Question

For each of the following alloy compositions indicate whether you would expect it to be a substitutional alloy, an interstitial alloy, or an intermetallic compound: (a) \(\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03}\), (b) \(\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}\), (c) \(\mathrm{Cu}_{3} \mathrm{Au}\).

Short answer

The alloy composition Fe\(_{0.97}\) Si\(_{0.03}\) would be a substitutional alloy. The alloy composition Fe\(_{0.60}\) Ni\(_{0.40}\) would also be a substitutional alloy. Meanwhile, the alloy composition Cu\(_3\) Au would be an intermetallic compound.

Step-by-step solution

Alloy type for Fe\(_{0.97}\) Si\(_{0.03}\)

The alloy composition Fe\(_{0.97}\) Si\(_{0.03}\) would be a substitutional alloy. #Step 2: Determine type of alloy for Fe\(_{0.60}\) Ni\(_{0.40}\)# Both Iron (Fe) and Nickel (Ni) have similar lattice structures, with both being Body-Centered Cubic (BCC). The atomic radii of Fe and Ni are very close, with Fe having an atomic radius of 1.24 Å and Ni having an atomic radius of 1.25 Å. Given that the concentration of both elements in the alloy is comparable, a substitutional solid solution will occur where Fe and Ni occupy the lattice sites interchangeably.

Alloy type for Fe\(_{0.60}\) Ni\(_{0.40}\)

The alloy composition Fe\(_{0.60}\) Ni\(_{0.40}\) would be a substitutional alloy. #Step 3: Determine type of alloy for Cu\(_3\) Au# Copper (Cu) and Gold (Au) have the same lattice structure, which is Face-Centered Cubic (FCC). Their atomic radii are also very similar: Cu has an atomic radius of 1.28 Å and Au has an atomic radius of 1.44 Å. The alloy has a fixed stoichiometry of Cu\(_3\) Au, which indicates the formation of an intermetallic compound, as opposed to the random distribution expected in substitutional or interstitial alloys.

Alloy type for Cu\(_3\) Au

The alloy composition Cu\(_3\) Au would be an intermetallic compound.

Key concepts

Substitutional Alloy

Substitutional alloys are fascinating mixtures where one kind of atom is replaced by another within the lattice of a metal. This occurs when the substituting elements are similar in atomic size and lattice structure.
For example, in the alloy composition Fe _{0.97} Si _{0.03}, silicon (Si) atoms replace iron (Fe) atoms within the iron lattice. This is possible because Si and Fe have comparable atomic sizes, allowing them to easily switch places in the metal lattice structure.

• These alloys contribute significantly to material strength and versatility by enhancing properties like corrosion resistance and conductivity.
• An easy way to remember: think of substitutional as "substituting" one atom for another within the metal's framework.
• Examples include brass (copper and zinc) and bronze (copper and tin).

To form a substitutional alloy, the difference in atomic radii of the two elements should generally be less than 15%, and they should have similar electronegativity and valency. This ensures that the new alloy maintains appropriate physical properties for practical use.

Interstitial Alloy

Interstitial alloys are formed when smaller atoms fit into the spaces, or "interstices," between the atoms in a metal lattice. Unlike substitutional alloys, the primary metal remains the same, but its lattice becomes more densely packed with additional, smaller atoms.
This type of alloy is typically formed with an element possessing significantly smaller atomic size, such as carbon, which can fit into the interstitial spaces of iron in steel production.

• By "filling" the gaps, these small atoms can increase material properties such as hardness and tensile strength.
• An interstitial alloy example is carbon steel, where carbon atoms strengthen iron.
• These alloys often exhibit increased density, due to the smaller atoms present in the metal lattice.

The classic property enhancement in interstitial alloys is increased hardness, which occurs because the smaller atoms can restrict the movement of the larger metal atoms when force is applied, acting as a structural reinforcement.

Intermetallic Compound

Intermetallic compounds are unique alloys that are more ordered than typical substitutional or interstitial alloys due to their definite stoichiometry and crystal structure. This means that these compounds have a fixed composition ratio, and the atoms occupy specific lattice sites.
The Cu _{3} Au alloy stands as a prime example of an intermetallic compound, where copper and gold atoms are arranged in a precise and repeating pattern that is not random or interchangeable.

• Intermetallic compounds often possess distinctive properties, such as high melting points and unusual magnetism, due to their ordered arrangements.
• They are rather like a "chemical compound" among metals, having defined compositions.
• These compounds are frequently used in high-temperature applications due to their structural stability under intense conditions.

Understanding how these defined ratios improve material characteristics can help engineers and scientists to tailor materials for particular needs, including superalloys for aircraft engines or unique materials for electronics.