Effects of the alumina scale on the room-temperature tensile behavior of preoxidized MA 956
- PDF / 1,255,542 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 41 Downloads / 219 Views
INTRODUCTION
THE MA 956 is a ferritic oxide dispersion strengthened (ODS) FeCrAl alloy, processed by mechanical alloying (MA). Particularly attractive is its excellent oxidation/corrosion resistance, which relies on the capability to develop a compact and adherent a-Al2O3 scale at high temperatures.[1] Consequently, this alloy is widely used for high-temperature applications. If the alloy is to be used either at low temperatures or under low-oxygen environments, the alloy has to be preoxidized at a high temperature (1100 7C) in pure oxygen or in air to develop the a-Al2O3 scale prior to operation.[2] Beneficial effects of such preoxidation treatment have been mainly addressed for coal gasification environments at intermediate temperatures,[3,4,5] but other novel applications are envisaged for lower temperatures. For example, it has been recently proposed that preoxidized MA 956 could be a good candidate for surgical implants because it combines the good mechanical properties of the metal substrate with the protective capacity of the alumina scale against physiological solutions.[6] Unfortunately, data on the mechanical behavior of preoxidized material at room temperature are limited.[7,8,9] Therefore, factors affecting the mechanical behavior of preoxidized material at room temperature should be studied in more detail. The MA 956 exhibits a ductile to brittle transition (DBT), as is typical in bcc materials. The transition temperature for this alloy, determined from tensile tests of notched specimens, is about room temperature.[2] On the other hand, it has been demonstrated that high-temperature exposure in oxygen-containing environments causes degradation of the mechanical properties of many alloys, particularly at intermediate or low temperatures.[10] For ´ LEZ-CARRASCO, J. IBA ´N ˜ EZ, M.L. ESCUDERO, J. CHAO, J.L. GONZA ´ LEZ-DONCEL, Research Scientists, are with CSIC, Centro and G. GONZA Nacional de Investigaciones Metalu´rgicas, Avda. Gregorio del Amo 8, 28040 Madrid, Spain. Manuscript submitted April 13, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
preoxidized MA 956 sheet, embrittlement has been related to the formation of the outer alumina scale.[7,8,9] Both features, the DBT and the postexposure damage, affect the tensile ductility at room temperature. However, clear conclusions about the operating mechanism accounting for the occurrence of fracture were not proposed. Consequently, the effect of the oxide scale on the room-temperature tensile behavior of the MA 956 alloy under different strain rates will be analyzed in this study.
II.
EXPERIMENTAL PROCEDURE
The alloy MA 956, of nominal composition Fe-20Cr5Al-0.5 Y2O3 (in wt pct), was supplied from a single batch by Inco Alloys International (Hereford, United Kingdom) as hot-rolled bars 9.5 mm in diameter without any heat treatment. Usually, the preoxidation treatment of this alloy consists of annealing at 1100 7C for 2 hours in air.[2] In this investigation, however, 100 hours exposure was selected to assure the development of a thick oxide layer
Data Loading...
