High temperature creep of alpha iron in terms of effective stress and dislocation dynamics
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in which xI, is a geometric factor and b the Burger's vector of the moving dislocations. However, the successful application of this relation was mostly restricted to low strains and low temperatures, at which it is frequently possible to measure the dislocation velocity directly in a well annealed crystal and to suppose that the effective stress is equal to the applied stress. N o direct measurements of dislocation velocity at high temperatures have been performed as yet. Such measurements will undoubtedly be extremely difficult even at small strains. At relatively large strains, such as those which correspond to steady state creep, it is necessary to determine the stress dependence of the dislocation velocity indirectly. However, an assumption must again be made about the stress dependence of the mobile dislocation density as it is in principle impossible to separate the influence of the stress on dislocation velocity from the influence of stress on the mobile density in strain rate sensitivity or stress sensitivity experiments. The aim of the present work was to investigate the application of the above microdynamic approach to the high temperature creep of alpha iron. K. H. GEORGY is with the Solid State Physics Department, National Research Center, Dokki, Giza, Egypt. J. (SADEK is with the Czechoslovak Academy of Sciences, Institute of Physical Metallurgy, Brno, Czechoslovakia. Manuscript submitted December 20, 1977.
2. M E C H A N I C A L E Q U A T I O N O F S T A T E A N D D E F I N I T I O N S OF EXPERIMENTAL PARAMETERS Taking into consideration the fact that the dislocation cannot move in a thermodynamic subsystem unless the external stress 0 differs f r o m the internal stress 0,, two different techniques of mean effective stress measurement can be used. These techniques consist of the determination of the external stress decrement Ao to which a zero rate corresponds. This can be achieved either by stress relaxation (stress transient dip test technique) 2,3 or by strain relaxation (strain transient dip test technique), 4-5 as measured immediately after a stress or strain change so as to avoid recovery effects. It is clear that Ao = o* and 0 - Ao = 0,, respectively where o, is the m e a n internal stress. Although, anelastic p h e n o m e n o n occurs usually after stress reductions. It was reported by several authors 6-8 to avoid any real validity of the dip test techniques for measuring the recovery effects during creep. This anelastic strains contribute to strain transients at all levels of stress decrements Ao, with a corresponding contractions increase linearly with the increasing reductions Ao. 7 However, experimentally the strain rate occurs immediately after the stress reduction. Whether, this new rate of strain is positive or negative, it is a function of the residual applied stress o - Ao, and the dislocation parameters (mobile dislocation density Pm and the glide velocity of dislocations us)" When the combined effect of these parameters overcome the effect of the instantaneou
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