Creep deformation and damage in a continuous fiber-reinforced Ti-6Al-4V composite
- PDF / 1,563,587 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 103 Downloads / 171 Views
I.
INTRODUCTION
TITANIUM matrix composites (TMCs) are currently being developed to meet the increasing requirements of high-performance aerospace vehicles and propulsion systems. By reinforcing titanium (Ti) alloys with high-strength, high-stiffness silicon carbide (SiC) fibers, it is possible to substantially improve their performance. In particular, high specific strength and stiffness can be achieved. The SiC fibers are also elastic and exhibit no appreciable creep deformation up to 900 7C;[1,2] therefore, they can significantly improve the creep resistance of Ti alloys. Despite the current interest in these materials for use in elevated temperature aerospace applications, there have been few studies on their creep behavior.[3–9] In particular, little work has been conducted on the fundamental mechanisms of creep deformation and failure and the effects of damage accumulation on creep response. Earlier studies have indicated that TMCs are susceptible to environmental damage at elevated temperatures, which can severely affect their creep behavior.[3,4] Concern has also been expressed over the microstructural stability of the matrix alloys and the fiber/matrix interfaces when subjected to long-term creep exposure at high temperature and stress. The stability of the interface and environmental interactions during creep will ultimately limit the upper use temperature of these materials. Predictive models for creep also need to be developed for TMCs. Simple micromechanical models have been proposed for longitudinal creep in continuous fiber metal-matrix composites.[10–14] However, it is not known if these models are applicable to TMCs, or if they can adequately predict creep response over the full range of anticipated S.W. SCHWENKER, Materials Engineer, is with the United State Air Force, Wright Laboratory, Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817. D. EYLON, Professor, is with the Graduate Materials Engineering Department, School of Engineering, University of Dayton, Dayton, OH 45469-0240. Manuscript submitted March 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
service temperatures and stresses. Based on a survey of available creep models, the model proposed by McLean[12,13] is the most applicable to TMCs, where it can be assumed that the fibers deform elastically in a creeping matrix. This model assumes that the matrix and fibers are constrained to deform at equal rates and that the creep rate of the matrix εz can be described by a power-law expression:
εz 5 Bm s n
[1]
where s is the applied stress and Bm and n are the preexponential and stress exponent, respectively. Furthermore, if it is assumed that load is partitioned between the fiber and matrix according to a rule-of-mixtures, one obtains the following differential equation for creep rate, εz , of the composite: Ef Vf εz 5 a Bm s cn (1 2 ε)n sc
[2]
where
a5
@
EmVm EmVm 1 Ef Vf
#
Vm2n
Equation [2] predicts a decreasing creep rate that asymptotically approaches zero as the matrix relaxes and redistributes its load
Data Loading...
