A Recovery-Athermal Glide Creep Model

Measurements of internal friction and of modulus relaxation are made during creep of Al specimens. In the transition from primary to secondary creep both the quantities show peaks: TEM observations show that the peaks are connected with cell formation. Experimental data are used to obtain indications on the distribution of the link distances on the dislocations directly during the deformation and on the rate determining processes accompanying the dislocation rearrangements in cellular structures.

Experimental observations on dislocation link distribution and supersaturation during creep of Al

Bonetti

Evangelìsta

Gondi

1981

Initial and Steady-State Creep

Shi

Northwood

1992

Creep tests on pure polycrystalline magnesium and an AISI type 310 stainless steel show that the creep rate ratio, F, between the initial creep rate, ii, and the steady-state creep rate, is, is temperature and applied stress dependent. At lower test temperatures and/or applied stresses, F is large with values up to about 100, while at higher temperatures and/or applied stresses, it tends to unity. These observed temperature and stress dependencies of F provide support for Li's dislocation mechanism model for creep, in that the general creep equation can be reduced to two formulations, one with a large F ratio and one with F = 1. The apparent activation energy for creep, Qc, for both materials is also measured, and it is found that at the onset of the creep deformation (creep strain around the measured Q, value is smaller than that measured in steady-state. The origins of this high creep rate ratio, F, and the low apparent activation energy, Qc, upon loading are discussed in terms of internal stress, a generalised deformation equation, and recent refinements to network creep theories.Kriechtests an reinem polykristallinem Magnesium und an rostfreiem AISI-310-Stahl zeigen, daB das Kriechratenverhaltnis F, zwischen der anfanglichen Kriechrate, 8, und der stationaren Kriechrate 8% temperatur-und spannungsabhangig ist. Bei niedrigen Priiftemperaturen und/oder angelegten Spannungen ist F grol3, mit Werten bis zu etwa 100, wahrend bei hoheren Temperaturen und/oder angelegten Spannungen es gegen eins geht. Diese beobachtete Temperatur-und Spannungsabhangigkeit von F liefert eine Unterstutzung fur das Lische Versetzungsmodell des Kriechens, in dem die allgemeine Kriechgleichung auf zwei Formulierungen reduziert werden kann, eine mit einem groDen F-Verhaltnis und eine mit F = 1. Die auftretende Kriechaktivierungsenergie Q, beider Materialien wird ebenfalls gemessen und es wird gefunden, daR beim Einsetzen der Kriechdeformation (Kriechdehnung etwa der gemessene Q,-Wert kleiner ist als der gemessene stationare Wert. Die Ursachen dieses hohen Kriechratenverhaltnisses F und der niedrigen Aktivierungsenergie Q, bei Belastung werden mit inneren Spannungen, einer verallgemeinerten Deformationsgleichung und neueren Verbesserungen der Kriech-Netzwerk-Theorien diskutiert.') 401 Sunset, Windsor, Ontario N9B 3P4, Canada. 5*

On Dislocation Link Length Statistics for Plastic Deformation of Crystals

Shi¹,

Northwood²

1993

The statistical distribution function for dislocation link lengths in a unit volume of plastically deformed crystalline materials as proposed by Wang et al. is used to derive the relationship between the average dislocation link length, 〈γ〉, and the dislocation density, Q. It is shown that 〈γ〉 = βϱ−½: β = (2/πc)½ where c is the dislocation network geometrical factor. Also, the mobile dislocation density, Qm, is shown to be directly proportional to the total dislocation density, Q, and the proportionality constant is determined by such factors as mobile dislocation fraction factor, fp, the geometrical factor, c, the Taylor factor, M, and a constant, α. The applicability of the distribution function, φ(λ), is also demonstrated with respect to the statistical constraints for the normalized distribution function, Φ (u), where u = λ/〈λ〉 is the normalized dislocation link length. The analysis is then applied to the strain‐hardening and recovery processes during plastic deformation. It is shown that the strain‐hardening coefficient, H, is related to the geometry of the three‐dimensional dislocation network and is of the order of Young's modulus, E, which is in good agreement with other strain‐hardening models and experimental measurements made by the stress change test. A creep rate equation is also obtained for high temperature recovery creep deformation.