Fatigue crack propagation behavior of titanium alloys 6242S and 5621 S at elevated temperature

The influence of test environment on the fatigue crack propagation performance of titanium alloys Ti-6A1-2Sn-4Zr-2Mo-O. 1Si (6242S) and Ti-5A1-6Sn-2Zr-0.8Mo-0.25Si (5621 S) was investigated at 450 °C. The results show that the crack propagation resistance of 6242S and 5621S was reduced in air compared to that in vacuum at a loading frequency of 0.17 Hz. A similar environmental behavior was observed when a one-minute hold period was included in the fatigue cycle. Both alloys exhibited excellent creep-fatigue resistance at 450 °C as little effect of hold time was observed. Overall, the fatigue crack propagation resistance of 6242S was found to be slightly superior to that of 5621S. These results are discussed in terms of the effects of thermal-mechanical history and microstructure on the fatigue crack propagation characteristics of titanium alloys. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Microstructure-ultrasonic inspectability relationships in Ti6242: Signal-to-noise in fine-grain-processed Ti6242

The ultrasonic behavior of titanium alloy Ti6242 was determined as a function of ultrasonic frequency and material microstructure. The signal-to-noise ratio for synthetic flaws machined in the Ti6242 blocks was strongly influenced by microstructural condition, particularly as it was defined by electron backscatter diffraction pattern (EBDP) analysis. Ti6242 blocks with a microstructure consisting of uniform, fine, texture-free αTi particles had signal-to-noise ratios about 20 dB greater than blocks with microstructures consisting of colonies of crystallographically aligned αTi particles.     

Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10^-8 to 10^-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK₀ values reported to date, and certainly the highest combination of strength and ΔK₀ for steels (i.e., ΔK₀ values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations. Formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California     

Interface microstructures in the diffusion bonding of a titanium alloy Ti 6242 to an INCONEL 625

A Ti6242 alloy has been diffusion bonded to a superalloy INCONEL 625. The microstructures of the as-processed products have been analyzed using optical metallography, scanning electron microscope (SEM), and scanning transmission electron microscope (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy-dispersive spectroscopy (EDS) microanalysis in both a SEM and a STEM. Several regions around the original interface have been observed. Starting from the superalloy INCONEL 625, first a sigma phase (Cr₄Ni₃Mo₂), followed by several phases like NbNi₃, Ŋ/Ni₃Ti, Cr(20 pct Mo), β Cr₂Ti, NiTi, TiO, TiNi, and Ti₂Ni intermetallics, just before the Ti6242 have been identified. Because the diffusion of Ni in Ti is faster than the diffusion of Ti in the superalloy, a Kirkendall effect was produced. The sequence of formation of the different phases were in agreement with the ternary Ti-Cr-Ni diagram.     

Fatigue crack growth behaviour of nickel based superalloys at ambient temperature

The present investigation deals with the study of fatigue crack growth rate (FCGR) behaviour of conventional IN 718 (Ni-0.02%C-19.0%Cr-19.35%Fe-3.0%Mo-5.10%Nb-0.50%Al-1.00%Ti-0.0033%B, all in wt%) and modified IN 718 (Ni-0.02%C-19.04%Cr-19.31%Fe-3.04%Mo-4.73%Nb-1.01%Al-1.16%Ti-0.0033%B, all in wt%) alloys at ambient temperature. Modified IN 718 alloy exhibits enhanced crack growth resistance due to roughness induced crack closure as compared to conventional IN 718 alloy.     

Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loading

The transverse fatigue crack growth resistance of unidirectional 8 and 35 pct 1140+/Ti-6-4 fiber-reinforced composites has been investigated. It has been found that, at a low fiber volume fraction, the transverse fatigue crack growth resistance of metal-matrix composites (MMCs) is improved with respect to the monolithic matrix alloy. This occurs because “holes” from debonded interfaces can trap the crack and reduce the average fatigue crack growth rates by periodically increasing the effective crack-tip radius. However, an increase of fiber volume fraction from 8 to 35 pct decreases the fatigue crack growth resistance dramatically, due to the significant increase of the frequency of interaction and coalescence between the main crack, the debonded interfaces, and microcracks.     

Effect of α phase on fatigue crack growth of Ti-6242 alloy

Fatigue crack growth as a function ofαphase volume fraction in Ti-6Al-2Sn-4Zr-2Mo(Ti-6242)alloy was investigated using fatigue testing,optical microscopy,scanning electron microscopy,and transmission electron microscopy.Theα+βannealing treatments with different solid solution temperatures and cooling rates were conducted in order to tailor microstructure with differentαphase features in the Ti-6242 alloy,and fatigue crack growth mechanism was discussed after detailed microstructure characterization.The results showed that fatigue crack growth rate of Ti-6242 alloy decreased with the decrease in volume fraction of the primaryαphase(αp).Samples with a large-sizedαgrain microstructure treated at high solid solution temperature and slow cooling rate have lower fatigue crack growth rate.The appearance of secondaryαphase(αs)with the increase of solid solution temperature led to crack deflection.Moreover,a fatigue crack growth transition phenomenon was observed in the Paris regime of Ti-6242 alloy with 29.8% αp(typical bi-modal microstructure)and large-sizedαgrain microstructure,owing to the change of fatigue crack growth mechanism.     

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m^1/2) in deaerated deionized water and 0.3 M Na₂SO₄ solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na₂SO₄ solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.     

Fatigue crack growth rates in a pressure vessel steel under various conditions of loading and the environment

Corrosion fatigue (CF) tests have been carried out on SA508 Cl 3 pressure vessel steel, in simulated P.W.R. environments. The test variables investigated included air and P.W.R. water environments, frequency variation over the range 1 Hz to 10 Hz, transverse and longitudinal crack growth directions, temperatures of 20 °C and 50 °C, andR-ratios of 0.2 and 0.7. It was found that decreasing the test frequency increased fatigue crack growth rates (FCGR) in P.W.R. environments, P.W.R. environment testing gave enhanced crack growth(vs air tests), FCGRs were greater for cracks growing in the longitudinal direction, slight increases in temperature gave noticeable accelerations in FCGR, and several air tests gave FCGR greater than those predicted by the existing ASME codes. Fractographic evidence indicates that FCGRs were accelerated by a hydrogen embrittlement mechanism. The presence of elongated MnS inclusions aided both mechanical fatigue and hydrogen embrittlement processes, thus producing synergistically fast FCGRs. Both anodic dissolution and hydrogen embrittlement mechanisms have been proposed for the environmental enhancement of crack growth rates. Electrochemical potential measurements and potentiostatic tests have shown that sample isolation of the test specimens from the clevises in the apparatus is not essential during low temperature corrosion fatigue testing. P. D. HICKS, formerly Graduate Student at The University of The Witwatersrand, South Africa     

Fatigue crack growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔK _th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (C _cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C _cr , ΔK _th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔK _th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.     

Fatigue crack growth in fiber-reinforced metal-matrix composites

Fatigue crack growth in fiber-reinforced metal-matrix composites is modeled based on a crack tip shielding analysis. The fiber/matrix interface is assumed to be weak, allowing interfacial debonding and sliding to occur readily during matrix cracking. The presence of intact fibers in the wake of the matrix crack shields the crack tip from the applied stresses and reduces the stress intensity factors and the matrix crack growth rate. Two regimes of fatigue cracking have been simulated. The first is the case where the applied load is low, so that all the fibers between the original notch tip and the current crack tip remain intact. The crack growth rate decreases markedly with crack extension, and approaches a “steady-state”. The second regime occurs if the fibers fail when the stress on them reaches a unique fiber strength. The fiber breakage reduces the shielding contribution, resulting in a significant acceleration in the crack growth rate. It is suggested that a criterion based on the onset of fiber failure may be used for a conservative lifetime prediction. The results of the calculations have been summarized in calibrated functions which represent the crack tip stress intensity factor and the applied load for fiber failure.     

Fatigue crack growth in Si−Fe

Fatigue crack growth rates were measured in vacuum in the temperature range from 100° to 250°C. At 200°C and above, crack growth occurs by a ductile mechanism. The ductile crack growth rate is proportional to the stress intensity factor amplitude raised to the 5/2 power, is equal to about one-tenth of the crack opening displacement per cycle, and is not measurably dependent on the peak stress intensity factor in the range measured. Local crack growth occasionally becomes arrested by large scale blunting of the crack tip. The fracture surface has many “river markings” and often adheres to specific growth planes on a scale of the grain size. The overall crack growth is approximately in the plane of maximum tensile stress. Inclusions and grain boundaries are not of importance in the growth mechanism in the range of crack growth rates measured. Growth rates in air at 200°C are at most twice those in vacuum. Although a mechanism, using continuum mechanical concepts, can explain the external features of the ductile crack growth rate, the local features showing a combination of all three modes of straining suggests that the actual process of crack growth must be far more complicated than hitherto realized. At 150°C and below, the ductile growth mechanism is augmented by the wholesale cleavage of single grains or grain clusters. The growth rates at these temperatures are higher and more sensitive to stress intensity factor amplitude.     

Fatigue crack growth mechanisms at the microstructure scale in Al-Si-Mg cast alloys: Mechanisms in regions II and III

The fatigue crack growth behavior in Regions II and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions II and III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively through Al-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress ratio,R=0.1, in ambient conditions.     

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.     

Fatigue crack initiation and microcrack growth in 4140 steel

Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions: as-quenched, tempered at 400°C, and tempered at 650°C. Micro-scopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an x-y micrometer base mounted directly on an MTS machine. Following Barsom and McNicol, the cycles to fatigue crack initiationN _i were plottedvs ΔK/√p and threshhold values of gDK√p were determined. The data of logN _i vs log [ΔK/√p-ΔK/√p|_th] fit on a straight line. Microcracks grew most rapidly in as-quenched specimens and least rapidly in 650°C tempered specimens at the same ΔK/√p. In as-quenched specimens, fatigue cracks initiated at grain boundaries but in the 400 and 650°C tempered specimens they initiated at intrusions-extrusions. They are also associated with Northwestern’s Materials Research Center.     

Fatigue crack growth in an as-rolled dual-phase steel

Fatigue crack propagation behaviour of an as-rolled dual-phase steel is investigated over a wide range from 10^?10 to 10^?6 m/cycle in a 3.5% NaCI solution and laboratory air under different load ratios. It was found that the as-rolled dual-phase steel studied in the present investigation shows good resistance to fatigue crack growth in laboratory air. The threshold values decrease with increasing stress ratios, which is consistent with the competition model proposed previously. The threshold value obtained using a 3.5% NaCI solution is higher than that in the laboratory air owing to the wedge effect of corrosion products within the crack. Fatigue crack growth rates in the higher ΔK range obey the Paris formula in each case within this study.     

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute