STRAIN-AGING AND BREAKAWAY STRAIN AMPLITUDE OF DAMPING IN NIAL AND NIALZR

Extruded NiAl and NiAlZr alloys often show discontinuous yielding on s train aging in compression at room temperature. Two sets of experiment s were conducted to understand the reasons for this yield-point behavi or. First, strain-aging experiments were carried out on NiAl alloys co ntaining 0 to 0.1 at. % Zr. The specimens were all deformed in compres sion at room temperature at a nominal initial strain rate of 1.1 X 10( -4) s-1, and the effect of annealing at 700 and 1200 K on the stress-s train curves and the yield strength was studied after an initial prest rain. While annealing at 700 and 1200 K consistently reduced the yield strength of both NiAl and NiAlZr, the effects were quite different. I n the case of NiAl, annealing at 1200 K did not result in discontinuou s yielding, whereas it generally resulted in a sharp yield point for t he Zr containing alloys. Second, the PUCOT (piezoelectric ultrasonic c omposite oscillator technique) was used to measure the dynamic Young m odulus, breakaway strain amplitude, and damping for the alloys. Only s mall differences were observed in the values of Young's modulus, but t he breakaway strain was at least a factor of 2 to 3 lower for NiAl tha n for NiAlZr. The experimentally determined values of damping were use d in the Granato-Lucke model to estimate the binding energy for NiAl. While the binding energy values were found to be in agreement with the calculated values of dislocation kink nucleation and migration energi es in this material, to within an order of magnitude, other effects, s uch as dislocation pinning by quenched-in vacancies, cannot be ruled o ut. The observations made in this study suggest that the yield-point b ehavior in NiAl may be due to several factors, such as difficulties in double kink nucleation, and single kink migration, as well as disloca tion-vacancy interactions; whereas, the yield-point behavior in the Zr -alloyed material is due at least in part to dislocation-solute intera ction.