A spiral timepiece spring with a two-phase structure, made of a niobium and titanium alloy, and method for manufacturing this spring, including producing a binary alloy containing niobium and titanium, with niobium: the remainder to 100%; titanium between 45.0% and 48.0% by mass of the total, traces of components among O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, of between 0 and 1600 ppm by mass of the total individually, and less than 0.3% by mass combined; applying deformations alternated with heat treatments until a two-phase microstructure is obtained including a solid solution of niobium with β-phase titanium and a solid solution of niobium with α-phase titanium, the α-phase titanium content being greater than 10% by volume, with an elastic limit higher than 1000 MPa, and a modulus of elasticity higher than 60 GPa and less than 80 GPa; wire drawing to obtain wire able to be calendered; calendering or winding.