Microstructural Influences on Very-High-Cycle Fatigue-Crack Initiation in Ti-6246

Abstract

The fatigue behavior of an alpha + beta titanium alloy, Ti-6Al-2Sn-4Zr-6Mo, has been characterized in the very-high-cycle fatigue (VHCF) regime using ultrasonic-fatigue (20 kHz) techniques. Stress levels (r max ) of 40 to 60 pct of the yield strength of this alloy have been examined. Fatigue lifetimes in the range of 10 6 to 10 9 cycles are observed, and fatigue cracks initiate from both surface and subsurface sites. This study examines the mechanisms of fatiguecrack formation by quantifying critical microstructural features observed in the fatigue-crack initiation region. The fracture surface near the fatigue-crack-initiation site was crystallographic in nature. Facets, which result from the fracture of primary alpha (a p ) grains, are associated with the crack-initiation process. The a p grains that form facets are typically larger in size than average. The spatial distribution of a p grains relative to each other observed near the initiation site did not correlate with fatigue life. Furthermore, the spatial distribution of a p grains did not provide a suitable means for discerning crack-initiation sites from randomly selected nominal areas. Stereofractography measurements have shown that the facets observed at or near the initiation sites are oriented for high shear stress; i.e., they are oriented close to 45 deg with respect to the loading axis. Furthermore, a large majority of the grains and laths near the site of crack initiation are preferentially oriented for either basal or prism slip, suggesting that regions where a p grains and a laths have similar crystallographic orientations favor crack initiation. Microtextured regions with favorable and similar orientations of a p grains and the lath a are believed to promote cyclic-strain accumulation by basal and prism slip. Orientation imaging microscopy (OIM) indicates that these facets form on the basal plane of a p grains. The absence of a significant role of spatial clustering of a p grains, coupled with the observation of regions of microtexture on the order of 300 to 500 lm supports the idea that variability in fatigue life in the very-high-cycle fatigue regime results from the variability in the nature (intensity, coherence, and size) of these microtextured regions.

Reducing uncertainty in fatigue life limits of turbine engine alloys

“…4), while some of the specimens failed from internal initiation sites under maximum stresses of 860 MPa or lower. Further details on the microstructure, experimental methods, results, and analysis are provided in [37,[39][40][41]54], which served as the backbone of the results to be presented below. residual-stress-free electropolished surfaces.…”

“…In this approach, the lifetime distribution was modeled as a superposition of the probability densities of the crack-growth-dominated lifetime (Type I) and the crack-initiation-dominated, mean lifetime (Type II). The underlying initiation and cracking mechanisms for this material have been reported elsewhere, and a key factor in the fatigue variability of this material was microtexture [40][41][42][54][55][56]. The total lifetime density was represented by:…”

“…2. This separation in fatigue lifetimes, known as bimodal or competing-modes fatigue, has been observed in a wide range of alloys, including the superalloys: Rene'95 [21,22], Rene'88DT [23], IN100 [8,[24][25][26][27][28][29], Waspaloy [30], the single crystal alloy PWA 1484 [31], the titanium alloys Ti-10-2-3 [23,[32][33][34], Ti-6Al-2Sn-4Zr-6Mo [35][36][37][38][39][40][41][42], Ti-6Al-4V [43][44][45][46][47], gamma titanium aluminides [48,49], the aluminum alloy 7075-T651 [50], and others. Although such a separation of fatigue response has been known for some time [51], the significance of this behavior has not yet been generally captured in the strategies for fatigue design of turbine engine materials.…”

Analysis of Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

“…1,2) The subsurface fatigue crack initiation sites in near-α and α-β type titanium alloys commonly appear crystallographic transgranular facet or facets. [1][2][3][4][5][6][7][8] Mostly the facets formed on or near the {0001} basal plane regardless their inclination to the principal stress axis. Dislocation movement in α phase is restricted on the primary slip plane and fairly planar so that dislocation arrays on {01 1 0} < 11 2 0 > are piled-up in the vicinity of grain boundaries and a local stress concentration generates near α grain boundary.…”

Cracks in superalloys

“…Crack initiation transition from surface to sub-surface/internal pores and inclusions have been reported to occur during giga-cycle fatigue testing [45,57]. In addition, texture can affect fatigue crack initiation in Ni-based superalloys and Ti alloys [58][59][60][61]. Clustered grains can act more like a single grain exhibiting similar crystallographic orientation and enable slip across low angle grain boundaries, thereby creating the possibility for slip to occur easier over longer distances, resulting in dislocation pile-ups and stress concentrations at grain boundaries.…”