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: $$ (a)\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03}, \quad(\mathbf{b})\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}, \quad(\mathbf{c}) \mathrm{Sm} \mathrm{Co}_{5} $$

Short answer

For the given alloy compositions: (a) \(\mathrm{Fe}_{0.97}\mathrm{Si}_{0.03}\) is an interstitial alloy (b) \(\mathrm{Fe}_{0.60}\mathrm{Ni}_{0.40}\) is a substitutional alloy (c) \(\mathrm{Sm}\mathrm{Co}_{5}\) is an intermetallic compound

Step-by-step solution

- Identify elements and analyze their atomic sizes and characteristics

In the first step, we need to identify the elements in each alloy composition and examine their atomic sizes and other characteristics: (a) \(\mathrm{Fe}_{0.97}\mathrm{Si}_{0.03}\) - This composition has Iron (Fe) and Silicon (Si) as its elements. Iron has a slightly larger atomic size than Silicon, and both are metallic elements. (b) \(\mathrm{Fe}_{0.60}\mathrm{Ni}_{0.40}\) - This composition has Iron (Fe) and Nickel (Ni) as its elements. Both Iron and Nickel have similar atomic sizes and characteristics, as they belong to the same period in the periodic table. (c) \(\mathrm{Sm}\mathrm{Co}_{5}\) - This composition has Samarium (Sm) and Cobalt (Co) as its elements. Samarium is a rare-earth metal and Cobalt is a transition metal.

- Determine the nature of the alloys

Based on atomic sizes, characteristics, and the compositions provided, we can determine the nature of each alloy: (a) \(\mathrm{Fe}_{0.97}\mathrm{Si}_{0.03}\) - Since Iron and Silicon are metallic elements with different atomic sizes, one would expect that this composition will form an interstitial alloy. In an interstitial alloy, the smaller atoms (such as Silicon) occupy the interstices or "holes" between the atoms of the larger element (Iron). (b) \(\mathrm{Fe}_{0.60}\mathrm{Ni}_{0.40}\) - Iron and Nickel have similar atomic sizes and characteristics. When elements with similar atomic sizes form alloys, they tend to create substitutional alloys, where one atom type replaces the other atom type within the lattice structure. Thus, this composition is likely to form a substitutional alloy. (c) \(\mathrm{Sm}\mathrm{Co}_{5}\) - Samarium and Cobalt have a specific stoichiometric ratio in this composition (1:5), which suggests the formation of an intermetallic compound. Intermetallic compounds are formed when metals with different electronegativities combine in specific proportions and have a unique crystal structure. In conclusion, for the given alloy compositions: (a) \(\mathrm{Fe}_{0.97}\mathrm{Si}_{0.03}\) is an interstitial alloy (b) \(\mathrm{Fe}_{0.60}\mathrm{Ni}_{0.40}\) is a substitutional alloy (c) \(\mathrm{Sm}\mathrm{Co}_{5}\) is an intermetallic compound

Key concepts

Substitutional Alloy

A substitutional alloy is a type of metal alloy in which atoms of a component element replace or substitute for the atoms of the base metal.

Imagine lining up a group of marbles, all of similar size but different colors—this represents the crystal lattice of the base metal. Now, if you pick some marbles and replace them with other marbles of a different color but the same size, you essentially have a substitutional alloy. This can occur when the replacing element's atomic radius differs by no more than around 15% from the base metal's atomic radius, to fit snugly into the crystal lattice without distorting it too much.

Key Characteristics of Substitutional Alloys

• Components have similar electronegativities.
• The components are usually within the same group or period in the periodic table.
• The alloy exhibits atomic radius size differences less than 15%.

Substitutional alloys are quite common, and one primary example is brass, which is a mixture of copper and zinc where the atoms of the two elements are approximately similar in size and can substitute each other in the crystal structure.

Interstitial Alloy

An interstitial alloy is another form of an alloy where the smaller atoms fit into the spaces (interstices) between the larger atoms in a metal's crystal lattice structure. In simpler terms, if you took a sponge made up of tightly packed balls, the tiny pores within the sponge are like the interstices.

Small atoms, like carbon in steel, can squeeze into these gaps without displacing the larger iron atoms. This characteristic distinguishes interstitial alloys from substitutional alloys and can lead to significant changes in the properties of the metal, such as increased hardness and strength. However, since they can distort the lattice structure to some degree, there's a limit to how much of the smaller atom can be interjected.

Key Characteristics of Interstitial Alloys

• Smaller atoms (non-metals or small metals) fill the gaps in the crystal structure of the larger atoms.
• They often result in hard and strong materials.
• Examples include steel, which is an alloy of iron and carbon.

The ability to significantly alter mechanical properties is one reason why interstitial alloys, like steel, are crucial in construction and manufacturing.

Intermetallic Compound

Intermetallic compounds stand out from the aforementioned types of alloys due to their very distinctive properties and stoichiometric composition.

These are not merely mixtures but rather new compounds with unique crystal structures and properties that are often much different from their constituent elements. Unlike substitutional or interstitial alloys, where atoms are mixed with each other, intermetallic compounds often form at precise proportions and exhibit a high degree of order in their crystal lattice.

Key Characteristics of Intermetallic Compounds

• They have specific stoichiometric ratios.
• The crystal structure is ordered and unique, often different from the structures of the pure metals involved.
• These compounds can show high levels of hardness and brittleness.

Examples include alloys like aluminides and silicides, which have high-temperature resistance and are used in aerospace applications. Their precise atomic ratios and robust structures offer specialized properties desired in advanced technical applications.