Initial Hardening Stage Research Articles

An analysis of cracks in ductile single crystals—II. Mode I loading

A geometrically rigorous formulation of crystalline plasticity is used to analyze the crack-tip deformation and stress fields in ductile single crystals subjected to mode I loading. The theory accounts for finite deformations and finite lattice rotations, as well as for the full three-dimensional crystallographic geometry of the crystal. An experimentally based self-hardening rule exhibiting an initial stage of rapid hardening followed by a saturation stage is also adopted. The problem of a stationary semi-infinite crack in FCC and BCC crystals is considered. As regards the dominant modes of deformation, the results are in partial agreement with earlier analytical and numerical solutions, but in excellent qualitative agreement with recent experimental observations. The results suggest that both finite-deformation and lattice rotation effects, as well as the details of the hardening law, strongly influence the structure of the solution.

New magnetic alloys for magnetic recording heads

A study was made of the magnetic and mechanical properties of the alloys obtained by adding titanium and niobium to 4Mo-79Ni Permalloy. The mechanical hardness markedly increases with the addition of titanium, being about 240 in Vicker's hardness at 3% titanium, and about 300 at 3.5% titanium. One of the new alloys has μ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> = 12000 (0.22 mm, 1 kHz), Hc = 0.01 Oe, B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</inf> = 5000 G, and ρ = 98 μΩ cm at the composition of 3% Ti-2.8% Nb-3.8% Mo-10.5% Fe-0.5% Mn-balance Ni. Moreover, it is supposed that this alloy is hardened not only by solid solution hardening, but also by precipitation hardening because the coercive force decreases as hardness increases at the initial stage of hardening.

The strength and fracture toughness of 18 Ni (350) maraging steel

The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni3Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe2Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.

Experiments on the deformation of niobium single crystals. I. Stress versus strain curves and slip systems in compression and tension

Abstract Single crystals of niobium were purified by heating in high vacuum and then tested in tension or compression at various temperatures. Three-stage hardening curves were found at room temperature for single slip orientations, irrespective of whether the primary slip plane was of {101} or {211} type. An initial rapid hardening (stage 0) was also found for {101} slip, and was often associated with low or negative stage I hardening rates. Microstrain measurements showed that the much less pronounced stage 0 observed for {211} slip was mainly attributable to the initial build-up of plastic strain rate caused by machine softness. Another difference between {101} and {211} slip was the latent hardening on the conjugate slip plane, which caused large overshoot in tension when primary slip was on {101}, but not on {211}. At 158°K. stress versus strain curves were parabolic, except for ideal {211} single slip orientations tested in compression. At still lower temperatures, most crystals yielded by twinning (especially in compression) unless first pre-strained above 90°K.

A debris mechanism of cyclic strain hardening for F.C.C. metals

Abstract Prismatic dislocation loops (dislocation debris), formed by fatigue or cyclic straining, have been observed in a number of metals. To account for these observations a mechanism of cyclic strain hardening is proposed which depends on the ease of loop formation and the behaviour of the loops after they are formed. In the initial rapid hardening stage, the rate of cyclic strain hardening is determined by the rate of formation of debris obstacles. Since the debris is thought to be formed by some variant of the double cross slip mecachism, changes of variable which lead to easier cross-slip result in higher hardening rates (because the rate of debris obstacle formation is increased). As the crystal fills up with debris, the motion of screw dislocations diminishes and the enforced strain is accommodated by the motion of the prismatic loops already present in the crystal. The basic motion of the prismatic dislocation loops is a flip-flop motion from one stable equilibrium position to another which is re...

Annealed metals under alternating plastic strain

Annealed metals (copper, nickel, aluminium and α -brass) were subjected to cycles of small alternating torsion in which the amplitude of plastic strain was held constant; measurements were made of the proof stresses required to impose the successive reversals of strain. Varia­tions in this proof stress, which shows how the metal strain-hardens at each amplitude, are correlated with structural changes shown by X-ray diffraction and metallographic ex­amination. The results show that the rate of increase of the strain-hardening which occurs during the early cycles decreases during subsequent cycles, indicating that the plastic strain in cyclic deformation may be non-hardening. Further, when hardening thus ceases the deformation still proceeds by slip, indicating that slip movements in cyclic deformation may also be non-hardening. Again, fragmentation of the grains which increases in the initial hardening stage, depending on amplitude, also ceases during the subsequent non-hardening stage, confirming the modified mode of deformation implied by the two foregoing observa­tions. Finally, the total plastic strain, the sum of the reversals added irrespective of sign, obtain­able in the non-hardening stage without fracture, may be many orders higher than the strain obtainable in static deformation. The observations are interpreted on the view that to-and-fro slip in cyclic straining m ay build up in the slip bands surface disturbances that relax excessive internal stresses and start fatigue cracks by the geometry of their formation.