Aluminum slip systems

For crystals of reasonably pure, well-annealed metals at a given temperature, slip begins when the resolved shear stress reaches a certain critical value, which is characteristic of each metal. In the case of aluminum, for example, the observed critical shear stress Uco is usually about 4x10 N/m ( 4 bars = 0.4 MPa). Theoretically, for a perfect crystal, the resolved shear stress is expected to vary periodically as the lattice planes slide over each other and to have a maximum value that is simply related to the elastic shear modulus /t. This was first pointed out in 1926 by Frenkel who, on the basis of a simple model, estimated that the critical resolved shear stress was approximately equal to h/Itt (see Kittel 1968). In the case of aluminum (which is approximately elastically isotropic), = C44 = 2.7x10 N/m, so the theoretical critical resolved shear stress is about lO wco for the slip system <100>(100). 

Since the number of slip systems is not usually a function of temperature, the ductility of face-centered cubic metals is relatively insensitive to a decrease in temperature. Metals of other crystal lattice types tend to become brittle at low temperatures. Crystal structure and ductility are related because the face-centered cubic lattice has more slip systems than the other crystal structures. In addition, the slip planes of body-centered cubic and hexagonal close-packed crystals tend to change at low temperature, which is not the case for face-centered cubic metals. Therefore, copper, nickel, all of the copper-nickel alloys, aluminum and its alloys, and the austenitic stainless steels that contain more than approximately 7% nickel, all face-centered cubic, remain ductile down to the low temperatures, if they are ductile at room temperature. Iron, carbon and low-alloy steels, molybdenum, and niobium, all body-centered cubic, become brittle at low temperatures. The hexagonal close-packed metals occupy an intermediate place between fee and bcc behavior. Zinc undergoes a transition to brittle behavior in tension, zirconium and pure titanium remain ductile. 

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