Accession Number:

ADA179245

Title:

Effect of Alloying, Rapid Solidification, and Surface Kinetics on the High Temperature Environmental Resistance of Niobium.

Descriptive Note:

Annual technical rept. Nov 85-Jan 86,

Corporate Author:

LOCKHEED MISSILES AND SPACE CO INC PALO ALTO CA RESEARCH AND DEVELOPMENT DIV

Personal Author(s):

Report Date:

1987-02-20

Pagination or Media Count:

84.0

Abstract:

A model derived from first principles for the transition from internal to external oxidation has been shown to be applicable to the oxidation of alloys. The feasibility of forming protective alumina scales by the selective oxidation of aluminum at greatly reduced AL contents has been demonstrated. The critical NAL for the transition has been lowered to between 25 and 30 at. AL by the balanced addition of elements that simultaneously reduce NOs and DO and increase DAL in the alloy. The model indicates that a low DAL is the major factor limiting the selective oxidation of AL and that an increase of about 6 orders of magnitude would be required to effect a transition to external scale information at 1100 C. A major increase in DAL was achieved by adding Ti to increase the solubility of AL in b.c.c Nb. This resulted in formation of alumina at an NAL of 0.4 in air at 1400 C. By adding Cr and V to reduce NOs and DO, the critical NAL was reduced further to between 0.25 and 0.3 at 1400 C. An alloy o fNiobium-29 Titanium-38 Aluminum-3 Chromium-4 Vanadium at. formed highly protective alumina in air at 1400 C. The ability to form alumina decreased as temperature decreased due to the high activation energy of alumina formation and rapid kinetics of transient oxidation. Further modification of both composition and alloy structure is needed.

Subject Categories:

  • Metallurgy and Metallography

Distribution Statement:

APPROVED FOR PUBLIC RELEASE