High-Temperature Compression Strength of Directionally Solidified Nb-Mo-W-Ti-Si In-Situ Composites
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heating floating zone melting technique (FZ), respectively. EBFZ was done at a growth rate of 30 mm/h in vacuum, and FZ at a growth rate of 15 mm/h in a 0.1 MPa He atmosphere. Each specimen was annealed at 1870 K for 100 h in vacuum, and then the microstructure was examined by means of a scanning electron microscope (SEM). The composition of each phase in the specimen was analyzed by energy-dispersion X-ray spectroscopy (EDS) under the SEM. The bulk composition of the Nb-xMo-22Ti-18Si alloys (prepared by EBFZ) was analyzed by inductively coupled plasma spectroscopy (ICP). Compression tests were performed at 1670 and 1770 K in vacuum at an initial strain rate of 1 x 10-4 s-1. Constant load compression creep tests were carried out at 1570, 1670 and 1770 K at initial stresses of 120, 200, 300 and 400 MPa in a 0.1 MPa Ar atmosphere. The dimensions of the compression and creep test pieces were 2 mm x 2 mm in cross-section and 5 mm in length. Each test piece was compressed parallel to the direction of growth. During the EBFZ of the Nb-xMo-22Ti-18Si alloys, a relatively large weight loss was observed. The bulk composition of the specimens with x = 10, 20 and 30 mol% was Nb-10.4Mo16.3Ti-19.8Si, Nb-21.0Mo-14.1Ti-21.2Si and Nb-31.6Mo-15.7Ti-20.4Si, respectively. Thus, the actual composition of each of these specimens is slightly different from the nominal composition, and this is mainly due to the evaporation of Ti during the EBFZ. We refer to each alloy by the nominal composition, however. As for specimens of Nb-10Mo-yW-10Ti-18Si alloys that were prepared by FZ, no weight loss was observed. For this reason, we assume that the bulk chemistry of these alloys did not change during processing. RESULTS AND DISCUSSION Microstructure and Composition of Each Phase Back-scattered electron images of representative samples are shown in figure 1. All of the images were obtained from a cross-section parallel to the direction of growth. The bright phase is Nb solid solution (referred to as Nbss hereafter) and the dark phase is (Nb, Mo, (W,) Ti)5Si3 silicide (referred to as 5-3 silicide hereafter). As seen in this figure, the microstructure is more or less oriented in the direction of growth, although it is not the microstructure typically observed in the case of directionally solidified materials. By adding a substantial amount of W, primary Nbss appears and the microstructure becomes coarser (see figure 1 (d) for example). As seen in figure 1 (a) and (b), the addition of Mo results in a finer microstructure. As seen in figure 1 (a) and (c), Nb-10Mo-10Ti-18Si grown at 15 mm/h shows a finer microstructure than Nb-10Mo-22Ti-18Si grown at 30 mm/h. There is a possibility that the addition of Ti results in a coarser microstructure. Small precipitates of Nbss are observed in the 5-3 silicide in the samples with coarser microstructure (see figure 1 (d)). In each annealed sample, the Nbss has an almost uniform composition. The 5-3 silicide has an almost constant Si concentration of 35 - 38 mol%, whereas the distribution of Ti, Nb and Mo is not ho
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