What are the strengthening mechanisms commonly used in engineering materials? describe and explain each mechanism concisely. use sketches as needed elaborating on your answer (ii) why are ceramics brittle while metals are ductile? explain by using the peierls-nabarro equation (iii) why does dislocation slip occur on closed packed planes and close packed directions? explain by using the peierls- nabarro equation (iv) why are polycrystalline solids more advantageous than single crystals in the context of plastic deformation at ambient temperatures? explain by using the schmid's law