• PDF / 2,455,545 Bytes
• 9 Pages / 597.28 x 785 pts Page_size
• 46 Downloads / 286 Views

DOWNLOAD

REPORT

---

I.

INTRODUCTION

THE plastic deformation in metals and alloys is characterized by the generation, movement, and interaction o f dislocations on one o r more slip planes. In the early stages o f plastic deformation, slip takes place essentially on the primary slip system, and the dislocations form coplanar arrays. As the deformation proceeds, heterogeneous structure and dislocation tangles separated by dislocation-free regions form, which develop into a threedimensional network. 11-Sj The cell diameters decrease as the extent o f imposed deformation increases, while the cell walls tend to become progressively sharper as the misorientation between two neighboring cells increases. Several authors have presented details o f substructural features during various deformation processes, indic'ating the characterization o f cell boundaries at different levels o f deformation, cell size refinement, and the relationship between the yield strength and cell sizes. [6-151 The driving force for the dislocation clustering and substructure formation is the reduction in the strain energy o f the material. Essentially, two approaches have been used to explain the substructure formation,t~a~ Holt TM has treated dislocation clustering as analogous to spinodal decomposition o f a supersaturated solution. Kuhlmann-Wilsdorl ~2] ( K - W ) has investigated in considerable detail the energies and stresses associated with a dislocation cell structure m o d e l composed o f idealized cell walls o f low-angle boundary type, the basis o f popular mesh length theory. Hansen and K - W 131 have made some quantitative estimation o f the energy per unit length o f simple dislocation arrays, such as dipolar m a t s , tilt walls, and dipolar walls to substantiate the mesh length theory. The specific energy o f the dislocation cell boundary with a given angle o f misorientation decreases with an increase in the n u m b e r o f types o f Burgers vector present. All o f the factors that contribute to slip on DEEPAK SIL, Graduate Student, and S.K. VARMA, Professor, are with the Department of Metallurgical and Materials Engineering, The University of Texas at E1 P a s o , El P a s o , T X 79968-0520. Manuscript submitted July 31, 1992. METALLURGICAL TRANSACTIONS A

more than one glide system decide the ease o f substructure formation and its k i n d . The factors that f a v o r the cell formation are: high symmetry orientation, l o w frictional stress, large deformation, the ability o f the dislocations to cross slip (i.e., stacking fault energy), and temperature, ta,5,~6,17,~s~ Kuhlmann-Wilsdorf's principle o f similitude predicts a constant ratio o f the cell volume to the dislocation cell width. [19J Similar features were reported in copper by Knoesen and Kritzinger ~2°~ and in nickel by Murr and K-W. t2q However, Griffith and Rileyt22~ noticed an increase in the cell boundary width with a decrease in cell size in iron-silicon alloys during rolling. S t a k e r and Holtf231 observed an increase in the ratio o f the cell volume to the dislocation cell w