Method for manufacturing a balance spring for a timepiece movement

A method for manufacturing a balance spring intended to be fitted to a balance of a timepiece movement, comprising:

• a step of creating a blank from a niobium and titanium alloy containing:
• niobium:
• the remainder to 100 wt %,
• titanium:
• between 40 and 60 wt %,
• traces of elements selected from the group formed of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount formed by all of said elements being comprised between 0 and 0.3 wt %,
• a step of n-quenching said blank with a given diameter, such that the titanium of said alloy is essentially in solid solution form with β-phase niobium, the α-phase titanium content being less than or equal to 5% by volume,
• at least one deformation step of said alloy alternated with at least one heat treatment step such that the niobium and titanium alloy obtained has an elastic limit higher than or equal to 600 MPa and a modulus of elasticity lower than or equal to 100 GPa, a winding step to form the balance spring being performed prior to the final heat treatment step, wherein, prior to the deformation step, the method comprises a step of depositing, on the alloy blank, a surface layer of a ductile material chosen from the group including copper, nickel, cupronickel, cupro manganese, gold, silver, nickel phosphorus NiP and nickel boron NiB, to facilitate the wire shaping process, the thickness of the deposited ductile material layer is chosen such that the ratio of the area of ductile material to the area of NbTi alloy for a given cross-section of wire is less than 1.