A method for manufacturing a spiral spring intended to equip a balance wheel of a horological movement, comprising: a) a step of providing a blank with an Nb-Ti core made of an alloy consisting of: - niobium: 100% balance by weight, - titanium: between 5 and 95% by weight, - traces of one or more elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being comprised between 0% and 0.3% by weight, b) a step of beta-quenching said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the beta-phase niobium, c) a step of deforming the blank in several sequences, d) a step of winding in order to form the spiral spring, e) a step of final heat treatment on the spiral spring, - the blank of step a) comprising, around the Nb-Ti core, a layer of X with a material X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr or an alloy of one of these elements, or the method comprises a step of supplying said material X around the Nb-Ti core to form the layer of X, said step being carried out between step a) and step c), the method being characterised in that: - it comprises a heat treatment step for 15 minutes to 100 hours at a temperature comprised between 200°C and 900°C to partially transform said layer of X into a layer of X,Ti intermetals around the Nb-Ti core, the blank thus successively comprising the Nb-Ti core, the layer of X,Ti intermetals and part of the layer of X, said step being carried out between step b) and step c) or between two sequences of the deforming step c), - it comprises a step of removing said part of the layer of X, said step being carried out between step b) and step c), between two sequences of the deforming step c) or between step c) and step d). - the layer of intermetals has a thickness comprised between 20 nm and 10 μm. a) a step of providing a blank with an Nb-Ti core made of an alloy consisting of: - niobium: 100% balance by weight, - titanium: between 5 and 95% by weight, - traces of one or more elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being comprised between 0% and 0.3% by weight, - niobium: 100% balance by weight, - titanium: between 5 and 95% by weight, - traces of one or more elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being comprised between 0% and 0.3% by weight, b) a step of beta-quenching said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the beta-phase niobium, c) a step of deforming the blank in several sequences, d) a step of winding in order to form the spiral spring, e) a step of final heat treatment on the spiral spring, - the blank of step a) comprising, around the Nb-Ti core, a layer of X with a material X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr or an alloy of one of these elements, or the method comprises a step of supplying said material X around the Nb-Ti core to form the layer of X, said step being carried out between step a) and step c), the method being characterised in that: - it comprises a heat treatment step for 15 minutes to 100 hours at a temperature comprised between 200°C and 900°C to partially transform said layer of X into a layer of X,Ti intermetals around the Nb-Ti core, the blank thus successively comprising the Nb-Ti core, the layer of X,Ti intermetals and part of the layer of X, said step being carried out between step b) and step c) or between two sequences of the deforming step c), - it comprises a step of removing said part of the layer of X, said step being carried out between step b) and step c), between two sequences of the deforming step c) or between step c) and step d). - the layer of intermetals has a thickness comprised between 20 nm and 10 μm. - the blank of step a) comprising, around the Nb-Ti core, a layer of X with a material X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr or an alloy of one of these elements, or the method comprises a step of supplying said material X around the Nb-Ti core to form the layer of X, said step being carried out between step a) and step c), the method being characterised in that: - it comprises a heat treatment step for 15 minutes to 100 hours at a temperature comprised between 200°C and 900°C to partially transform said layer of X into a layer of X,Ti intermetals around the Nb-Ti core, the blank thus successively comprising the Nb-Ti core, the layer of X,Ti intermetals and part of the layer of X, said step being carried out between step b) and step c) or between two sequences of the deforming step c), - it comprises a step of removing said part of the layer of X, said step being carried out between step b) and step c), between two sequences of the deforming step c) or between step c) and step d). - the layer of intermetals has a thickness comprised between 20 nm and 10 μm.