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) \(\mathrm{Fe}_{0.97} \mathrm{Si}_{0.03 \text {, }}\) (b) \(\mathrm{Fe}_{0.60} \mathrm{Ni}_{0.40}\) (c) \(\mathrm{SmCo}_{s}\).

Short answer

a) Fe0.97 Si0.03 forms an interstitial alloy due to their different atomic sizes. b) Fe0.60 Ni0.40 forms a substitutional alloy due to their similar atomic sizes and structures. c) SmCo5 forms an intermetallic compound due to their fixed ratio and the combination of their properties, resulting in strong permanent magnets.

Step-by-step solution

In this alloy composition, we are given Iron (Fe) with 0.97 and Silicon (Si) with 0.03. #Step 2: Classifying the type of alloy#

Iron (Fe) and Silicon (Si) have different atomic sizes. Since they are not very close, it is likely that the smaller element (Si) will fit into the vacant space between the larger element (Fe) atoms. Thus, this composition can be classified as an interstitial alloy. #b) Fe0.60 Ni0.40# #Step 1: Identifying the elements#

In this alloy composition, we are given Iron (Fe) with 0.60 and Nickel (Ni) with 0.40. #Step 2: Classifying the type of alloy#

Iron (Fe) and Nickel (Ni) are both transition elements with similar atomic sizes and structures. This suggests that they can easily replace each other in the crystal lattice, forming a substitutional alloy. Thus, this composition can be classified as a substitutional alloy. #c) SmCo5# #Step 1: Identifying the elements#

In this alloy composition, we are given Samarium (Sm) and Cobalt (Co) with a stoichiometric ratio of 1:5. #Step 2: Classifying the type of alloy#

Samarium (Sm) and Cobalt (Co) form a stoichiometric compound with a fixed ratio of 1:5, which indicates that this composition may be an intermetallic compound. Furthermore, for intermetallic compounds, the individual properties of the elements are combined to create new properties in the compound. Samarium (Sm) is a rare earth element, while Cobalt (Co) is a transition element. The combination of these elements results in the formation of strong permanent magnets, which may not be seen in substitutional or interstitial alloys. Therefore, this composition can be classified as an intermetallic compound.

Key concepts

Substitutional Alloy

A substitutional alloy forms when atoms of different elements have similar atomic sizes and structures. In these alloys, one element's atoms can replace or substitute the atoms of another element within the crystal lattice.
This substitution often occurs without significantly altering the overall structure of the metal. Substitutional alloys enjoy certain benefits:

• Enhanced mechanical properties, like increased strength or hardness.
• Improved corrosion resistance compared to their pure metal counterparts.
• Retained conductivity characteristics of the base metals.

An example of this type of alloy is brass, where zinc atoms replace some of the copper atoms. Such substitutions result in a new material with desirable properties stemming from both metals.

Interstitial Alloy

Interstitial alloys are made when smaller atoms fit into the spaces or interstices between larger metal atoms in a crystal lattice. These smaller atoms do not replace the larger atoms but instead occupy the little gaps that naturally occur within the metallic structure.
This often leads to enhanced material properties. Key traits of interstitial alloys include:

• Increased hardness and strength due to the presence of smaller atoms filling gaps.
• Reduction in malleability and ductility, making these alloys tougher.
• They may exhibit different magnetic or electrical properties than the base metal.

An example is steel, where carbon atoms occupy interstitial spaces between iron atoms, enhancing the metal's strength and durability.

Intermetallic Compound

Intermetallic compounds are unique materials combining metals in a defined stoichiometric ratio, leading to ordered structures. These compounds do not merely mix; instead, they create a new set of properties not present in the individual metals.
In intermetallic compounds, the elemental properties merge to produce materials with unique characteristics. Notable features include:

• Precise chemical formulas are followed to form these compounds, giving them fixed proportions.
• They often exhibit high temperature resilience and other extraordinary mechanical properties.
• Many have unique magnetic or electronic properties due to their ordered structures.

An example is NiAl, where nickel and aluminum form a compound with a characteristic structure, known for its high-temperature strength and oxidation resistance.