An analytical description for the deformation response of Ni 3 Al alloys in the anomalous regime
- PDF / 482,676 Bytes
- 5 Pages / 597.28 x 785 pts Page_size
- 16 Downloads / 242 Views
I.
INTRODUCTION
UNDERSTANDING the anomalous flow characteristics o f Ni3A1 alloys and several other Llz materials represents a great challenge to the materials community. Considerable effort has been made over the course o f the last 30 years to increase our insight into the behavior o f these materials. Now it appears that we are at a point where a complete understanding is not far off. This is important, since commercial superalloys, used in virtually all high-temperature applications, are strengthened by Ni3AI precipitates. In addition, single-phase Ni3A1 alloys have been investigated as candidate materials for several high-temperature engineering applications. Iq Experiments on single crystals o f Ni3A1 and similar L12 materials have shown that these materials have the following unusual characteristics: (1) the strength increases with temperature; t2~ (2) Schmid's law is not obeyed; t3,4~ (3) different strengths are obtained in tension and compression; tS,6J (4) strength is virtually independent of strain rate; andtT~ (5) deformation is temperature-path history independent (TPHI).f2j These macroscopic observations have been combined with the observation ~2-7J o f the predominance o f screw dislocations in postmortem transmission electron microscopic foils o f deformed Ni3A1 to develop several analytical theories. The most advanced o f these theories can describe thermal strengthening, the violation o f Schmid's law, and tension/compression asymmetry. GRAHAM WEBB and ARNAUD de 13USSAC, formerly Graduate Students, School of Materials Engineering, Georgia Institute of Technology, are collaborateur temporaire etranger, Centre D'l~tudes Nucl6aires de Grenoble, Grenoble, France, and ing6nieur de Socirt6 Nationale d'Etude et de Construction de Moteurs d'Aviation, Paris, France, respectively. STEPHEN D. ANTOLOVICH, formerly Director and Professor of Materials Engineering, is currently Professor and Chairman, Department of Mechanical and Materials Engineering, Washington State University, Pullman, W A 99164. Manuscript submitted October 14, 1991. METALLURGICAL TRANSACTIONS A
This theory, commonly known as the cross-slip pinning (CSP) model,E8] contends that thermal strengthening o f Ni3A1 and like L12 materials is derived from cross slip o f screw dislocations from {111} to {010}. C o m p u t e r simulations]9,~o] suggested that only a small portion o f the dislocation core cross slips from {111} to {010} (although the length o f this segment was left undefined). This result was used to suggest that localized cross slip o f small segments o f screw dislocations onto {010} decreases the mobility o f screw dislocations by locally pinning the glissile ( i . e . , octahedral) portions. These localized obstacles were assumed to be in the form o f a small double kink. A rather complex mechanism was specified for the formation o f such kinks (Figure 1), which was subsequently used to derive an activation enthalpy for the process. Incorporation o f this activation energy into the Arrenhius rate expression for the kinetics of
Data Loading...
