Thermal expansion — select the engineering material with near-zero coefficient Which of the following engineering materials exhibits a nearly zero coefficient of linear thermal expansion around room temperature, making it ideal for precision instruments?

Introduction / Context:
Dimensional stability with temperature is critical in precision devices such as clock pendulums, interferometer frames, and instrument scales. Engineers therefore select alloys whose length changes minimally with thermal fluctuations. One famous alloy is designed for an ultra-low coefficient of expansion near ambient temperature.

Given Data / Assumptions:

• Room-temperature applications and modest temperature excursions.
• Comparing common steels and a specialized Fe-Ni alloy.

Concept / Approach:
Invar (approximately 36% nickel and balance iron) displays anomalously low thermal expansion due to magneto-volume effects: changes in magnetic ordering partly counterbalance lattice expansion, yielding a near-zero net coefficient over a limited temperature range. Most steels, including stainless, high-speed, and heat-resisting types, have significantly higher coefficients (on the order of 10 to 17 microstrain per metre per degree Celsius), rendering them less suitable for ultra-stable dimensions.

Step-by-Step Solution:

Verification / Alternative check:
Precision pendulum rods, seismic instrument frames, and metrology gauge blocks often use Invar or related low-expansion Fe-Ni-Co alloys (e.g., Kovar), confirming practical usage consistent with the material property.

Why Other Options Are Wrong:

• Stainless steel, high-speed steel, heat-resisting steel, mild steel: none exhibit near-zero expansion; they expand appreciably with temperature, leading to measurement drift.

Common Pitfalls:
Confusing corrosion resistance (stainless) or high-temperature strength (heat-resisting) with low expansion; assuming hardness (high-speed steel) implies dimensional stability under temperature change.

Final Answer: