2009 Al-Abdallat Properties of Eng. Material 1 (3) Interfacial defects Interfacial defects: Types: External surfaces, Grain boundaries, Twin boundaries 2009 Al-Abdallat Properties of Eng. Material 2 1. External surfaces Surfaces along which crystal structure terminates. Surface atoms are in high energy states (WHY? They are not bound to maximum number of nearest neighbors). To reduce surface energy (J/m 2 ), material tend to minimize surface area 2009 Al-Abdallat Properties of Eng. Material 3 2. Grain boundaries Boundary separating two small grains or crystals having different crystallographic orientations. Within boundary region, there is atomic mismatch in a transition from crystalline orientation of one grain to adjacent one. 2009 Al-Abdallat Properties of Eng. Material 4 2. Grain boundaries (Cont.) Types: High angle grain boundary. Small (Low) angle grain boundary: Described in terms of dislocation arrays, e.g., tilt boundary (resulting from arrays of edge dislocations). Twist boundary results from arrays of screw dislocations. Since atoms are bonded less regularly along grain boundary, interfacial energy exists. 2009 Al-Abdallat Properties of Eng. Material 5 Small & high-angle grain boundaries 2009 Al-Abdallat Properties of Eng. Material 6 2. Grain boundaries (Cont.) Due to interfacial energy, grain boundaries are more chemically reactive than grains. Impurity atoms preferentially segregate along grain boundaries. 2009 Al-Abdallat Properties of Eng. Material 7 Tilt boundary having an angle of misorientation resulting from alignment of edge Dislocations Twist boundary Due to an array of screw dislocations Interfacial Defects (Contd.) 2009 Al-Abdallat Properties of Eng. Material 8 3. Twin boundaries Special type of grain boundary across which there is a specific mirror lattice symmetry. Atoms on one side of the boundary are located in mirror image positions of the atoms on the other side. Region of material between these boundaries is termed a Twin. Twin results from atomic displacements produced from mechanical shear force (mechanical twin, found in BCC & HCP) or annealing heat treatment (annealing twin, found in FCC). Twining occurs along definite crystallographic plane and in specific directions. 2009 Al-Abdallat Properties of Eng. Material 9 Twin plane or boundary and the adjacent atom positions. 2009 Al-Abdallat Properties of Eng. Material 10 (3) Interfacial defects (Cont.) Interfacial energy: Interfacial energy is associated with each interfacial defect. Interfacial energy depends on boundary defect type and varies from material to another. 2009 Al-Abdallat Properties of Eng. Material 11 (4) Bulk (Volume defects) Examples: Pores, Cracks, Foreign inclusions, Other phases. Introduced during processing and fabrication steps. 2009 Al-Abdallat Properties of Eng. Material 12 Atomic vibrations Atomic vibrations are imperfections. At any instant of time, not all atoms vibrate at the same frequency and amplitude, nor with the same energy. 2009 Al-Abdallat Properties of Eng. Material 13 Atomic vibrations (Cont.) Over time, vibrational energy of any specific atom will vary in a random manner. With rising temperature, average vibrational energy increases. At room temperature, typical vibrational frequency is on the order of vibrations per second. Melting occur when vibrations are enough to rupture large number of atomic bonds. 2009 Al-Abdallat Properties of Eng. Material 14 Microscopy Careful and meticulous surface preparations are necessary First, ground and polished to a smooth and mirror like finish. Accomplished by using successively papers and powders Etching : microstructure is revealed by a surface treatment using an appropriate chemical reagent (a) Polished and etched grains (b) Showing etching characteristics and resulting surface texture (c) Photomicrograph of Brass