Improving the fracture toughness of constituent phases and Nb-based in-situ composites by a computational alloy design a
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31/7/03
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Improving the Fracture Toughness of Constituent Phases and Nb-Based In-Situ Composites by a Computational Alloy Design Approach KWAI S. CHAN and DAVID L. DAVIDSON A computational alloy design approach has been used to identify a ductile matrix for Nb-based in-situ composites containing Ti, Hf, Cr, Si, and Ge additions. Candidate alloys in the form of cast buttons were fabricated by arc melting. Coupon specimens were prepared and heated treated to vary the microstructure. Backscattered electron (BSE) microscopy, quantitative metallography, energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were utilized to characterize the morphology, volume fraction, composition, and crystallography of individual phases in the microstructure. The fracture toughness of the composites was characterized by three-point bending and compact-tension techniques, while the fracture toughness of individual phases in the in-situ composites was determined by an indentation technique. The composition, crystallography, and volume fraction of individual phases were correlated with the fracture-toughness results to assess (1) the role of constituent properties in the overall fracture resistance of the composites and (2) the effectiveness of the computational design approach. The results indicated that the effects of alloy addition and plastic constraint on fracture toughness were reasonably predicted, but the conditions for relaxing plastic constraint to attain higher fracture toughness were not achieved.
I. INTRODUCTION
NB-BASED in-situ composites are multiphase alloys that contain an Nb (bcc) solid-solution phase and brittle intermetallic phases such as silicide and Laves phases.[1–12] Depending on the alloy composition, as many as four silicide (Nb5Si 3, Nb3Si, Ti5Si 3, and Ti 3Si) and two Laves phases (C14 and C15 NbCr2) in alloyed forms can exist in the microstructure.[4–6,9–12] NbCr2 is normally C15 at ambient temperature, but transforms to C14 when alloyed with 2 at pct Si or greater.[12] The silicide and Laves phases are intended for providing high-temperature creep and oxidation resistance, while the Nb solid solution (Nbss) is intended to improve the ambient-temperature fracture resistance. Extensive research has demonstrated that alloy additions can impart fracture resistance in the Nb solidsolution, silicide, and Laves phases.[4–11,13–20] The large number of potential alloying elements, however, makes the discovery of beneficial alloy additions a daunting task if undertaken via empirical means. There is considerable interest in developing computational tools for designing materials with a desired composition, microstructure, and performance. For designing against brittle fracture, some investigators[21,22,23] have focused on alloy effects on the unstable stacking energy[24] and the crack-tip dislocation-emission process, while others[17,25–29] have emphasized the influence of alloying additions on the Peierls–Nabarro (P–N) barrier energy[30,31] and the mobility of dislocations moving away
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