Correlation of creep elongation and substructure in aluminum-stainless steel composites
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Fig. 1--Schematic representation for the prediction of creep rates in continuous fiber-reinforced m e t a l - m a t r i x composites; after McDanels, et al.~ 2 (a-l) Fiber; (a-2) matrix; (b) log s t r e s s - l o g c r e e p plots; (c) composite s t r e s s - f i b e r content plots for s e v e r a l c r e e p rates.
with 0.006 in. diam cold-drawn stainless steel wire. The wire (15.5 pet Cr, 4.3 pet Ni, 2.7 Mo) is bee (lattice parameter 2.88A) and possesses a tensile strength ->425,000 psi. Composites containing 4.1, 15.3, and 32.8 pet by volume wire reinforcement were prepared at Drexel University. Placement of the fibers was accomplished by continuously winding the wire over layers of aluminum sheet on a lathe. A uniform wire spacing within layers was maintained by the lateral feed rate of the lathe carriage. The wound sample was vacuum hot pressed at 930~ for 4 to 8 hr to complete the diffusion bond. The composite was then cut from the winding mandrel with a diamond slitting wheel and repressed for 2 hr at 900~ to ensure flatness of the specimen. A procedure of slow cooling to 200~ under pressure was used to cause a continual small flow of the matrLx and thereby relieve residual thermal stresses. Optical metallography of the as-pressed composites on sections cut perpendicular to the wire axis reveals a uniform interracial zone (< 1 p thickness) around the wires ;,z the aluminum-aluminum bond is continuous. This prewinding procedure gives excellent fiber distribution for the three volume fractions examined. 2) T e n s i l e C r e e p T e s t i n g The uniaxial time-dependent deformation behavior was monitored at 20~ for constant applied loads in the range 1.5 to 4 times the 0.1 pet yield stress, as determined in uniaxial tension for that particular volume fraction. A tensile loading fixture ~I mounted in an angle-iron support frame was used to provide precision uniaxial loading. Specimen elongation was monitored by two linear variable differential transformers (LVDT) mounted at 90 deg to each other, but parallel to the stress and fiber axes. Signals from the transformers were fed to an X-Y plotter to monitor elongation as a function of time. Elongation was monitored at a sensitivity of 5 • 10-4 in. per in. of chart. During the initial stages of creep (2 to 6 hr, depending on volume fraction of reinforcement) the elongation was recorded every 15 rain on both LVDT's. As the creep rate decreased, the monitoring interval was increased to one or two hours. Following achievement 1416-VOLUME 2,MAY 1971
Fig. 2--Strain-time curve for unreinforeed aluminum loaded to approximately twice the 0.1 pet yield stress level at room temperature. of s t e a d y - s t a t e c r e e p , t h e d a t a w e r e r e c o r d e d s e v e r a l t i m e s a d a y . T h e t o t a l t e s t t i m e w a s g e n e r a l l y in t h e r a n g e of 100 to 500 h r , w i t h a total s t r a i n of 0.5 to 1.0 pct. 3) Preparation of Electron-Transparent Composites The procedure for the preparation of electron-transparent foils of the composites has b
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