Creep and fracture of a Laves phase strengthened ferritic alloy
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EXPERIMENTAL PROCEDURE Alloy heats of approximately 20 lbs (9 kg) were induction melted in an argon atmosphere, cast in heavy copper molds, and subsequently annealed in vacuum for 3 days at 1l00°C. The nominal composition of the alloy, referred to hereafter as Ta7Cr, was Fe, 1 at. pet Ta, 7 at. pet Cr. The alloy ingots were forged at 1l00°C into plates 3 x 9/16 in. (76 x 14 mm) in cross section, and blanks for creep specimens were cut from these plates. Specimen blanks were solution treated for 1 hat 1320°C and quenched in 70°C water. The solution treated blanks were aged at 700°C for 40 min to attain peak hardness. They were subsequently given a spheroidizing treatmenr'" that consisted of holding at 1l00°C (in the y + Fe 2Ta phase field) for 10 min. Constant load creep and stress rupture tests were made on cylindrical specimens with 0.5 in. (12.7 mm) diam threaded ends, gage section diameter of 0.25 in. (6.4 mm), and a gage length of 1.5 in. (38.1 mm). A conventional extensometer fitted with dial gages was used for measurement of extension. Specimens for optical metallography were prepared by conventional mechanical grinding and polishing, followed by chemical etching. Thin foils for transmission electron microscopy were prepared by twin jet electropolishing using a 1 : 4 mixture of perchloric and acetic acids. A Hitachi HU-125 microscope operated at 100 kv was used for transmission electron microscopy. Fracture surfaces of creep specimens were examined in the JEOLCO JSM-U3 scanning electron microscope. RESULTS AND DISCUSSION
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M. DlLIP BHANDARKAR is with NASA Langley Research Center, Materials Division, Materials Research Branch, Hampton, VA 23665. M. SHANTHIDAS BHAT is Graduate Student and EARL R PARKER and VICTOR F. ZACKAY are Professors of Metallurgy, University of California, Lawrence Berkeley Laboratory, Berkeley, CA 94720, where M. Dilip Bhandarkar was formerly associated. Manuscript submitted September 4, 1974. METALLURGICAL TRANSACTIONS A
Microstructure of Spheroidized Ta7Cr Alloy Before Creep An optical micrograph of the microstructure of the spheroidized alloy is shown in Fig. 1. The microstructure was characterized by a dispersion of the Laves phase within the grains of a (bee) solid solution. The VOLUME 7A, MAY 1976-753
a phase had irregular grain boundaries that resulted from the y -- a transformation that occurred during cooling from llOO°C. Similar microstructures have been observed in several iron-base alloys that exhibit a y -- a massive transformation," Transmission electron micrographs obtained from thin foils of aged and spheroidized Ta7Cr alloy are shown in Figs. 2 and 3. The Laves phase particles were uniformly distributed. The low magnification micrograph (Fig. 2) illustrates the lath-like structure representative of the alloy. Similar structures have been reported in low carbon and noncarbon-containing iron-base alloys that exhibit a massive austenite to ferrite transformation." The high magnification micrograph (Fig. 3) illustrates the alloy substructure consisting of
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