Low cycle fatigue of niobium–zirconium and niobium–zirconium–carbon alloys

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 278; no. 1; pp. 121 - 134
• Main Authors: Dickerson, Scott L, Gibeling, Jeffery C
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.02.2000; Elsevier
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Elasticity and anelasticity; Elasticity and anelasticity, stress-strain relations; Exact sciences and technology; Fatigue, corrosion fatigue, embrittlement, cracking, fracture and failure; Fatigue, embrittlement, and fracture; Low cycle fatigue; Materials science; Metals. Metallurgy; Niobium alloys; Physics; Strain rate; Treatment of materials and its effects on microstructure and properties; Niobium alloys; Low cycle fatigue; Strain rate; Hardening; Carbon alloys; Plastic deformation; Experimental study; Microstructure; Stress-strain relations; Heat treatments; Transition element alloys; Zirconium alloys
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/S0921-5093(99)00591-2

Constant plastic strain amplitude low cycle fatigue tests were performed on two niobium alloys at both room temperature and at 573 K. In order to fully explore the influence of strain rate on the cyclic stress–strain response, a new testing procedure was developed to conduct tests at constant true plastic strain rate. The two alloys tested were a solid solution strengthened alloy (Nb–1Zr) and a precipitation strengthened alloy (Nb–1Zr–0.1C, also known by the commercial designation PWC-11). The PWC-11 alloy was heat treated to produce two different microstructures: fine grained (PWC-11(A), 20–50 μm grain size), and coarse grained (PWC-11(B), 70–100 μm grain size). The Nb–1Zr alloy generally exhibited hardening to a stable, saturated state in each test. Tests conducted on Nb–1Zr at 573 K at plastic strain rates of 10 −4 s −1 and 10 −3 s −1 had essentially the same saturation stress, confirming that strain rate does not influence the cyclic deformation behavior of this alloy under these test conditions. The PWC-11 alloys exhibited stable saturation in room temperature tests, but at 573 K they exhibited continuous softening. Furthermore, at 573 K the cyclic stress–strain curves for the PWC-11 alloys had shallower slopes than those of Nb–1Zr, while the monotonic stress–strain curves had greater hardening rates than did those of Nb–1Zr. These observations are consistent with precipitate strengthening that is diminished at elevated temperatures and by cyclic deformation. In terms of practical significance, however, these effects are minor, and the PWC-11 alloys exhibited low cycle fatigue behavior that was very similar to that of Nb–1Zr. Both alloys exhibited grain shape changes leading to intergranular cracking as the dominant failure mechanism.