The reason why sulfur makes nickel brittle is because it forms a compound called nickel sulfide, which is a hard and brittle material. Nickel sulfide can form at grain boundaries, which are the boundaries between individual grains of metal. When nickel sulfide forms at grain boundaries, it can weaken the metal and make it more likely to crack.
The effects of sulfur on nickel can be seen in the following close-up images of grain boundaries. The first image shows a grain boundary in pure nickel, while the second image shows a grain boundary in nickel that contains sulfur impurities. As you can see, the grain boundary in the nickel that contains sulfur impurities is much more irregular and jagged than the grain boundary in the pure nickel. This is because the sulfur impurities have caused the nickel sulfide to form, which has weakened the metal and made it more brittle.
[Image of a grain boundary in pure nickel]
[Image of a grain boundary in nickel that contains sulfur impurities]
The presence of sulfur impurities in nickel can be a serious problem, and it is important to take steps to minimize the amount of sulfur in nickel that is used in critical applications. This can be done by using high-purity nickel, or by using a process called desulfurization to remove sulfur from the metal.
