The Fracture of Ordered (Fe, Co) 3 V

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I.

INTRODUCTION

EARLIER l it was established that a partial replacement of cobalt with iron in C03V alters the crystallography from a hexagonal ordered (BaPb3-type) to a cubic ordered (Cu3Au or Llz-type) structure. This change suppresses brittle behavior and imparts tensile ductility in excess of 35 pct at room temperature. Little information was reported, however, on the nature of fracture. Other than noting that a ductile transgranular mode accompanies the brittle-to-ductile transition, no observations were cited concerning the effects of temperature and strain rate, and mechanisms were not proposed. These points are considered in this paper. While the emphasis is on the behavior of one of these new longrange-ordered (LRO) alloys, previously designated 2 as LRO-1 and written (Fe22Co78)3V,. the behavior to be described appears to be typical of the whole group of cubicordered alloys based on (Fe, Ni, Co)3V. The (Fe, Co)3V and related alloys have potential as hightemperature structural materials. They possess creep resistance, 2 high-temperature strength, 3 and attractive fatigue properties. 4 Moreover, they are resistant to irradiationinduced swelling 5 and, unlike conventional alloys, become stronger with increasing temperature at intermediate temperatures6'7 (in this case from 300 ~ to Tc -~ 950 ~ where sudden disordering 2 occurs).

II.

EXPERIMENTAL

The alloy under discussion was processed and tested as described elsewhere. 1 Briefly, it was prepared by arcmelting and electron-beam drop casting, using high-purity vanadium (total impurities less than 700 ppm by weight) and electron-beam purified iron and cobalt. The ingot was hot rolled at 1100 ~ between molybdenum cover sheets to a thickness of 3 mm and was further reduced to 0.8 mm by cold rolling. Table I shows the nominal and analyzed alloy compositions. The final sheet contained less than 400 ppm total of interstitial impurities, mainly oxygen, carbon, and nitrogen. C.T. LIU is Group Leader, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830. E.M. SCHULSON is Associate Professor, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755. Manuscript submitted January 17, 1983. METALLURGICALTRANSACTIONS A

Table I.

Elements Fe Co V O C N H

Alloy Composition and Chemical Analysis of LRO-1 (Fe22COTs)3V Atomic Concentration, Pct Weight Concentration Nominal Analyzed (ppm) 16.5 17.3 bal. bal. 25.0 25.1 107 82 134