Abstract
An investigation has been made into the types of product that are produced by the gamma - alpha transformation in pure iron nitrogen alloys, and the resulting structures were classified in terms of product morphology as either irregular ferrite or massive martensite. The effect of these structures, together with that of a tempered massive martensite, on the recovery of transient damping in pure iron and the quench ageing and strain-ageing rates of the iron nitrogen alloys have been investigated and the results compared with parallel tensile experiments. The recovery of transient damping in pure iron can be described by the equation P = exp (-B t[n]), where n varies between 0.15 and 0.45 and the activation energy of the recovery process was found to be between 8.5 and 10.5 Kcal/mol. The mechanism of recovery is tentatively attributed to a rearrangement of dislocations. Quench ageing results show that massive martensite ages at a rate which is three to five times faster than the tempered massive and that the kinetics of ageing of massive martensite can be fitted to a t[2/3] relationship whereas that of tempered massive martensite, can only be fitted to the normal quench ageing time dependent relationship. The difference is ascribed to the reduction in dislocation density due to anihilation during the tempering treatment. Equivalent strain ageing experiments, using a 2.5% pre-strain show a reduction in the difference in ageing rate between the massive martensite and tempered massive martensite and that the kinetics of ageing of both structures can be described by the t[2/3] relationship. Dislocation density calculations show that a quenched massive martensite has a density of 9 x 1011 lines/cm[2] and that the strained massive martensite and tempered massive have densities of 1,4 x 10 [12] lines/cm and 12[2] 1,1 x 10 lines/cm respectively. The activation energy for both quench ageing and strain ageing is 15 Kcals/mol. which is slightly lower than previous values and is possibly due to a small degree of tetragonality produced by the transformation.