Final answer:
The pair most likely to form an interstitial alloy is Titanium and Copper, as interstitial alloys occur when smaller atoms fit into the spaces between larger metal atoms, and Copper's smaller atomic radius can potentially fit into the interstices of Titanium's crystal structure.
Step-by-step explanation:
To determine which pair of elements is most likely to create an interstitial alloy, we need to consider the size difference between the atoms and the possibility of smaller atoms fitting into the interstices (spaces) between larger metal atoms in the crystal structure.
Among the options provided, Titanium (Ti) and Copper (Cu) is most likely to form an interstitial alloy. Titanium has a relatively large atomic radius and is often used as the host metal, while smaller non-metal atoms such as carbon or nitrogen—and in some cases smaller metal atoms—can occupy the interstitial sites. Copper, while not a small non-metal, has a smaller atomic radius compared to titanium and can, under specific conditions, diffuse into the interstices of titanium's crystalline structure. This cannot be easily achieved with metals of similar sizes or larger atomic radii.
Aluminum (Al) and Lead (Pb), Silver (Ag) and Tin (Sn), and Magnesium (Mg) and Calcium (Ca) are not suitable pairs for interstitial alloys as they typically form substitutional alloys, where the atoms replace each other in the crystal structure due to their similar atomic sizes.