13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- The evaluation of internal resistance without the knowledge of the pre-existing internal stress due to the prior high temperature deformation is shown in Fig. 7 (a). The combination of ND measured internal stresses in section 3.2 and the determined internal resistance produces the correct magnitude of the internal resistance, Fig. 7 (b). The internal stresses determined via Rietveld refinement was used to correct the macro-yield strength, read from each bulk stress-strain curve for the corresponding specimens. In general, the internal resistance of the material increased with an increase in the level of the prior strain induced at high temperature. The points with an upper arrow in Fig. 7 (a) and (b) indicates that the {220} grain family did not yield with the applied stress, as shown by specimen 4 in Fig. 4 (b). 4. Discussion and Concluding Remarks Internal stress is a consequence of strain incompatibility between grains which deform differently due to their specific orientation. This deformation, arising from slip on {111} <110> system, is accommodated elastically within the various crystallographic grain families. Internal resistance is a reflection of the material internal microstructure that resists plastic deformation. These two terms have been measured using the present ND technique and the success is attributed to the separation of internal stress and internal resistance. The former can be measured after unloading from high temperature deformation by using a microstructure freezing technique. The latter can be measured with the applied stress to evaluate the flow stress. Mecking and Kocks [15], and Follansbee and Kocks [16] proposed a model and experimental method to measure the internal resistance (called a threshold strength) at temperatures of <300K. However, in this case the presence of the internal stress was not taken into account when the internal resistance was determined. The technique proposed in this paper measures the internal resistance and internal stress at both macro-scale and at the scale of grain families. The latter is very important when providing a crystal plasticity based self-consistent model, see Ref [12]. Using this approach it is found that the internal stress is dependent on the specific crystallographic orientation of each grain family, as shown in Fig. 6. The increase in the magnitude of the internal stress, {200} in tension and {220} in compression, corresponds to the increase in the inhomogeneous strain induced by high temperature deformation, summarised in Table 2. High temperature recovery may play a role in accommodating the strain incompatibility, and lead to a small decrease in the measured internal stress, see {200} grain family for specimens 3 and 4 shown in Fig. 6. This indicates that the creep deformation rate is grain orientation dependent, otherwise the accommodation will not decrease strain incompatibility in a polycrystalline material. The crystallographic orientation dependence of the elastic lattice strain on the applied stress, Fig. 3 (a), is consistent with a previous study by Daymond and Bouchard [17]. The present work specifically explored the influence of the prior deformation at high temperature on the measured elastic lattice strain. It was shown that the {220} grain family no longer yielded after secondary creep deformation, see specimen 4 in Fig. 4 (b). The residual lattice strain, measured after each unloading step of the incremental tensile deformation, revealed a strong influence of creep deformation on the ability of the material to plastically deform in the {220} and {200} grain families, Fig. 5 (a) and (b). Finally, we have described a method to distinguish between the internal stress and internal resistance based on neutron diffraction measurements combined with in-situ incremental deformation. This could be used to validate the threshold strength concept, proposed by Kocks, Mecking and their co-workers [9, 15, 16]. This has been discussed more fully in a review by Kocks and Mecking [18].
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