The Mechanism of Hardening in Nickel-Silver by Low Temperature Annealing

Abstract

Cu-Ni-Zn alloy shows an anomalous increase in hardness by low temperature annealing: In this investigation, the structure of the anomaly was discussed in detail using a 18 wt%Ni alloy. Changes in hardness, elastic modulus and internal friction were measured at room temperature after water-quenching from several temperatures during isochronal and isothermal annealing. (1) The hardness of a specimen furnace-cooled from 700°C was greater than that of a specimen water-quenched from the same temperature. Both of the specimens showed a maximum hardness at about 300°C and it decreased gradualy with further increase of temperature during isochronal annealing. (2) During isothermal annealing at 300°C, the hardness of the water-quenched and cold-worked specimens increased with the annealing time, and became greater with the degree of cold-work, the overall change in hardness. Result (1) sugests the hardening by low temperature annealing to be due to short range ordering; result (2), however, could not be intepreted only by a simple sum of the short range order-hardening and the work-hardening, but it meant that another type hardening mechanism promoted by increasing of the dislocation density existed in this alloy. (3) The change in elastic modulus was nearly the same as that in hardness shown in (2). Activation energies calculated from the change of elastic modulus in the cold-worked and water-quenched specimens were 22∼26 kcal/mol and 17 kcal/mol, respectively. The results of the quenched specimen showed that the short range ordering increased the elastic modulus and was improved with quenched-in excess vacancies. The results of the cold-worked specimen, however, could not be interpreted only by the short range ordering, but it might be reasonably conceived that the strengthening of interaction between high density dislocations and solute atoms would increase the elastic modulus and hardness. (4) The strain-amplitude independent internal friction of the quenched specimen showed a minimum value at about 350°C during isochronal annealing indicating the strongest pinning effect of dislocations at this temperature. The interaction between dislocations and solute atoms was known to serve as the pinnig effect.
Therefore, the authers conclud that the mechanism of hardening in the cold-rolled nickel-silver containing about 18 wt%Ni by low temperature annealing depends not only on the short range ordering but also on the interaction between dislocations and solute atoms, i.e., the segregation of solute atoms into dislocation atmosphere.