Spinodal decomposition, ordering transformation, and discontinuous precipitation in gold-nickel and copper-nickel-tin alloys

Transmission electron microscopy (TEM) is employed to study the microstructure and transformation kinetics of Cu-15Ni-8Sn (wt%) and Cu-7.5Ni-5Sn (wt%) alloys. For the Cu-15Ni-8Sn alloy, it is found that spinodal decomposition occurs during the early stage of decomposition below {dollar}{lcub}sim{rcub}500spcircrm C.{dollar} {dollar}rm DOsb{lcub}22{rcub}{dollar} ordering appeared after spinodal decomposition, and {dollar}rm L1sb2{dollar} ordering appeared after {dollar}rm DOsb{lcub}22{rcub}{dollar} ordering. Discontinuous precipitation took place at the late stage of the decomposition process. Grain boundary/intragranular {dollar}gamma{dollar} {dollar}rm (DOsb3){dollar} precipitates were observed at temperatures higher than {dollar}{lcub}sim{rcub}650spcircrm C;{dollar} A time-temperature-transformation (TTT) diagram was established and five separate C-curves were found. This TTT diagram is very helpful in clarifying the inconsistency in the literature concerning the precipitation process of this alloy. For the Cu-7.5Ni-5Sn alloy, only four kinds of transformation products were observed and no {dollar}rm DOsb{lcub}22{rcub}{dollar} to {dollar}rm L1sb2{dollar} transition was found. The determined TTT diagram provides valuable information for understanding the general features of the precipitation kinetics and for determining the optimum heat treatment procedures.; This work is one of the first experimental studies which report that spinodal decomposition can take place before ordering. It also provides definite evidence for the co-existence of both {dollar}rm DOsb{lcub}22{rcub}{dollar} and {dollar}rm L1sb2{dollar} long-range order (LRO) structures in the Cu-15Ni-8Sn alloy. In addition, three isopleths of the Cu-Ni-Sn ternary phase diagram were constructed.; A systematic study was also performed on Au-40at%Ni and Au-50at%Ni alloys in order to test for the possible existence of transient LRO states within disordered miscibility gaps. An {dollar}rm L1sb0{dollar} LRO phase was found in the Au-50at%Ni alloy when spinodally decomposed specimens were re-annealed at {dollar}{lcub}sim{rcub}490spcircrm C{dollar} on a TEM hot-stage. This observation together with the literature results indicate that transient ordering does exist in the Au-Ni system, although it appears only during reversion processes. This study also found that an {dollar}rm L1sb0{dollar} short-range order (SRO) effect co-existed with spinodal decomposition in specimens annealed at low temperatures ({dollar}{lcub}<{rcub}225spcircrm C).{dollar}; TTT diagrams for the two Au-Ni alloys were constructed. In bulk annealed samples, only discontinuous precipitation and spinodal decomposition were observed.