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METALLURGICAL TRANSACTIONS A

to contain small amounts (up to ~5 pet) of c~-precipitates the deformation mode and LCF behavior remains almost unchanged. However, with further aging the volume fraction of ~ increases, and the deformation behavior changes to planar slip. During LCF the a s quenched and low ~ 24 pct V alloys deform initially by twinning and subsequently by wavy multiple slip. The as-quenched 32 pct V alloy usually deforms by planar slip. The behavior of the as-quenched 28 pct V alloy is somewhat intermediate between the 24 and 32 pct V alloys. By using the Ti-V system, one may independently vary m i c r o s t r u c t u r e or deformation mode by suitably varying the heat treatment and/or composition. This allows an examination of the effect of each p a r a m e t e r on the FCP resistance. EXPERIMENTAL

Three Ti-V alloys containing 24, 28, and 32 wt pet vanadium were prepared by Titanium Metals C o r p o r a tion (Henderson, Nevada), where 14 cm diam ingots were hot forged and c r o s s - r o l l e d to produce 15 x 15 x 0.8 cm plates having a random texture. The chemical analysis and grain intercept lengths of these materials are shown in Table I. Samples from the a s - r e ceived plates were solutionized at 850~ in dry argon and quenched in iced brine. Some samples were aged to produce the desired microstructure and deformaion mode (Table I). Compact tension 25 samples with H/W of 0.486, W = 44.5 mm, and B = 6.4 mm were used to measure the fatigue crack growth rate (FCGR). FCGR measurements were performed on a servohydraulic closed-loop MTS testing machine using tension-tension sinusoidal loading with a minimum/maximum load ratio (R) of 0.1 at a frequency of 10 Hz. The specimens were first precracked at AK = 15 MPa~/-m and then the AK was gradually reduced to ~5 MPaV~m where crack growth essentially stopped. The zXK was then increased to a value corresponding to a FCGR of ~2 x 10-3 m / cycle where FCP measurements commenced and the load amplitude was henceforth kept constant. Multiple specimens were employed to define FCP curves of each material. Accurate threshold measurements were not made and no emphasis is placed on the AK value corresponding to the initial FCGR. The tests were carried out in laboratory air with a relative

ISSN 0360-2133/79/1211-1901500.75/0 9 1979 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME

VOLUME 10A, DECEMBER 1979-1901

h u m i d i t y of 27 to 30 pet and at a t e m p e r a t u r e of 23 to 25~ T h e c r a c k length was m e a s u r e d within • 0.01 m m on the p o l i s h e d s u r f a c e of the s p e c i m e n using a G a e r t n e r t r a v e l l i n g m i c r o s c o p e . The m i c r o s t r u c t u r e ,

d e f o r m a t i o n and f r a c t u r e b e h a v i o r was c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n , and o p t i c a l and e l e c t r o n m i croscopy.

RESULTS Table I. Composition, Heat Treatment, Microstructure and Deformation Modes of Alloys Used for Fatigue Crack Propagation Studies

Designation and Group Compositi