Study On Microstructures,Properties And Age Hardening Behavior Of Cu-Be-Co-Ni Alloys

The performance of beryllium copper alloy depends much on the micro structure, especially the nature of precipitates. The alloy with better performance can be obtained through the optimizing the precipitation state. This paper systematically investigated the high temperature compressive deformation behavior, micro structure under different single and double stage aging conditions, phase transformation law, fracture toughness, fatigue behavior and age hardening mechanism of Cu-Be-Co-Ni alloy. The main conclusions are summarized as follows:The compressive deformation of Cu-Be-Co-Ni alloy at high temperature was a thermal activation process, which could be divided into work hardening, dynamic recovery and dynamic recrystallization. The peak stress of the alloy increased with the strain rate and decreased with deformation temperature.The precipitation kinetics of soft-state and hard-state Cu-Be-Co-Ni alloy was studied by XRD and electric conductivity method, and the corresponding precipitation kinetics equations were obtained. The precipitation transformation of soft-state alloy during aging at 320? followed the mechanism of homogeneous nucleation, with a long period of incubation. The precipitation transformation of hard-state alloy presented the interface nucleation mechanism and achieved a high phase transformation ratio at the early stage of aging.The fracture toughness of Cu-Be-Co-Ni alloy could be significantly improved by interrupted aging. The aging process was underaging at 320? for 30 min and then followed by interrupted aging at 280? for 360 min. Compared with the alloy subjected to normal peak aging, the ultimate tensile strength of interrupted aging alloy was only 3.3% lower but the uniform elongation and plane stress fracture toughness were increased by 17.1% and 23%, respectively. The interrupted aging also raised the crack initiation energy by 84% and doubled the crack propagation energy of the alloy. When the precipitates with small size distributed densely in the alloy and interacted with dislocations by the precipitate shearing mechanism, the alloy would achieve higher plasticity and fracture toughness.The fatigue crack growth (FCG) behavior of the Cu-Be-Co-Ni alloy was closely related to the microstructures, which could be successfully explained by the model based on the reversible plastic zone (RPZ) size at the crack tip and microstructural factors. At relatively low stress intensity factor range, RPZ size was smaller than the grain size of the alloy, so the fatigue crack propagates by a mechanism that follows the individual deformation mode of the localized area inside the grain and adjacent to the grain boundaries (GBs). The FCG rate was mainly influenced by the precipitates inside the grains and adjacent to the GBs, and decreased with the increase of the volume fraction of shearable precipitates in the alloy. When the stress intensity factor range became higher, the RPZ size increased to 1-2 times of the corresponding grain size. Consequently, the crack trended to grow along the GBs and the features of the GBs became the main factor that influenced the FCG rate. The continuous interfaces between the discontinuous precipitation (DP) cells and parent phase would hinder the propagation of fatigue cracks effectively, resulting in the decrease of FCG rate.The yield stress model of the Cu-Be-Co-Ni alloy was established based on the quantitative analysis results of the precipitates and thus revealed the precipitation strengthening mechanism of the alloy. For the normal peak aging and overaging alloy that just contained non-shearable precipitates, the contribution of precipitation hardening only came from the Orowan mechanism. The double aging and underaging alloy contained both shearable and non-shearable preciptates, the critical transition radius between the two is 1.5 nm. The precpitation hardening was contributed by the combination of precipitate shearing and Orowan mechanism, and the latter played a more important role.The stain hardening contribution from isotropic hardening and kinematic hardening was investigated by the tensile and tension-compression (Bauschinger) behavior of the Cu-Be-Co-Ni alloy. A flow stress model with the integration of yield stress, iso tropic and kinematic hardening behavior was established. The model indicated that the strength and ductility were determined by two contradictory factors:the dislocation dynamic recovery rate and the dislocation storage rate. Optimization of strength and ductility relied on the balance of these two contradictory factors. It was suggested that the alloy containing the mixture of shearble/non-shearable precpitates would achieve the optimization.