Method for manufacturing a spiral timepiece spring, characterized in that the following steps are implemented in succession: - producing a blank from a binary alloy containing niobium and titanium, and which contains: - niobium: the remainder to 100%; - a proportion by mass of titanium greater than or equal to 60.0% of the total and less than or equal to 85.0% of the total, - traces of other components from among O H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said trace components being comprised between 0 and 1600 ppm by mass of the total, and the sum of said traces being less than or equal to 0.3% by mass; - performing a treatment cycle including a prior beta quenching treatment at a given diameter, such that the entire structure of the alloy is beta, then applying to said alloy a succession of the pairs of deformation/precipitation heat treatment sequences, comprising the application of deformations alternating with heat treatments until a two-phase microstructure is obtained comprising 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 or equal to 1000 MPa, and a modulus of elasticity higher than 60 GPa and less than or equal to 80 GPa; - wire drawing to obtain a wire of round cross-section, and flat rolling compatible with the entry cross-section of a roller press or of a winder arbor or with insertion in a ring; - forming coils in the shape of a treble clef to form a mainspring prior to its first winding, or winding to form a balance spring, or insertion in a ring and heat treatment to form a mainspring. - producing a blank from a binary alloy containing niobium and titanium, and which contains: - niobium: the remainder to 100%; - a proportion by mass of titanium greater than or equal to 60.0% of the total and less than or equal to 85.0% of the total, - niobium: the remainder to 100%; - a proportion by mass of titanium greater than or equal to 60.0% of the total and less than or equal to 85.0% of the total, - traces of other components from among O H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said trace components being comprised between 0 and 1600 ppm by mass of the total, and the sum of said traces being less than or equal to 0.3% by mass; - performing a treatment cycle including a prior beta quenching treatment at a given diameter, such that the entire structure of the alloy is beta, then applying to said alloy a succession of the pairs of deformation/precipitation heat treatment sequences, comprising the application of deformations alternating with heat treatments until a two-phase microstructure is obtained comprising 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 or equal to 1000 MPa, and a modulus of elasticity higher than 60 GPa and less than or equal to 80 GPa; - wire drawing to obtain a wire of round cross-section, and flat rolling compatible with the entry cross-section of a roller press or of a winder arbor or with insertion in a ring; - forming coils in the shape of a treble clef to form a mainspring prior to its first winding, or winding to form a balance spring, or insertion in a ring and heat treatment to form a mainspring.