Question

Explain briefly the four main microstructural mechanisms by which metals are hardened. Identify which mechanisms account for the following increases in yield stress (NB there may be more than one): (i) Pure annealed aluminium: 25 MPa; cold rolled non-heat-treatable Al-Mn-Mg alloy: 200 MPa. (ii) Pure annealed copper: 20 MPa; cast 60-40 brass (60% Cu, 40% Zn): 105 MPa. (iii) Pure annealed iron: 140 MPa; quenched and tempered medium- carbon steel: 600 MPa. (iv) Pure annealed copper: 20 MPa; nanocrystalline pure Cu: 500 MPa. (b) For each of mechanisms (i)-(iii) in part (a), give other examples of important engineering alloys which exploit these mechanisms.

Answer 1

Metals are hardened through strain hardening, grain boundary strengthening, solid solution strengthening, and precipitation hardening. Engineering alloys like stainless steel and aluminum alloys use these mechanisms to improve strength and durability.

Step-by-step explanation:

The four main microstructural mechanisms by which metals are hardened include:

1. Strain hardening
2. Grain boundary strengthening
3. Solid solution strengthening
4. Precipitation hardening

Increases in yield stress for different metals and alloys can be explained by these mechanisms:

• (i) Pure annealed aluminium (25 MPa) is hardened primarily by grain boundary strengthening while cold rolled non-heat-treatable Al-Mn-Mg alloy (200 MPa) is hardened through strain hardening and solid solution strengthening.
• (ii) Pure annealed copper (20 MPa) increases in strength due to grain boundary strengthening while cast 60-40 brass (105 MPa) benefits from both solid solution strengthening and possibly precipitation hardening.
• (iii) Pure annealed iron (140 MPa) is hardened by grain boundary strengthening, whereas quenched and tempered medium-carbon steel (600 MPa) utilizes mechanisms such as precipitation hardening and the formation of martensite.
• (iv) Pure annealed copper (20 MPa) compared to nanocrystalline pure Cu (500 MPa) is an example of grain boundary strengthening due to the presence of a high volume of grain boundaries in nanocrystalline materials.

Engineering alloys that exploit these mechanisms include:

• Strain hardening: cold-worked stainless steel, rolled aluminum alloys
• Grain boundary strengthening: high-strength, low-alloy (HSLA) steels, ferritic stainless steels
• Solid solution strengthening: nickel-based superalloys, bronze
• Precipitation hardening: 6000 and 7000 series aluminum alloys, 17-4 PH stainless steel