Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

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I. INTRODUCTION

BECAUSE of the attractive properties and, in particular, the excellent creep resistance associated with the orthorhombic (O) phase, Ti-Al-Nb alloys containing a significant amount of the O phase (henceforth termed O alloys) are being considered for use in structural applications both as monolithic materials as well as matrix alloys in continuously reinforced metal matrix composites. The creep behavior of O alloys has been the focus of several recent studies.[1–14] First-generation O alloys such as Ti-25Al-17Nb and Ti22Al-23Nb,* which contain the a2 (hcp DO19 structure), *All alloy compositions are given in atomic percent.

bcc, and ordered O (based on Ti2AlNb) phases, show a strong creep-microstructure correlation. The creep resistance increased with increasing volume percent of the O phase and decreased with decreasing volume percent of the a2 phase.[1,2,3] As a first attempt, the observations may be explained by the superior creep resistance found for fully O microstructures[4] compared to that for fully a2 microstructures.[15] However, grain-size effects, which have proven to be important in a2-based Ti-Al-Nb alloys,[16] may be significant, as the three-phase microstructures that contained a higher volume fraction of O phase also contained larger

C.J. BOEHLERT, Postdoctoral Fellow, is with the Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218. D.B. MIRACLE, Research Group Leader, is with the Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433-7817. Manuscript submitted September 24, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A

prior bcc grains.[3] Also important are morphological effects, which may have played a role, as the O phase was typically in a lenticular morphology, while the a2 phase was equiaxed. A fourth possibility is that the order/disorder nature of the bcc phase, which depends on alloy content and heat treatment, is important. Due to the large number of microstructural factors potentially affecting the creep resistance, a more-thorough investigation of microstructure-creep relations and, in particular, the creep mechanisms is necessary. The literature reporting intermediate-temperature (650 8C to 760 8C) creep exponent (n) and apparent activation energy (Qapp) values, based on the secondary creep rates, for O alloys has indicated the possibility of several different mechanisms.[4,5,10–14] Table I lists the creep parameter values. Nandy et al.[4,5] have examined the high-stress compression creep characteristics of Ti-27Al-21Nb, Ti-27Al-16Nb, and Ti-24Al-16Nb. For applied stresses between 240 and 500 MPa, the stress exponent values obtained were between 4.3 and 6.0. The corresponding activation energies ranged between 340 and 376 kJ/mol and a climb-controlled mechanism was suggested. The creep parameters obtained in other studies are in agreement with such a mechanism.[10,11] At low applied stresses (s , 100 MPa), Smith et al.[11] and Woodard and Pollock[13] have recorded steady-state creep ex