 3 Al-Nb Laminated Composites
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THE SYNTHESIS AND EVALUATION OF Nb3A1-Nb LAMINATED COMPOSITES R.G. ROWE and D.W. SKELLY GE Corporate Research and Development, Schenectady, NY 12309 ABSTRACT Microlanminates of Nb3 AI and Nb were synthesized in-situ by high rate magnetron sputtering. Three composites were fabricated. They had thicknesses ranging from 19 to 146 •tm (5.5 mils) with 11 to 91 layers. The volume fraction of the Nb lamellae was approximately 0.4 for two and 0.5 for the third. Room temperature fracture of the composites revealed that some cleavage cracking in the Nb3 Al intermetallic layers was arrested by the ductile Nb layer. The Nb layers failed by chisel point fracture and shear. INTRODUCTION High temperature intermetallic systems, which have strength and resist creep deformation at temperatures above 10000 C are brittle at low temperatures. [1] One approach to the utilization of these high temperature intermetallics is to combine them with ductile alloys for toughening at low temperatures. The toughening of a brittle material by ductile filaments or sheets has been demonstrated theoretically and experimentally for a number of systems. [2-5] In addition to toughening, the low temperature strength of a composite with a given fracture toughness can be increased by reducing the size of the largest cracks or defects. Since cracking across the intermetallic layer is the most likely defect, this can be accomplished by reducing the thickness of the intermetallic lamellae. The disadvantage of thin intermetallic layers is high interstitial element contamination for fabrication processes that begin with thin laminates or fine powder. Interstitial contamination causes higher ductile to brittle transition temperatures in both the toughening and the intermetallic phases. [6-8] In-situ fabrication is one technique that can be used to produce low interstitial high temperature intermetallic laminated composites with thin intermetallic layers. Vapor phase deposition by high rate magnetron sputtering provides a means of producing low interstitial laminates with the freedom to control ductile and intermetallic phase compositions, volume fraction and laminate dimensions independently. This paper describes the synthesis and evaluation of multi-layer laminates of the intermetallic composite system Nb 3AI-Nb. EXPERIMENTAL PROCEDURE Figure 1. is a schematic of the magnetron sputtering process that was used to produce intermetallic Nb3 AI plus ductile Nb laminates. The two layers were produced by alternately sputtering from two different targets. An MRC 603 magnetron sputtering system, with Inset® magnetron cathodes was used. The sputtering target was produced by inserting segments of aluminum with bars of pure niobium as in Figure 2. The target composition was adjusted by changing the width of the aluminum segments. A target with 20 vol.% Al gave a film composition of Nb-21 at.% Al. An 11-layer composite of Nb and Nb-20AI (Nb3AI) was fabricated by sputtering onto a sapphire substrate. The composite consisted of six layers of Nb and five layers of Nb-20A1. Th
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