Fracture and Fatigue of Refractory Metal Intermetallic Composites

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igure 1. 3D Backscattered SEM view of Nb-10Si Composite.

Very little information exists in the literature on the fatigue behavior of Nb alloys in general. Work by Salama, et al (13) on thin sheets (i.e. 1 mm thick) of both polycrystalline Nb and hydrogen charged polycrystalline Nb revealed an effect of hydrogen charging and R-ratio on the fatigue threshKK6.10.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society

old over a limited range of R-ratios. Recent work (18,19) on some Nb toughened intermetallics has revealed that the fatigue performance of some of these systems is not much better than that of the brittle intermetallic, although as noted in those works this is clearly microstructure, chemistry, and composite architecture dependent. The present work on refractory metals was conducted on both pure Nb as well as Nb alloys containing either Zr or Si in solid solution, as well as the Nb-Si composite shown in Figure 1. References to the preliminary work conducted on the fatigue of the monolithic Nb alloys and the Nb-lOSi composites are provided (9,10,21). EXPERIMENTAL PROCEDURES The pure Nb and Nb-1%Zr alloys were obtained from Cabot in the hot rolled condition and were subsequently annealed in Ar in order to produce materials with grain sizes in the range 50 - 165 gim. The chemical composition and impurity levels are summarized elsewhere as are the details of the heat treatment process (11,12). The mechanical properties of the monolithic Nb alloys, including the strength, cleavage fracture stress, and fracture toughness are also provided elsewhere (6,7,11,12,14). The Nb-Si monolithic alloy (14) and the Nb-Si arc cast and extruded composite shown in Figure 1 were obtained from Wright Patterson Air Force Base and the detailed microstructures are summarized elsewhere (1,2,14). In addition, higher alloy variants of the Nb-10Si system were prepared via directional solidification (DS) at GE Corporate Research and Development Laboratory in Schenectady, NY (4). A representative microstructure of a DS Hf-Cr-Al-Ti-Nb-Si composite is shown in Figure 2.

Figure 2. 3D Backscattered SEM view of DS Nb-Hf-CrAI-Ti-Si composite.

Single edge notched bend bars were tool or electrodischarge machined from the bulk extrusions with the bar axis parallel to the extrusion direction. The width (W)-by-thickness (B) dimensions of the composite specimens were typically 12 x 6 mm, while those of the monolithic Nb alloys were 12 x 12 mm. Specimens were notched to an a/W = 0.45. One side of each specimen was polished to a 0.05gtm finish to facilitate observations of crack/microstructure interactions at the surface. Fracture toughness tests were conducted in three point bending at both 77K and 298K at loading rates ranging from 15 N/sec to 5 x 104 N/sec. The load vs displacement data in the highest rate tests were obtained via the use of a storage oscilloscope as detailed elsewhere (2). The peak toughness was calculated using the maximum load reached during the tests. Other details are provided elsewhere

(2). Fatigue crack g