On the cyclic behavior of a directionally solidified eutectic alloy containing ductile fibers
- PDF / 2,627,806 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 28 Downloads / 252 Views
growth direction, several mm in length and approximately one mm in transverse dimension. The molybdenum fibers contained within the grains were of the order of one micron in edge dimension and 1000 microns in length. The volume fraction of the fibers, VI, was 0.2 and the spacing between fibers was 2 to 3 microns as determined by metallographic analysis. In order to facilitate observations of the specimens by light microscopy, flat sided specimens for monotonic and cyclic testing were prepared by a combination of grinding, electro-discharge machining (EDM) and final polishing with 0.3 micron alumina. The specimens were approximately 100 mm in overall length with gripping areas 35 x 10 x 5 mm and a gage section 5 x 5 mm in cross-section and 15 mm in length which was parallel to the growth direction. Axial strain was measured with an extensometer which had a gage length of 12.7 mm and a strain sensitivity of 10-5 . Cyclic tests were carried out under straincontrolled conditions in either a screw-driven Instron machine at 0.2 Hz, or, for tests of longer duration, in an electro-hydraulic closed-loop fatigue system at 3 Hz. Samples from the as-fabricated alloy as well as the tested specimens were examined by both optical microscopy and by transmission electron microscopy (TEM). Thin foils for TEM were prepared from discs which were 3 mm in diam and 0.3 mm in thickness. These discs were cut either parallel to or perpendicular to the growth direction by EDM. The discs were then reduced further in thickness by jet-machining and electropolishing in a solution containing 20 pct H 2 S O 4 in methanol. The resultant foils were examined in a 100 kV Phillips electron microscope which was equipped with a tilt-rotation stage. Electron micrographs were obtained under two-beam imaging conditions and analyses were carried out to determine the operative slip systems. The initial microstructure of the 3"-Mo eutectic alloy is shown in Fig. 1. On cooling from the melting temperature plastic deformation primarily occurred within the matrix at the fiber-matrix interface, Fig. l(a). Typically two sets of 3" dislocations (indicated
ISSN 0360-2133/81 / 1111-1909500.75/0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12A, NOVEMBER 1981--1909 THE METALLURGICAL SOCIETY OF AIME
Fig. l--Illustration of as-grown dislocations. (a) 3" dislocations near planar fiber-matrix interface. (b) Dislocations in Mo fiber. (c) and (d) Dislocation networks (N) and arrays (P) in "t' near irregular fiber shapes. by a and b in Fig. l(a)) were found distributed on the octahedral slip planes along an interface. In addition to the interfacial dislocations, dislocation networks (N) and dislocation arrays (P) were present in the asgrowth matrix, features associated with either fiber terminations or jogs in the fibers (Fig. l(c) and (d)). The Mo fibers were relatively free of dislocations as seen in Fig. l(b), with the dislocation density therein estimated to be 5 x 10 7 c m -2. Diffraction contrast and trace analyses indicated 1910--VOLUME
Data Loading...
