Assisted by ab initio calculation, an n-body potential is derived for the equilibrium immiscible Cu–Nb system. Based on the potential, molecular dynamics simulations reveal that among the nine Cu∕Nb interfaces stacked by possible combinations of (100), (110), and (111) atomic planes, only the (111)Cu∕(110)Nb and (100)Cu∕(110)Nb interfaces could remain stable up to a temperature of 873K, and that introducing a disordered interlayer at these two interfaces could trigger solid-state amorphization, suggesting that the Cu∕Nb close-packed plane does not serve as a growth barrier, which was frequently the case observed previously in many other systems.