Jawaharlal Nehru Technological University Hyderabad B.Tech II METALLURGY AND MATERIALS SCIENCE Previous Question Paper (August/September - 2022 - R15)

Grain size significantly influences the mechanical properties of metals and alloys, primarily governed by the Hall-Petch relationship. As grain size decreases, the number of grain boundaries increases, which act as effective barriers to dislocation motion.

Key effects include:

• Yield Strength and Hardness: Smaller grains provide more grain boundaries that impede dislocation movement, requiring higher stress for plastic deformation. This increases both yield strength and hardness. The relationship is defined as:

• Ductility and Toughness: Unlike most strengthening mechanisms, grain refinement simultaneously improves strength and toughness. Finer grains promote more uniform deformation, delaying the onset of necking and increasing impact resistance.
• Creep Resistance: At elevated temperatures, grain boundaries become sites for sliding and diffusion. Consequently, coarser grains are often preferred for high-temperature applications to minimize grain boundary area and reduce creep rates.

Alloying is the process of combining a base metal with one or more other elements (metallic or non-metallic) to form a new material with superior properties. This is achieved through melting and mixing, resulting in a solid solution or intermetallic compound upon solidification.

The importance of alloying includes:

• Enhanced Mechanical Properties: Alloying elements improve strength, hardness, and wear resistance through solid solution strengthening or precipitation hardening.
• Corrosion Resistance: Elements like chromium are added to iron to form a protective oxide layer, significantly increasing resistance to oxidation and environmental degradation.
• Improved Fabricability: Alloying can modify the melting point, castability, and machinability of a metal, making it easier to process into desired shapes.
• Specialized Functionality: It allows for the tailoring of physical properties such as electrical conductivity, thermal expansion coefficients, and magnetic permeability to meet specific engineering requirements.