A Dislocation Constriction, Pinning Point Model for the Flow Stress Anomaly in Iron Aluminides

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ABSTRACT Iron aluminides with compositions from Fe 3A1 to FeAl show an anomalous stress rise at temperatures about 600-800K. Many models have been suggested to explain this behaviour, with the suggestion of hardening by vacancies presently receiving much attention. An alternative model is suggested here, valid for those alloys where the simultaneous reduction of DO 3 order does not interfere, based on the density of constrictions produced on mobile superdislocations and their subsequent pinning by decomposition or by their cross slip to produce jogs. The exponential increase in stress with temperature can be related to the pinning point density and the absolute values of strengthening correspond well with those predicted by this model. The orientation dependence of the stress anomaly and the tension-compression asymmetry can be related to the role of shear stresses in assisting constriction of the dislocation partials by modifying the core configuration and by assisting the expansion of the new, decomposed dislocations. INTRODUCTION Iron aluminides with compositions over almost the entire range from Fe 3A1 to FeAl show increasing flow stresses over the temperature range about 600-800K. Mechanical behaviour has been

examined on many occasions (1-7),and many theories proposed, including the change from perfect superdislocations to an imperfect variant (1), increased cross slip (1,2), the onset of cross slip

of superdislocations for {110} to {112} planes (1,2,8), local climb locking (4), relaxation of the APB (9), decomposition of superdislocations into perfect and dislocations (7), and vacancysolute hardening (10). The fall of flow stress at high temperatures is explained by the onset of dislocation glide (4,5) which may be controlled by Peierls forces (4), creep-lie diffusive effects (10), or re-enhanced mobility of superdislocations (8). The present report suggests an alternative mechanism for the anomalous stress increase based on the concentration of jogs on near-screw dislocations, taken to be in thermal equilibrium for each temperature and produced by partial constriction events. Several typical characteristics of the stress anomaly are first noted, and used to test the supplicability of the model proposed. (i) while TEM has provided evidence of the operation of many pinning mechanisms, e.g. cross slip (2,8), climb (4), and decomposition (7), it is difficult to distinguish between the occasional operation of a given mechanism and its critical role in determining strengthening. (ii) the flow stress increase (Au') shows an exponential change with temperature (T) (10,11), which can be written Aae = A exp (-Qa/RT), with A a constant, Qa the activation energy describing the

anomalous stress increase, and R the gas constant. The values of Qa change only slightly as the composition changes from near stoichiometric Fe3AAto near stoichiometric FeAl (11). For the vacancy hardening model (10) Qa has been given the meaning E!2, where Ef is the enthalpy of vacancy formation. For alloys of composition near Fe-40%Al, the v