Formation Of Dislocation Substructure Research Articles

Cyclic work hardening and formation of dislocation substructures during low‐cycle fatigue of tempered dual‐phase steels

A C‐Mn‐Si dual‐phase steel, containing 33.8% lath martensite of 0.45% C content (called steel 1), was tempered at 200, 460 and 650°C and designated steels J, K and L, respectively. Sheet specimens of 3 mm thickness and 10 mm gauge length pertaining to steels I, J, K and L were subjected to tensile and low cycle fatigue testing at room temperature. The steels I, K and L show cyclic softening while the steel J exhibits a small cyclic hardening. The cyclic stress response varies significantly at higher and lower applied strain amplitudes for each of the steels which has been discussed in terms of competing processes of hardening and softening during strain cycling. The value of the cyclic strain hardening exponent decreases continuously in the order steels I>J>K>L. Varied dislocation substructures form at higher and lower strain amplitudes during cyclic deformation.

Martensite formation, strain rate sensitivity, and deformation behavior of type 304 stainless steel sheet

The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed.

Dislocation substructures formed during the flow stress recovery of high purity aluminum

Flow stress recovery experiments were performed on polycrystalline high purity aluminum following room temperature tensile prestrains of 5%, 10%, 15%, 20% and 25%. Isothermal recovery temperatures of 120, 160 and 200°C were used for recovery times up to 300 h. It was found that work hardening at room temperature is accompanied by the development of dislocation tangles that form the boundaries of a dislocation cell structure. These tangles were found to develop along {100},{110} and {111} planes. The cells formed decrease in size with increasing strain up to 15%, thereafter remaining constant at an average size of about 1.8 μm. The cell structure converts to a sharply defined subgrain structure during recovery as the tangles are rearranged into networks. The network planes are the same as those on which the original tangles were formed. During recovery following tensile prestrains less than about 15%, the subgrain size tends to grow by two mechanisms: (1) subgrain coalescence and (2) subboundary coalescence. The flow stress related to formation of dislocation substructures has been found to be inversely proportional to either the cell or the subgrain size for prestrains less than 15%, where the dislocation density in, and the misorientation angle of, the cell or subgrain boundaries was found to be nearly constant. In this region, conversion from cell to subgrain by recovery annealing results in a 24% loss of flow stress, independent of the prestrain used. Above about 15% tensile prestrain, the flow stress continues to rise without a significant decrease in the cell or subgrain size but with an increase in the density of dislocations in the cell walls and the average misorientation angle of the subboundaries. Furthermore, the subgrains formed after strains greater than 15% are more thermally stable, i.e. they do not grow appreciably during recovery annealing for 300 h at a temperature of 160°C.

Formation of dislocation substructure and hardening mechanism in cyclic strain of FCC metals

1. A cellular dislocation substructure is formed at the moment of hardening saturation in cyclic strain of fcc metals having a high stacking fault energy. 2. The influence of strain amplitude is manifested in a different structure of the cell boundaries; after low-amplitude stressing they represent principally pile-ups of dipolar prismatic loops, after high-amplitude stressing, dislocation networks. 3. The influence of a reduction in stacking fault energy has been followed; it is shown by the example of nickel that it leads to an increase in the stress ensuring the formation of a given dislocation configuration. 4. An examination is made of the possible mechanisms of hardening in cyclic cycles, and also a model of the formation of intrusions and extrusions, taking into account the specific character of the dislocation structure in cyclically strained fcc metals.

Creep of Dense, Polycrystalline Magnesium Oxide

Creep of polycrystalline MgO was studied using four‐point transverse bending at 1380° to 1800°K and stresses from 1000 to 5000 psi. The effects of temperature, stress, and grain size on the creep rate were determined for grain sizes from 2 to 20μ. Activation energies for creep decreased sharply with increasing grain size from 96,000 cal/mole at 2μ to 54,100 cal/mole at 5.5μ and then remained constant over the grain‐size range 5.5 to 20μ. Creep was attributed in part to a stress‐directed diffusional mechanism controlled by extrinsic oxygen ion diffusion in the 5.5 to 20μ grain sizes, although the calculated ionic self‐diffusion rates were higher than those predicted by the Nabarro‐Herring theory. It is suggested that the discrepancy may be due to a vacancy formation mechanism, which is consistent with the observed formation of dislocation substructure and preferentially distributed porosity during creep, as well as with the observed decrease in creep rate with increasing creep strain.

On the formation of dislocation substructure during growth of a crystal from its melt

Dislocation networks in as-grown lithium fluoride crystals were studied by etch pit and decoration techniques in an attempt to obtain experimental evidence concerning the origin of grown-in dislocations. Several mechanisms for formation of new dislocation loops at or near a growing crystal face were considered. The kinds of distribution observed, and the insensitivity of the grown-in networks to the growth rate and temperature gradient in the solid, were considered to be consistent with the hypothesis that stresses are responsible for most of the grown-in dislocations.