Simulating microstructural evolution during the hot working of alloy 718

The simulation of microstructural evolution during the primary breakdown of production-sized alloy 718 ingots and billets by radial forging was accomplished in the laboratory via multiple-stroke axial compression testing of cylindrical specimens. The dwell or hold time between strokes was varied to simulate the deformation-time history for three different locations along the radial-forging work piece: lead-end, mid-length, and tail-end positions. The microstructural evolution varied with simulated work piece position. Static, rather than dynamic, recrystallization was responsible for the observed grain-size refinement, and its repetitive occurrence during consecutive dwell periods resulted in the maintenance of a fine-grain microstructure during multiple-stroke deformation sequences. For comparison, the total plastic strain was also applied in a single-stroke test. The single- and multiple-stroke techniques gave differing microstructural results, indicating that multiple-stroke testing is necessary in modeling microstructural evolution during primary breakdown. Martin C. Mataya earned his Ph.D. in metallurgical engineering at Marquette University in 1974. He is currently a research professor at the Advanced Steel Processing and Products Research Center at the Colorado School of Mines and a staff member in the Materials Science and Technology Division at Los Alamos National Laboratory. Dr. Mataya is a member of TMS. For more information regarding the Advanced Steel Processing and Products Research Center, contact D. Matlock at (303) 273-3775.     

Fatigue behavior of a nial precipitation hardening medium carbon steel

The fatigue behavior of an Fe-0.30C-4.48Ni-l.32Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, X-ray line broadening and stress relaxation measurements. The three different heat treatments produced the following structures: I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, II) a microstructure of high dislocation density, coarse carbides and fine coherent NiAl precipitates and III) a highly tempered micro-structure with a recovered dislocation substructure, coarse carbides and fine coherent NiAl precipitates. In low cycle, strain controlled fatigue cyclic softening in Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain. Treatment II, which remained cyclically stable during the initial portion of the fatigue life, showed little change in the internal stress and dislocation density and showed a slight increase in lattice microstrain. Treat-ment III, which initially cyclically hardened, exhibited a rise in internal stress, lattice microstrain and dislocation density. The behavior of Treatments II and III is attributed in part to the presence of the fine NiAl precipitates which appear to reduce the tendency of the transformation induced dislocation substructure to rearrange itself into a cell structure during fatigue. In high cycle, stress controlled fatigue Treatment II showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation. Formerly Graduate Student, Marquette University,