Improving the fracture toughness of constituent phases and Nb-based in-situ composites by a computational alloy design approach

Abstract

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.

Cyclic oxidation response of multiphase niobium-based alloys

“…Details of the experimental procedure were described earlier. [43] In the BSE images shown in Figures 1(a) through (f), the dark phase is the Laves phase, the gray phase is the silicide, and the light phase is the Nb-solid solution, Nbss. In Figure 1(d), the light phase along the grain boundary is a Ge-rich intermetallic, while the gray phase is the Nb-solid solution.…”

Ambient- to elevated-temperature fracture and fatigue properties of Mo-Si-B alloys: Role of microstructure

“…Furthermore, both of these mechanisms benefited from higher ␣-Mo volume fractions, leading to improved fracture and fatigue properties. [38][39][40][41][42] When assessed as a function of metallic-phase volume percent, the Mo-Si-B alloys appear in line with most Nb-Si alloys in terms of both K i and K max,TH . Note, the toughness values from Ref.…”

“…Also shown in Figure 10(a) are K c fracture toughness values (i.e., computed from the peak load at failure) from a study on Nb-Si-based multiphase alloys. [42] The R-curves were not measured in that study since little R-curve behavior was observed; accordingly, these K c values should approach the initiation toughness, K i , and be useful for comparison. These alloys, which often contained Laves phases as well as silicides, exhibit slightly superior toughness at low metallic volume percents, but in general appear to fall in line with the present Mo-Si-B alloys as the metallic phase volume percent increases.…”

“…[38][39][40][41][42] This system presents similar challenges to Mo-Si-B, with the Nb solid solution phase (Nb ss ) being ductile, but easily oxidized, and the niobium silicides being very brittle with relatively good oxidation resistance. Fracture 1m 1m 1m 1m and fatigue properties have been reported for alloys with 20 to 95 pct metallic Nb ss ; these properties are compared to Mo-Si-B alloys from the present study and References 8 and 9 in Figure 10.…”

“…[39,40] The latter can achieve toughnesses over 20 MPa after only a few hundreds of micrometers of growth [39,40] instead of several millimeters for the present alloys. Enhanced extrinsic toughening is often achieved through microstructural modification (e.g., References 36 and 43), and considering some Nb-Si-based alloys exhibit essentially no R-curve behavior, [42] there is most likely room for improvement in the Mo-Si-B alloys. For example, the Nb-Si-based alloys described in References 39 and 40 had highly anisotropic microstructures achieved by either 1m 1m 1m directional solidification or extrusion processing, with the fracture properties measured transverse to the extrusion or solidification direction.…”