Small non-metal atoms (H, C, N, B) fit into interstitial voids of transition metal lattices. Properties: (1) very hard, (2) very high melting points, (3) chemically inert, (4) retain metallic conductivity, (5) often non-stoichiometric (e.g., TiH1.7). Examples: TiC (hardest known carbide), steel (Fe3C, cementite), TiN (golden-yellow coating on drill bits). Alloy formation occurs because transition metals have similar atomic radii, allowing atomic substitution. Important alloys: brass (Cu+Zn), bronze (Cu+Sn), stainless steel (Fe+Cr+Ni), nichrome (Ni+Cr+Fe).