Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism

Abstract

Bulk-metallic glasses (BMGs) are now candidate materials for structural applications due to their exceptional strength and toughness. However, their fatigue resistance can be poor and inconsistent, severely limiting their potential as reliable structural materials. As fatigue limits are invariably governed by the local arrest of microscopically small cracks at microstructural features, the lack of microstructure in monolithic glasses, often coupled with other factors, such as the ease of crack formation in shear bands or a high susceptibility to corrosion, can lead to low fatigue limits (some ~1/20 of their tensile strengths) and highly variable fatigue lives. BMG-matrix composites can provide a solution here as their duplex microstructures can arrest shear bands at a second phase to prevent cracks from exceeding critical size; under these conditions, fatigue limits become comparable with those of crystalline alloys. Here, we report on a Pd-based glass that similarly has high fatigue resistance but without a second phase. This monolithic glass displays high intrinsic toughness from extensive shear-band proliferation with cavitation and cracking effectively obstructed. We find that this property can further promote fatigue resistance through extrinsic crack-tip shielding, a mechanism well known in crystalline metals but not previously reported in BMGs, whereby cyclically loaded cracks propagate in a highly "zig-zag" manner, creating a rough "staircase-like" profile. The resulting crack-surface contact (roughness-induced crack closure) elevates fatigue properties to those comparable to crystalline alloys, and the accompanying plasticity helps to reduce flaw sensitivity in the glass, thereby promoting structural reliability.

Fatigue endurance limit and crack growth behavior of a high-toughness Zr61Ti2Cu25Al12 bulk metallic glass

“…Such roughness-induced crack closure has an effect to reduce the driving force for crack propagation. As a matter of fact, such extrinsic toughening mechanism is in accord with the finding previously reported for the damage-tolerant Pd-based BMG [28]. An important message here is that these high-toughness BMGs characterized by an extensive crack-growth resistance (R-curve behavior) all exhibit a consistent behavior that their fatigue endurance is enhanced through a ''staircase-like fracture mechanism''.…”

“…In the phase of crack propagation, the resistance to crack growth is contributed by two sources, crack-tip plasticity via shear-banding and crack-deflection in a ''zigzag'' manner. This mechanism seems to be common to high-toughness BMGs, as it was also recently reported for the Pd-based BMG [28]. A higher propensity for multiple shear banding, similar to the case for high fracture toughness, improves the fatigue resistance in the high DK regime.…”

“…Recently, BMGs characterized by a high crack-growth toughness (crack-resistance curve) have been discovered, firstly a noble Pd-based Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 [25], and later a Zr-based Zr 61 Ti 2 Cu 25 Al 12 (in at.%, denoted as ZT1) [26,27]. It is encouraging that the Pd 79 Ag 3.5 P 6 Si 9.5 Ge 2 BMG manifests a higher-than-normal fatigue resistance, as indicated by the fatigue endurance strength of r a = 360 MPa, or 0.24r u (in four-point bending at R = 0.1), and the DK th is at 3.3 MPa p m [28]. A distinct feature for this BMG is that the fatigue crack propagates in a highly ''zigzag'' manner, creating a rough ''stair-like'' profile.…”

“…In the plot, the K IC of the in terms of the number of loading cycles, N f , versus the applied stress amplitude, r a , normalized by the ultimate tensile strength, r u . These data were all obtained in four-point bending tests [5,12,14,16,28,37].…”

Biomedical materials and techniques to improve the tribological, mechanical and biomedical properties of orthopedic implants – A review article

“…Generally, Zr-based BMG alloys are promising materials for biomedical applications because of their excellent mechanical and biocompatibility properties. However, Zr-based BMG alloys products are limited in size, as well as, they are very brittle in tension and show less plasticity in compression [53].…”