Thermal properties of materials
Which statement correctly explains why the linear thermal expansion coefficient of Fe-36%Ni alloy (Invar) is very small near room temperature, about 1×10⁻⁶ K⁻¹?
- ANi suppresses the lattice vibrations of Fe and raises the Debye temperature greatly, so at room temperature the atoms hardly vibrate
- BBelow the Curie temperature, the magnetic volume expansion shrinks as magnetization weakens on heating, canceling normal expansion
- COn heating, a gradual transformation from BCC to the more densely packed FCC phase proceeds, and the resulting shrinkage cancels the expansion
Show answer and explanation
Answer: B. Below the Curie temperature, the magnetic volume expansion shrinks as magnetization weakens on heating, canceling normal expansion
Invar is a ferromagnet with a large magnetovolume effect (spontaneous volume magnetostriction), in which spontaneous magnetization lengthens the interatomic distance and enlarges the volume. As temperature rises, the magnetization weakens, so this magnetic expansion decreases and nearly cancels the ordinary thermal expansion from lattice vibration. As a result, the linear expansion coefficient is as small as about 1×10⁻⁶ K⁻¹ below the Curie temperature, and Invar is used in precision measuring instruments, containers for liquefied natural gas and the like. Above the Curie temperature the cancellation disappears and it expands like an ordinary metal. Invar is single-phase FCC at room temperature; atomic vibration grows with temperature as usual, and no phase transformation that offsets the expansion takes place.