Question

At 0.4pt temperature, the grains and grain boundaries have equal strength.

• A. Curie
• B. Absolute zero
• C. Melting temperature
• D. equi-cohesive

Hint:

Remember the crossover effect: At low temperatures, boundaries are strong. At high temperatures, boundaries are weak. The temperature where their strength curves cross is the Equi-Cohesive Temperature (ECT). This is typically around 0.5 T\(_m\) (half the absolute melting temperature).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
In a polycrystalline metal, both the grains (crystalline interiors) and the grain boundaries (interfaces between grains) contribute to the overall strength. However, their relative strengths change significantly with temperature.

Step 2: Detailed Explanation:

• At Low Temperatures (e.g., room temp): Grain boundaries are disordered regions that act as strong obstacles to dislocation motion. This makes the grain boundaries stronger than the grain interiors. This is the basis of the Hall-Petch effect, where finer grain size (more grain boundaries) leads to higher strength.

• At High Temperatures: Atoms at the grain boundaries have higher energy and mobility. Processes like grain boundary sliding and diffusion become active. This makes the grain boundaries a source of weakness compared to the more orderly and stronger grain interiors. High-temperature failure, such as creep, often occurs along grain boundaries.

• (D) Equi-cohesive Temperature (ECT): This is the specific temperature at which the transition in relative strength occurs. By definition, the equi-cohesive temperature is the temperature at which the strength of the grains is equal to the strength of the grain boundaries. "Equi" means equal, and "cohesive" refers to cohesive strength. Above the ECT, grain boundaries are weaker; below the ECT, grains are weaker (and boundaries are stronger).

• Other Options: itemize

• (A) Curie temperature is related to the transition between ferromagnetic and paramagnetic behavior.

• (B) Absolute zero is the lowest possible temperature.

• (C) Melting temperature is when the material transitions from solid to liquid.
itemize

Step 3: Final Answer:
The temperature at which the strength of the grains and grain boundaries are equal is called the equi-cohesive temperature.