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THE TEMPERATURE DEPENDENCE of SUSTAINED LOAD CRACKING in Ti-6211 C.L

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THE TEMPERATURE DEPENDENCE of SUSTAINED LOAD CRACKING in Ti-6211 C.L 2557 THE TEMPERATURE DEPENDENCE OF SUSTAINED LOAD CRACKING IN Ti-6211 C.L. Hoffmann, Combustion Engineering, Windsor, CT 06095 USA J.E. Cox, R.W. Judy, Jr., and B.B. Rath, Washington, DC 20375 USA In recent years, the design of structures and components has appropriately e1nphasized fail-safe principles. The basis for these principles includes the recognition that cracks and voids that either are initially in the components or are formed early during the service 1 i fe can continue to grow during service. The durabi 1ity or useful 1ife. of a component is directly related to the rate of degradation of its load-bearing capacity by subcritical crack growth and is, therefore, a function of the material's resistance to subcritical crack growth [1-3]. Sustained load cracking (SLC) due to internal hydrogen in titanium alloys is very complex and depends on alloy composition, microstructure, hydrogen content, and applied stress intensity. One area of interest in investigations of SLC in the near-alpha alloy Ti-6Al-2Cb-1Ta-0.8Mo (Ti-6211) is the temperature dependence of crack growth; this aspect can provide information concerning t herma 1 ly activated rate control 1i ng processes which may determine SLC crack growth rates [4]. In work on the titanium alloy Ti-6Al-6V-2S·n, Moody and- Gerberich [5,6] calculated an activation energy for SLC crack growth in air of approximately 30 kJ/mol. The value was related to the diffusion of nydrogen through the beta phase. Threshold stress intensities for SLC crack growth decreased slightly as temperature was decreased below ambient temperature. A significant increase in threshold values was observed at temperatures greater than ambient temperature. The behavior at 1ow te1nµeratures was explained by the temperature dependence of beta phase hydrogen· diffusivity. Difficulty in nucleating hydrides at higher tenperatures was suggested as an explanation for the observed increase in threshold stress intensity. Another study of Ti-6Al-6V-2Sn [7] obtained an activation energy of 14.4 kJ/mol and attributed the temperature dependence of SLC to stress induced hydride precipitation. The proposed model consisted of stress-enhanced transport of hydrogen and nucleation of a cloud of hydrides at the crack tip. Cracking proceeded by a rupture process that linked up cleaved regions resulting fr~n the presence of hydride precipitates. In addition, activation energies have ·been calculated for stress corrosion and corrosion fatigue in T(-6Al-4V [8,9] and cracking of Ti-5Al-2.5Sn and Ti-5Al-4V in gaseous hydrogen [10,11]. All of these studies agree that the cracking process is related to the transport of hydrogen and hydride formation in titanium alloys [8-13]. In the present study, the tenperature dependence of Ti-6Al-2Cb-1Ta-0.8Mo (Ti-5211) alloy as a functio~ of hydrogen content is examined. SLC crack growth rates and threshold s~ress intensities were determined in an effort to define the rate controlling parameter for SLC in this alloy. Material and Test Methods Beta processed, 25 mm (one inch) thick plate material of Ti-6211 was used 2558 for this segment of the experimental work. A typical mi crostructure for this material is shown in Fig. 1. Crack growth rates as a function of applied stress intensity (K) were determined using side-grooved WOL specimens. Standard pin and clevis loadiny in static load frames was used for determinations of crack growtl1 rates at ambient temperatures [14]. I 40 Plllj Fig. 1: Microstructure of beta processed Ti-6211. To facilitate determinations of crack growth rates at temperatures other than ambient, a bolt-loaded WOL specimen was used. A constant displacement, decreasing load (and hence, a decreasing stress intensity) SLC test is performed by this method; in addition, the loading bolt is instrumented, allowing the loads to be monitored and the crack growth rates to be computed. A threshold stress intensity for SLC is also obtained since the stress intensity and the crack growth rate both decrease with increased crack length. When the crack growth rate approaches arrest conditions, the stress intensity approaches the threshold for SLC. Good agreement between crack growth rate data obtained by pin-loaded and bolt-loaded methods was demonstrated for the Ti-6211 material. SLC crack growth tests were perfonned for a temperature range of -7S°C to +120°C. Hydrogen was charged into the WOL specimens. using a modified Sieverts apparatus. The charging temperature was 760°C (1400°F), which was not high enough to cause modifications to the microstructure. Results and Discussion SLC crack growth rates as a function of temperature are compared in Figure 2 for a hydrogen content of 1000 ppm and in Figure 3 for a .hydrogen content of 2SO ppm. Increasing hydrogen content caused increased crack growth rates and decreased threshold stress intensity values for SLC. Similar effects of hydrogen have been observed for SLC crack growth and fatigue crack growth in other titanium alloys [4,7,12,16]. The 1000 ppm material exhibits the highest SLC crack growth rates and lowest threshold stress intensity near ambient temperature (2S°C) (Figure 2). Crack growth rate decreases and threshold stress intensity level increases when the temperature is lowered to -1S°C. Further decrease in the temperature to -7S°C raises the SLC threshold still higher. There is a crossover in crack growth rates at -7S°C, which is probably related to a decrease in fracture toughness at this temperature. The same trends are observed at temperatures higher than ambient. Raising the temperature .to 70°C increases the SLC threshold and decreases the growth rate;: At a tanperature of 120°C, resistance to sustained load cracking increases considerably. 2559 K (1111 ..ll'Fi:I K (ksi ,/Iii.) 20 30 40 1ci' 20 30 40 "° 60 70 Tl-6211 10° Ti-6211 1rf' 1000 ppm H 1 1 10- j 10- ~ 10-2 .§'. 10-2.::. 10-1 120-C 15 20'C 10-3 ~ 10-3 g 10-2 10-4 .~~:"_~. 10-4 10-3 10-• ~ .. 10 30 40 50 20 30 40 50 60 JD 80 K (MPo,/iii) K (MPo ./ml Fig. 2: SLC crack growth rate Fig. 3: SLC crack growth rate vs stress intensity as a func- vs stress intensity as a func- tion of tanperature with 1000 tion of temperature with 250 PJlll hydrogen. PJlll hydrogen. The same trend for SLC threshold behavior is observed for material containin\:) 250 ppm hydrogen (Fig. 3). The threshold reaches a minimum at 25°C. No detectable SLC crack growth occurred at temperatures above room temperature at the 250 ppm hydrogen level. Specimens were loaded to high stress intensities at 70°C for long times with no apparent crack growth takiny place. The crack growth rates for 250 PJlll hydrogen material do not exhibit any clear pattern because of the crossover in the data. At low stress intensities, maximum SLC growth rates are observed at 25°C. At higher stress intensity levels, maximum growth rates occur at -15°C. This behavior may be related to greater driving force for hydride formation at -15°C than at +25°C. At -75°C, the rate of diffusion of hydrogen may be slowed enough to reduce the rate of crack growth again. Curves of the type shown in Figure 4 are obtained when standard Arrhenius type plots are constructed for the SLC crack growth data. The SLC crack growth rates versus reciprocal absolute temperature are plotted at four stress intensity levels for 1000 PJlll hydrogen material (left) and at five stress intensity levels for 250 piJ11 hydrogen lnaterial (right). The curves show very strong dependence of SLC crack growth rates on stress intensity. Crack ~rowth rates should be relatively independent of temperature in order to determine a thermal activation energy for the.process. A weaker stress intensity dependence of activation energy was observed during stress corrosion cracking of Ti-6Al-4V [8]. The decrease in activation ener~ with increased stress' intensity was attributed to lattice dilation ahead of the· crack tip alrowing for faster diffusion rates. Increased lattice di lat ion also lowers the resistance to hydride nucleation, since the volume expansion of the hydride phase will be more easily accommodated [17-21]. The strong stress ·intensity dependence of SLC crack growth rates in Ti-6211 as a function of. temperature precludes the determination of a unique thermal activation erieryy for the SLC crack growth process. The dependence of SLC thresholds on temperature for hydrogen levels of 250 p'pm and 1000 ppm is shown in: Fig. 5. A minimum threshold stress intensity is observed near ambient temperature at both hydrogen levels. At higher temperatures the threshold .increases rapidly to the point where no SLC crack \:)rowth will occur, as in the case of the material containing 250 PiJTI hydrogen tested at 70°C. This observation is consistent with SLC results 2560 100-----------~ 1rfl SLC IN Ti-6211 AIR SLCN~H 250 ppm H ~~ 28 K=26MPovm 164 '--~---'---~---'--~-.l-----' 2.0 30 4.0 1 1/T (1000x"K- l Fig. 4: Arrhenius plot of SLC crack growth rate vs reciprocal temperature as a function of stress intensity for beta processed Ti -6211 with 1000 PJJTI hydrogen (left) and 250 pJJTI hydrogen (right). on other titanium alloys; other investigators have concluded that difficulty in nucleation of hydrides above ambient temperature is responsible for increased thresholds or for the need for higher stress intensities to produce a given crack growth rate at higher temperatures [5,7]. At temperatures below ambient, the SLC threshold increases for both hydrogen levels. However, as the temperature is decreased further, the threshold appears to become independent of temperature.
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