Polycrystalline metal whiskers are made by feeding carbonyl of ferromagnetic metals (such as Fe, Co or Ni) into an oxygen free decomposition chamber so as to give 10-4 to 10-10 moles per c.c. of the chamber, against a falling temperature gradient, the metal atoms produced forming chain like aggregates under the influence of a magnetic field. Further carbonyl vapours of ferro- or paramagnetic (such as Mo or W) metals are then fed through the chamber with the temperature of the chains being kept sufficient for coherent deposition of the metals until the whiskers are of the desired thickness. The carbonyl supplied to the vapourizer may be liquid or as a solid carried in an inert solvent. Non metals or metal compounds may be included in the carbonyl vapour for deposition on the chains simultaneously with the metal. Oxidation, carburization or nitration of the metals can be carried out simultaneously with the deposition of the metal or in between successive layers thereof. The fibres produced by the method of the invention may be cold rolled, cold pressed or cold welded and sintered.