Effects of temperature and environment on fatigue crack growth in ordered (Fe, Ni) 3 V-type alloys
- PDF / 1,899,060 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 211 Views
I.
INTRODUCTION
LONG range order significantly alters the mechanical properties of solid solution alloys. J,2,3Among the most noteworthy effects are the anomalous rise in yield stress with temperature, rapid strain hardening, improved resistance to high cycle fatigue and slowed recrystallization, grain growth, oxidation, and other diffusion-controlled phenomena. However, brittleness of many ordered alloys at low temperatures has hampered their development as structural materials. Recent progress in producing ductile ordered alloys by microalloying (boron doping of Ni3AI) 4'5 and macroalloying (stabilizing fcc L12 structure by adding Fe or Ni t o C 0 3 V ) 6'7 has led to renewed interest in ordered alloys as potential structural materials. While high cycle fatigue resistance of several alloys has been reported to be improved by introducing long range order, 8 there is no published information on the effects of order on crack growth under cyclic loading conditions, nor have crack growth data been reported for any ordered alloy. The present investigation was undertaken to study the influence of order, grain size, temperature, and environment on crack growth in two ductile long range ordered alloys of the (Fe, Ni)3V system. II.
EXPERIMENTAL PROCEDURE
Two alloys of nominal composition 39.5 wt pct Ni, 37 pct Fe, 22.4 pc t V, 0.4 pct Ti, designated LRO-42 and LRO60, were provided by Oak Ridge National Lab (ORNL) for this study. The essential difference between these two alloys, both of which form L12 superlattices, was in the level of trace elements added for improved high temperature properties. Most samples of the two alloys were annealed at 1100 ~ for 20 minutes to produce a grain size of 22/~m; a few samples of LRO-60 were annealed at 1200 ~ for 5 hours to produce a grain size of 120 /~m. All samples were water quenched to disorder them. Ordering was induced by a step heat treatment: 650 ~ hours, 600 ~ and 500 ~ hours. The critical temperature for long range A.K. KURUVILLA, Graduate Student, and N.S. STOLOFF, Professor, are with the Materials Engineering Department, Rensselaer PolytechnicInstitute, Troy, NY 12181. Manuscript submitted May 14, 1984. METALLURGICALTRANSACTIONSA
order, T,,, is reported to be between 675 ~ and 700 ~ for alloys of about the same composition. 9 All crack propagation experiments were performed on compact tension-type specimens whose dimensions were 3 cm square by 2.5 mm thick, at a frequency of 20 Hz and a stress ratio R = O'min/Orma x = 0.17. Testing was conducted on ordered material at 25 ~ 400 ~ 600 ~ and 650 ~ (all in argon); additional environments used were air and hydrogen gas. Tests could be performed only on disordered specimens at 25 ~ since any exposure to temperatures of 400 ~ or above would induce ordering during test. Hydrogen also was cathodically precharged from a 1 N H2SO4 solution at a current density of 50 mA/cm2; 0.05 gms/1 of sodium arsenite was added as a hydrogen recombination poison. Crack growth was monitored using either a traveling microscope or a D.C. potentia
Data Loading...
