Plastic deformation occurs when a material is stressed beyond its elastic limit, resulting in a permanent change in shape that is not reversible. Ductile materials like metals can undergo significant plastic deformation through stretching before fracturing. Alloys are mixtures of two or more metals or metals and non-metals that combine to form a new material with different properties than the individual components. Common alloys include brass, stainless steel, and superalloys used in jet engines.