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) \(\mathrm{Cu}_{0.66} \mathrm{Zn}_{0.34}\) (b) \(\mathrm{Ag}_{3} \mathrm{Sn}\), (c) \(\mathrm{Ti}_{0.99} \mathrm{O}_{0.01}\).

Short answer

(a) \(\mathrm{Cu}_{0.66} \mathrm{Zn}_{0.34}\): Substitutional Alloy, due to the similar atomic radii of Cu and Zn. (b) \(\mathrm{Ag}_{3} \mathrm{Sn}\): Intermetallic Compound, due to the fixed stoichiometric ratio between Ag and Sn. (c) \(\mathrm{Ti}_{0.99} \mathrm{O}_{0.01}\): Interstitial Alloy, due to the significant difference in sizes between Ti and O atoms.

Step-by-step solution

Understanding Different Types of Alloys

Before moving on to each alloy composition, we need to know the basic differences between the three types of alloys mentioned. 1. Substitutional Alloy: A substitutional alloy is formed when one element replaces (substitutes) another element of similar size in the crystal lattice. This usually occurs when the difference in atomic radii between the two elements is less than 15%. 2. Interstitial Alloy: An interstitial alloy is formed when atoms of one element occupy the interstitial spaces between the atoms of another element in the crystal lattice. It occurs when the atoms of the first element have significantly smaller sizes than the host atoms of the second element. 3. Intermetallic Compound: An intermetallic compound is a distinct chemical compound that forms between metallic elements. It has a fixed composition and specific crystal structure, often with a stoichiometric ratio between the elements. With this information, we can now proceed to examine each alloy composition.

Determine the type of alloy for Cu0.66Zn0.34

(a) \(\mathrm{Cu}_{0.66} \mathrm{Zn}_{0.34}\) Both Copper (Cu) and Zinc (Zn) are metallic elements, and their atomic radii are similar (Cu: 128 pm, Zn: 134 pm). Since the difference in atomic radii is small (around 4%), we can expect this alloy to be a substitutional alloy, where Zn atoms replace some of the Cu atoms in the crystal lattice.

Determine the type of alloy for Ag3Sn

(b) \(\mathrm{Ag}_{3} \mathrm{Sn}\) Silver (Ag) and Tin (Sn) are both metallic elements. In this case, the composition given has a fixed stoichiometric ratio of 3:1 between Ag and Sn. Since it has a fixed stoichiometric ratio, it is likely to be an intermetallic compound.

Determine the type of alloy for Ti0.99O0.01

(c) \(\mathrm{Ti}_{0.99} \mathrm{O}_{0.01}\) Titanium (Ti) is a metallic element, while Oxygen (O) is a non-metallic element. The composition given suggests that a small amount of Oxygen atoms are present within the crystal lattice of Titanium. The ionic radii of Ti and O are significantly different (Ti: 145 pm, O: 60 pm). Given the difference in sizes and the nature of the elements, we can expect this alloy to be an interstitial alloy, where O atoms occupy interstitial spaces within the Ti lattice. In conclusion, based on the analysis of the alloy compositions: (a) \(\mathrm{Cu}_{0.66} \mathrm{Zn}_{0.34}\): Substitutional Alloy (b) \(\mathrm{Ag}_{3} \mathrm{Sn}\): Intermetallic Compound (c) \(\mathrm{Ti}_{0.99} \mathrm{O}_{0.01}\): Interstitial Alloy.

Key concepts

Substitutional Alloys

Substitutional alloys form when atoms of one element in a metal crystal lattice are replaced by atoms of another element. This process happens if those atoms are similar in size, typically when the difference in their atomic radii is less than 15%. Think of it like swapping one friend for another in a group photo; the friend you swap in needs to be about the same size to keep the picture balanced and harmonious.

Key real-world examples include brass, which is a substitutional alloy of copper (Cu) and zinc (Zn). Here, zinc atoms substitute some of the copper atoms in the lattice structure. In practice, substitutional alloys usually enjoy good strength and corrosion resistance, which makes them widely useful in various industrial applications.

The formation of substitutional alloys is favored because the substitution does not disturb the overall structure significantly, so the alloy maintains stability while gaining new properties based on the added element. As a result, engineers can enhance or modify the mechanical, thermal, or electrical properties of metals by designing substitutional alloys.

Interstitial Alloys

Interstitial alloys form when smaller atoms fit into the empty spaces, or interstices, between larger metal atoms in the lattice structure. This occurs when the small atoms have radii significantly smaller than the host atoms, allowing them to 'hide' comfortably in between the larger atoms.

Carbon steel is a common example of an interstitial alloy where small carbon atoms fit into the gaps of an iron lattice. This ends up strengthening the iron without significantly expanding its volume, because the small carbon atoms slip into the pockets between the bulkier iron atoms.

The presence of interstitial atoms often increases the hardness and strength of the base metal. However, this can sometimes come at the cost of the material's ductility, which is its ability to deform under tensile stress. This makes interstitial alloys particularly useful in applications that require materials to be hard or wear-resistant.

Intermetallic Compounds

Intermetallic compounds differ from other types of alloys as they form specific, ordered structures with fixed ratios between participating elements, rather than random mixtures. They are stoichiometric compounds, which means the elements exist in fixed, definite proportions.

These compounds often have unique physical properties that are different from their constituent metals, such as improved strength at high temperatures or better corrosion resistance. The intermetallic compound Ag₃Sn, seen in soldering applications, is an example where silver and tin combine in a precise ratio, forming a distinct structure that adds to its utility.

Known for their brittleness and difficulty to plastically deform, intermetallic compounds are not usually used in situations where materials need to withstand significant impact but are valued for their stability, strength, and resistance to adverse conditions. Thus, they often find applications in aerospace, electronics, and high-temperature environments where these traits are advantageous.