HIGH-STRAIN-RATE BEHAVIOR OF POLYCRYSTALLINE a -IRON

1. Polycrystals ms Grains Martensite time µs Plasticity BCC Phase stability, elasticity HCP Energy barriers, paths ns Phase-boundary mobility nm µm mm length 100 mm Rectangular SCS geometry Material Composition P, d P, d Typical Dimensions (mm) Typical Dimensions (mm) D L L 23.95 h h w D D 12.7 12.7 L t t 2.54 2.54 w – – 2.60 – 1.30 – 0.50 45 t P, d P, d P, d 99 w/o pure Fe Pure Fe softens noticeably at high strain rates, in accord with Weston’s (1992) observations for pre-shocked iron By contrast, Fe hardens continuously at large strains in the quasi-static regime. HIGH-STRAIN-RATE BEHAVIOR OF POLYCRYSTALLINE a-IRON D. Rittel, M. Vural, M. Tao, S. Mizrach, A. Bhattacharyya, G. Ravichandran Goal: Investigate the thermo-mechanical behavior of pure a-iron (BCC), from quasi-static to dynamic strain rates: Techniques: Quasi-static compression, dynamic compression, non-contact, high speed infrared thermography, shear compression specimen (SCS). THERMOMECHANICAL BEHAVIOR Continuous recording of the temperature rise of the specimen allows for determination of the thermomechanical conversion of plastic work to heat. Define . Note that bint 1 while there is no such restriction for bdiff.As shown below, the behavior at moderate strain rateof pure Fe shows no anomalies in terms of b. By contrast, the high strain-rate response shows that bint > 1. Such an anomaly can be attributed to the operation of an additional heat source, such as the release of latent heat associated with a phase transformation. The a (BCC) e (HCP) martensitic phase transformation is well documented for pressure levels on the order of 13 GPa under shock loading conditions. It has not been reported for large strain, high strain-rate experiments. SPECIMEN GEOMETRY and MATERIAL MECHANICAL BEHAVIOR T EFFECTIVE STRESS-STRAIN DETERMINATION STRAIN RATE SENSITIVITY OF PURE Fe Pure Fe exhibits a marked strain rate sensitivity, as shown in the figure below for ep=0.1 flow stress level. The transition is noticeable at Results were obtained, using both cylindrical and SCS specimens. SCS vs. TORSION EXPERIMENTS Comparing results obtained with SCS specimens to those obtained in pure shear (Klepaczko, 1969) further validates the effective stress strain reduction technique. ki, k2 and k3 are material and geometry (w/t) dependent. They are determined from numerical simulations and verified by experiments. Both b are > 1 COMPARISON WITH PRIOR RESULTS Weston (1992) compared the dynamic behavior of as-received pure Fe to that of pre-shocked specimens. Our results indicate that pure Fe behaves as expected at low strain rates but reaches strength levels of pre-shocked Fe at high strain rates. MICROSTRUCTURAL OBSERVATIONS Twinning observed in the heavily sheared gage section. Hardness in the gage section for the material deformed at high strain rates is comparable to shock loaded iron. • WORK IN PROGRESS • Elucidate a possible shear induced • phase transition (TEM, X-ray). • Microstructural and material characterization. • Texture analysis using orientation imaging • microscopy (OIM). • M. Cuitino • R. A. Radovitzky M. Ortiz E. A. Carter R. E. Cohen Validation studies of the multi-scale computational models for a-iron.