Question

When iron surfaces are exposed to ammonia at high temperatures, "nitriding"-the incorporation of nitrogen into the iron lattice-occurs. The atomic radius of iron is \(124 \mathrm{pm}\). (a) Is the alloy interstitial or substitutional? Justify your answer. (b) How do you expect nitriding to change the properties of iron?

Short answer

The alloy formed by nitriding is interstitial because nitrogen atoms, which are smaller than iron atoms, occupy the interstitial spaces in the iron lattice. Nitriding is expected to make the iron harder, more wear-resistant, and possibly affect its corrosion resistance and fatigue strength.

Step-by-step solution

Understand Alloy Types

There are two main types of alloys: interstitial and substitutional. In an interstitial alloy, smaller atoms fit into the spaces (interstices) between the larger metal atoms. In a substitutional alloy, atoms of similar size replace each other in the lattice structure.

Analyzing the Atomic Size

To determine whether the alloy is interstitial or substitutional, compare the atomic sizes of the involved elements. If the non-metallic element, such as nitrogen in this case, is much smaller than the metal atoms, it is more likely to form an interstitial alloy.

Determine Alloy Type

Since nitrogen is a non-metal with a much smaller atomic radius than that of iron, when it is incorporated into the iron lattice at high temperatures, it will occupy the interstitial spaces. Therefore, the nitriding process forms an interstitial alloy.

Predict the Changes in Properties

Nitriding can be expected to affect the properties of iron by making it harder and more wear-resistant due to the formation of a harder surface layer. Additionally, the incorporation of nitrogen may also lead to a change in the iron's corrosion resistance and fatigue strength.

Key concepts

Interstitial Alloy

The process of nitriding iron results in the formation of what's known as an interstitial alloy. This peculiar name comes from the word 'interstice,' which refers to the tiny spaces in between larger atoms—in this case, iron atoms—within a metal's lattice structure.

In nitriding, nitrogen atoms, which are significantly smaller than iron atoms, diffuse into the iron at high temperatures and occupy these interstices. It's the difference in size that's crucial here. The atomic radius of nitrogen is approximately 70 picometers (pm), which is significantly smaller than iron's 124 pm. This disparity allows nitrogen to slip into the gaps in the iron crystal structure rather than replacing the iron atoms themselves.

This interstitial diffusion leads to changes in the metal's characteristics, such as increased hardness. Much like injecting air into a balloon makes it tighter and more rigid, infusing nitrogen into iron's lattice increases its strength and resilience.

Substitutional Alloy

In contrast to interstitial alloys, substitutional alloys are a different breed where the primary metal's atoms are actually replaced by atoms of another element. Imagine a crowd in a stadium, where some people get up and leave their seats, and others come in to sit down; that's the essence of substitution.

The key factor in the formation of a substitutional alloy is the size of the atomic radius. For an atom to replace another in the lattice, their radii must be similar—usually within about 15% of each other. If we take the case of iron, atoms that are too small, like nitrogen, would not typically create substitutional alloys, as they don't fit the ratio and would prefer to occupy the gaps instead.

Atomic Radius

The atomic radius plays a pivotal role in determining how elements will interact within an alloy. It is essentially the measure of the size of an atom, typically represented in picometers (pm).

The concept of atomic radius helps in predicting the type of alloy that will form between elements. It's a bit like trying to decide whether a small new couch will fit in your living room between the other furniture, or if you need to replace an old one. If the new piece is small enough, it fits in the extra space easily, just like interstitial elements do in metal lattices. But if it’s similar in size to your old couch, it needs to take its place, akin to how elements form substitutional alloys.

Alloy Properties

When we talk about the properties of alloys, we're referring to the distinct characteristics they take on as a result of their unique combinations of elements. The nitriding of iron, for instance, imparts several mechanical and chemical benefits.

One major improvement is hardness. By adding nitrogen to the iron, an iron-nitrogen compound forms on the surface, which is much harder than pure iron. This characteristic makes nitrided iron suitable for applications where abrasion resistance is critical. Wear resistance, fatigue life, and corrosion resistance often increase as well. These properties are not just crucial for the performance but also add to the durability, meaning your materials can withstand harsher conditions and last longer. It's akin to adding a protective cover to your smartphone screen—it fortifies the surface, making it more resilient to scratches and impacts.