3.1. Hardness measurements
The IFW process provided a joined sample with dissolved γ′ precipitates within the LSHR material (Fig. 1). The dissolved region extended approximately 100 mm into the LSHR material. Large γ′ precipitates within the Mar-M247 are still present up to the interface location. Because we were interested in the evolution of γ′ within LSHRmaterial under IFW conditions, we used the more heat resistant Mar-M247 as the joining material, forcing the majority of IFW deformation and strain to accumulate on the LSHR side. All the measurements reported below were conducted on the LSHR side of the welded specimen. Measured hardness values (HVN) are provided in Fig. 2. The error bars reflect the standard deviation in
the measurements taken at each position distance from the interface. The data shows an increase in hardness near the weld interface. The results agree with previous research which has shown a similar increase in a variety of inertia welded samples [19,20]. Interestingly, the errors associated to each of the hardness values, i.e. their standard deviations, decreased as the distance from the interface increased. Several issues affected this behavior, including the impact of stored energy and dislocation density on hardness. Both of these parameters increase at distances closer to the interface, with the added complexity of recrystallized grains forming very close to the interface. The hardness value errors directly relate to the homogeneity/heterogeneity spatial distribution of the stored work and dislocation density although. Additional
research is needed to fully understand this behavior.
To utilize the strength model,the γ grain size and composition, as well as the γ′ precipitate sizes,compositions,and volume fractions must be measured(or calculated from the processing history). Information for γ′ includes specific chemistry and area fraction values for tertiary,secondary and primary precipitates. Specifics of the strength model have been extensively detailed else where [10].