Effects of test temperature and grain size on the charpy impact toughness and dynamic toughness ( K ID ) of polycrystall
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ODUCTION
THE continuing desire for increasing the efficiency of turbine engines via operating at elevated temperatures has led to exploration into various materials systems. Niobium (Nb) is a refractory metal with the distinction of possessing excellent ductility at low temperature, hightemperature strength, and liquid metal corrosion resistance. These properties have encouraged researchers to investigate a variety of Nb-base systems for potential aerospace applications. Recently, a number of research groups[1–6] have investigated the fracture and fatigue behavior of Nb-Si systems, which combine a refractory metal intermetallic, Nb5Si3, with the terminal refractory metal phase (i.e., Nb with Si in solid solution). Recent research studies[1–6] have demonstrated that Nb can be used as a tough reinforcement in ductile-phasetoughened Nb5Si3 composites. The success of such a system is significantly affected by the mechanical behavior of the toughening phase (Nb) as well as the interfacial strength between the brittle constituent (Nb5Si3) and the toughening phase. Considerable efforts have been directed in the past toward understanding the effects of changes in grain size, test temperature, and strain rate on the tensile flow behavior, cleavage fracture stress, and static plane strain fracture toughness of Nb.[7–12] However, the ductileto-brittle transition and fracture toughness of Nb under dynamic testing conditions, key considerations for structural applications, have not been investigated to the same extent. The present investigation examines the effects of changes in test temperature and grain size on the Charpy impact and dynamic impact toughness (KID) of polycrysD. PADHI, Process Engineer-III, is with Applied Materials Inc., Santa Clara, CA 95054. J.J. LEWANDOWSKI, Leonard Case Jr. Professor of Engineering, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Contact e-mail: [email protected] and [email protected] Manuscript submitted June 19, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
talline Nb. The studies reported have been conducted on materials identical to those reported previously,[7,8] enabling direct comparison. II.
EXPERIMENTAL PROCEDURES
A. Materials Tested, Heat Treatments, and Specimen Details Commercial purity Nb (i.e., Nbcp) was obtained from Cabot Corporation (Bethlehem, PA) in the form of hotrolled square plates having a nominal thickness of 10.5 mm. The composition, determined via wet chemical analysis, was identical to that tested previously[7,8] (i.e., oxygen 165 ppm, nitrogen 63 ppm, silicon 0.03 at. pct, bal. Nb). Unnotched bend bar specimens (dimensions: 55 ⫻ 10.5 ⫻ 10.5 mm3) were machined from the plates so that the long axis of the bend bar was along the rolling direction. Each bend bar was wrapped in tantalum foil and heat treated in a vacuum of 10⫺5 torr to minimize oxidation. The temperature and time of annealing were varied in order to obtain a range of grain sizes, measured via the linear intercept method, and
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