Fatigue of Extruded Steel/Nial Composites

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M.K. BANNISTER, S.M. SPEARING, J.P.A. LOFVANDER, M. DE GRAEF High Performance Composite Center, Materials Department, College of Engineering, University of California, Santa Barbara CA 93106

ABSTRACT Fatigue tests were performed on a novel, extruded, stainless steel/NiAl composite having good impact and tensile properties. A high fatigue limit was observed to occur at approximately 67% of the aUTS. The fracture surface showed a distinct change in morphology between the fatigued and fast fracture areas and the formation and growth of microcracks was postulated as the initial fatigue mechanism. The microcrack development was monitored by intermittent measurement of the elastic modulus and associated hysteresis. Microstructural characterization by means of SEM, TEM and EDS revealed the existence of approximately 100nm diameter A120 3 particles decorating the interface between the NiA1 and the stainless steel tubes.

INTRODUCTION The B2 ordered material, NiAl, is one of a variety of systems under close examination for use as a high temperature, structural material [1, 2]. Its use is hampered by a lack of ductility and inadequate toughness at low temperatures. Previous work has shown the benefits of reinforcing brittle materials with ductile particles [3, 4]. Studies performed by Nardone et al [5, 6] provided evidence for a dramatic improvement in toughness and ductility through the use of a novel composite design. The material was formed by extrusion at 1066 *C and consisted of a NiAl matrix containing continuous, uniaxial tubes of 304 stainless steel and B 4 C particulates. However, previous work [7] has raised doubts concerning the fatigue properties of brittle materials reinforced with ductile particles or fibers. The results presented here outline some research into the fatigue properties of two types of materials' : NiAl/stainless steel with A1 20 3 (MT-90-18) or B 4 C (MT-90-19) particulates.

ANALYTICAL TECHNIQUES Electron transparent foils for analysis in a transmission electron microscope (TEM) were prepared by cutting thin slices of material with a low speed diamond saw perpendicular to the extrusion direction. These slices were ground to roughly 100 pm using diamond paste before discs 3 mm in diameter were ultrasonically drilled out. The samples were dimpled using 3 and 1 psm paste with 1/4 pm used for the final polish. Subsequent thinning was accomplished by ion milling with Ar at 5 kV, 1 mA and 140 incidence angle. Samples were examined in a JEOL 2000FX TEM equipped with a LINK eXL high take-off angle energy dispersive spectroscopy system; the microscope was operated at 200 kV. Indexing of diffraction patterns was aided by the Desktop Microscopist software 2 . Conventionally mounted and polished samples were also prepared and examined in a JEOL SM840A scanning electron microscope. The TEM samples were also characterized in the SEM.


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