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) \(\mathrm{SmCo}_{5}\)

Short answer

(a) \(\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03}\): Substitutional alloy (b) \(\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}\): Substitutional alloy (c) \(\mathrm{SmCo}_{5}\): Intermetallic compound

Step-by-step solution

(a) Identify the type of alloy for \(\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03}\)

In this case, we have iron (Fe) and silicon (Si). Both elements are metals with similar electronegativities and have a difference in atomic radii less than 15%. Therefore, we would expect the alloy to be a substitutional alloy.

(b) Identify the type of alloy for \(\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}\)

In this case, we have iron (Fe) and nickel (Ni). Both elements are metals, and have similar electronegativities and a difference in atomic radii less than 15%. As such, we would expect the alloy to be a substitutional alloy.

(c) Identify the type of alloy for \(\mathrm{SmCo}_{5}\)

This compound involves the elements samarium (Sm) and cobalt (Co) in a specific stoichiometric ratio of 1:5. Considering the ratio and the fact that these two elements exhibit mixed metallic and covalent bonding, we would expect this compound to be an intermetallic compound. In conclusion, the alloy types for each of the provided compositions are: (a) \(\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03}\): Substitutional alloy (b) \(\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}\): Substitutional alloy (c) \(\mathrm{SmCo}_{5}\): Intermetallic compound

Key concepts

Substitutional Alloys

Substitutional alloys are mixtures where atoms of the solute element substitute for the host metal atoms in their crystal lattice. This type of alloy forms when the atomic sizes of the two elements are close, typically within a 15% size difference. Additionally, both elements should have similar electronegativities.
This way, the overall structure remains robust and unchanged.
Some important features of substitutional alloys include:

• The similar atomic size ensures that the lattice remains stable and maintains its integrity.
• Common examples include brass, which is composed of copper and zinc.
• These alloys tend to be more ductile because the substitution of one atom for another does not lead to large distortions in the metal lattice.
• This means that the metal's properties, like strength and resistance to corrosion, can be enhanced.

In our example, both \(\mathrm{Fe_{0.97}Si_{0.03}}\) and \(\mathrm{Fe_{0.60}Ni_{0.40}}\) are substitutional alloys because the elements involved have similar atomic sizes and chemical properties.

Interstitial Alloys

Interstitial alloys are formed when smaller atoms fill the spaces, or interstices, between the larger atoms of a metal's crystal structure. The small size of these added atoms, often non-metal elements, allows them to fit snugly into the spaces without significantly altering the metal's overall structure.
This leads to some intriguing changes in the metal's properties.
Key characteristics of interstitial alloys include:

• The addition of small atoms increases the hardness and strength of the alloy.
• Typical examples are steel, where carbon atoms fit into iron's structure.
• The presence of interstitial atoms usually reduces lattice vibration, thereby improving the thermal properties of the alloy.
• These alloys are generally less ductile compared to substitutional alloys, due to the lattice's reduced capacity to deform.

Though our original examples didn't feature interstitial alloys, understanding them is critical for grasping the full scope of how alloys can be engineered.

Intermetallic Compounds

Intermetallic compounds are distinct in that they contain specific proportions of their constituent elements which form a unique crystal structure entirely different from that of the constituent metals. These alloys are usually characterized by a fixed stoichiometric ratio between the elements.
Intermetallic compounds often exhibit mixed metallic and covalent bonding, leading to unique properties.Some fascinating aspects of intermetallic compounds include:

• They often have high melting points and are generally more brittle than other types of alloys.
• These compounds may have desirable mechanical and thermal properties for specialized applications, such as in aerospace engineering.
• Their formation relies on specific ratios of elements, rather than just random mixtures.
• An example from our exercise is \(\mathrm{SmCo}_{5}\), where the elements, samarium and cobalt, combine in a precise 1:5 ratio.

Understanding what intermetallic compounds bring to the table in terms of performance characteristics helps in appreciating their significance in modern technological applications.