An analysis of frequency and amplitude effects on corrosion fatigue crack propagation in Ti-8Al-1Mo-1V
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material have only a minor influence on susceptibility. n-14 This background of general understanding of the potentially twofold nature of corrosion fatigue provides a basis for evaluating the corrosion fatigue characteristics of high strength alloys which are susciptible to SCC, such as many titanium alloys. To establish the cyclic and environmental parameters which are of importance in assessing the corrosion fatigue resistance of aluminum-containing a and a-/3 titanium alloys, an experimental program was initiated using Ti-SAI-IMo-lV, which exemplifies the group of high strength alloys susceptible to SCC. A wide range of cyclic amplitudes, with frequencies from 1/20 to 30 cps, was employed to bring out the effects of both Type A and Type B corrosion fatigue in salt water, methanol, distilled water, air, and, as an inert reference environment, vacuum. The results of crack growth rate measurements, together with fractographie and X-ray diffraction studies, are correlated to suggest the operating mechanisms. MATERIALS
AND
PROCEDURES
Most of the results to be presented and discussed were obtained using two heats of mill-annealed Ti8AI-Mo-IV sheet. Table I gives their compositions. The microstructure consisted of ol phase (hcp), with particles of/3 phase (bcc) distributed in a network throughout the alpha matrix, Fig. i. When coarse grains were necessary to index cleavage planes by X-ray methods, this coarsening was done by heating the 0.270 in. sheet to 1950~ for 4 hr and furnace cooling in a vacuum. The resulting microstructure was similar, except for the grain size, to that of the mill-annealed starting material, Fig. l(c). Most of the data were obtained from center-notched specimens, shown in Fig. 2. Some additional data were measured using specimens of the design shown in Fig. 3 for reasons of economy and ease of alignment. A closed-loop servo-controlled hydraulic universal testing system made it possible to use a wide range of VOLUME 2, MARCH 1971-853
Table I. Properties and Composition of Ti-8AI-IMo-IV Alloys
NRL No.
Thickness, in.
0.2 pct Offset Yield Strength, psi
TT-5* R-7* R-7V
0.150 0.270 0.220 to 0.235
138,000 134,000 t
Composition, wt pct Fe C
A1
Mo
V
7.9 7.4
1.0 1.1 1.16 0.86 ..... Same as R-7.....
0.06 0.14
0.02 0.08 NA
O
N
0.09 0.086 NA
0.006 0.004 NA
H, ppm 86 40 21 :i:
*Analysesby supplier. tHardness 33 Rc, compared with 38 Rc for R-7. :~NRL analysis.
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Fig. 2 - - C e n t e r - n o t c h e d s h e e t t e s t s p e c i m e n . The s t a r t i n g notch length. 2a 0, was about 0.20 in.
(a)
(b)
frequencies and loading programs, even on a single specimen, without disturbing the test arrangement. The cyclic waveform was always sinusoidal or approximately sinusoidal. The minimum and maximum load p e a k s w e r e m a i n t a i n e d w i t h i n 1 p c t of n o m i n a l v a l u e . The liquid environments were retained by a small polyethylene cup, with a glass microscope slide view port, cemented to the specimen using R.T.V. silicone compound. The resulting solution volume w
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