Physical Metallurgy - What is Slip systems and Surface Defects

A: In body-centered cubic (BCC) metals, the primary slip systems involve {110}, {112}, and {123} slip planes with <111> slip directions. Although BCC metals have several potential slip systems, their activation is influenced by temperature due to higher Peierls-Nabarro stress compared to FCC metals. In contrast, FCC metals primarily deform by slip on {111} planes in <110> directions, which are close-packed and allow for easier dislocation movement even at lower temperatures. This difference explains why BCC metals often exhibit a ductile-to-brittle transition temperature, while FCC metals remain ductile over a wide range of temperatures. More can be read in "Introduction to Dislocations" by D. Hull and D.J. Bacon.

A: Surface defects act as stress concentrators where cracks can initiate and propagate under cyclic loading, thereby influencing fatigue life. Even minor imperfections like scratches or pits can significantly reduce the number of cycles a metal can endure before failure. This is why surface finish and treatments (like polishing or coatings) are vital in components subjected to fatigue. The role of surface defects in fatigue is well-documented in the literature, for example in the book "Metal Fatigue in Engineering" by Ralph I. Stephens et al.

A: Slip systems are crucial because plastic deformation in metals primarily occurs through the motion of dislocations along slip systems. The ease of dislocation movement depends on the availability and orientation of slip systems relative to the applied stress. Materials with multiple active slip systems can accommodate deformation more uniformly, leading to better ductility and toughness. The concept of critical resolved shear stress (CRSS) helps quantify the stress needed to activate slip on a particular system. Understanding slip systems enables engineers to predict material behavior under mechanical loads and design alloys with desired mechanical properties. References include "Mechanical Behavior of Materials" by Norman E. Dowling.