LOW-TEMPERATURE AND HIGH-STRAIN-RATE DEFORMATION OF NICKEL AND NICKEL-CARBON ALLOYS AND ANALYSIS OF THE CONSTITUTIVE BEHAVIOR ACCORDING TO AN INTERNAL STATE VARIABLE MODEL

Analysis of the strain-rate sensitivity of the yield stress and the strain-rate sensitivity of strain hardening in annealed and quasi-statically and dynamically prestrained Ni270, Ni-510 wt ppm (0.25 at.%) C, and Ni-1900 wt ppm (0.92 at.%) C is presented. Measurements in compression are analyzed according to an internal state variable model where for a given obstacle type the state variable represents the stress required for deformation at 0 K. State variables are defined for dislocations interacting with interstitial carbon atoms and for dislocations interacting with other dislocations. The application of a power law to combine the contributions of separate obstacles is investigated. Evolution of the mechanical threshold stress characterizing dislocation/dislocation interactions is analyzed using a modified Voce equation. The Stage II hardening rate is shown to be insensitive to the carbon concentration, while the hardening at large strains is strongly dependent on the carbon concentration. Dynamic strain aging is found to influence both the temperature and strain-rate dependent yield stress and the hardening rate in the nickel-carbon alloys. Deformation microstructures are characterized using transmission electron microscopy, and the measured dislocation cell size is correlated with the internal state variable characterizing dislocation/dislocation interactions.