Method for manufacturing a balance spring for a horological movement

A method for manufacturing a balance spring intended to equip a balance of a horological movement, comprising:

• producing a blank made of a niobium and zirconium alloy containing:
• niobium:
• the remainder to 100 wt %,
• zirconium:
• between 10 and 30 wt %,
• impurities, the total percentage whereof lies in the range 0 to 0.5 wt %;
• annealing and cooling the blank;
• depositing a layer of a ductile material on the blank;
• deforming the annealed and cooled blank having the layer of ductile material to form a wire;
• winding the wire to obtain the balance spring;
• and
• heat treating the balance spring;
• wherein the ductile material is selected from the group consisting of copper, nickel, cupronickel, cupro-manganese, gold, silver, nickel-phosphorus Ni—P and nickel-boron Ni—B,
• a ratio of the area of ductile material to the area of the niobium and zirconium alloy for a given wire cross-section is less than 1, and
• annealing and cooling the blank comprises a dissolving treatment, with a duration that lies in the range 5 minutes to 2 hours at a temperature that lies in the range 700° C. to 1,500° C. in a vacuum, followed by quenching, in a gas or by natural cooling in a vacuum to obtain a supersaturated solid solution of Zr in Nb.