Conclusions and recommendations Chapter Conclusions and recommendations 7.1 Conclusions Physical, thermal and mechanical properties of nanostructured Mg-5Al-1AlN composite are studied in comparison with nanostructured pure Mg which is synthesized and tested with the same parameters as the composite. The Mg composite and pure Mg consolidated from MMed powders inherently contain processing defects. Combined effect of grain refinement, solid solution, reinforcement and in-situ formed second phase particles result in very complex microstructure and consequently very different mechanical properties at different milling durations. Based on the experimental results, some conclusions can be drawn. 1. Similar behavior of overall tensile deformation is observed in both material systems despite additional alloying and reinforcement in the composite. The only difference is that the composite samples possess higher strength due to additional solid solution strengthening and particle dispersion strengthening. 2. Short milling durations such as 10h and 20h produce microstructures with higher yield stress compared to those of the as-blended samples. Grain refinement, solid solution and particle dispersion strengthening resulted from the interactions between: (i) dislocations and grain boundaries and (ii) dislocations and particles are responsible for the strength enhancement in these composite samples. 3. Decline in yield stress but enhanced ductility with strain softening behaviors are found from those samples after 20h-MM. Extremely high ductility of 34% 155 Conclusions and recommendations with yield stress of 205 MPa is observed from the composite sample with grain size of about 33 nm after 40h-MM. Similar results of 33% ductility and 210 MPa yield stress are produced for 40h-MMed pure Mg samples. This finding implies that the grain refinement to a certain length scale diminishes the significance of solid solution strengthening and dispersion hardening. 30hMMed composite exhibited the best combination of strength and ductility. 4. Time dependence of mechanical properties of both material systems is verified by tensile tests at three different strain rates, 3.33x10-5 s-1, 3.33x10-4 s-1, 3.33x10-3 s-1. At room temperature, strain rate sensitivity factors of 0.087 and 0.106, and unusually small apparent activation volume of 41b3 and 34b3 are observed from the 40h-MMed composite and the 40h-MMed pure Mg samples respectively. 5. Apparent activation energy of 50 KJ mol-1 which is close to the activation energy for grain boundary diffusion in Mg has been estimated from creep curves of 120 MPa constant applied stress on the 40h-MMed composite samples at different temperatures. It implies that grain boundary deformation process is dominant in the nanoscaled grain size region. 6. Investigation of grain boundary deformation process in the nanostructured composite through constant stress creep tests at various temperatures less than 0.35 Tm show stress exponent n of and activation energy Q of 61 KJ mol-1. 7. Coble creep alone is not sufficient to describe the creep behavior of the present nano-composite samples. In addition to the AlN reinforcement, processing defects such as impurities and excess free volume render the grain boundaries imperfect making the experimental results deviate from theoretical prediction. Although creep deformation parameters (n and Q) favor grain boundary sliding 156 Conclusions and recommendations as dominant deformation mode, the co-existence of Coble creep is evident with two sets of competitive results which are quite close to each other especially at higher stress level. 7.2 Recommendations for future investigation In general, in order to understand the nature and origin of deformation mechanisms at high temperatures, information related to the dependence of strain rate on stress, temperature, and grain size is essential. In the case of nc materials, this type of information may be obtained by (a) conducting creep tests over a wide range of strain rates (at least five orders of magnitude) and at several temperatures to identify unambiguously the values of the stress exponent, n, and the activation energy for creep, Q, (b) adopting an effective method that would produce nc materials with different grain sizes and then performing systematic tests under identical conditions of temperature and stress for the purpose of determining the values of the grain size sensitivity, and (c) performing experiments that involve large strains to ensure the presence of steady-state creep. In addition, in order to avoid complications that may be caused by the presence of different alloying elements and impurities, it is desirable to perform such tests on pure nc metals. However, caution must be exercised in adopting such an approach due to the occurrence of significant grain growth in pure nc metals at elevated temperatures. From the experimental results in the present research, various milling durations produced different mechanical behaviors. It can be observed that quite extreme 157 Conclusions and recommendations combinations of tensile properties such as high strength and low ductility to low strength and high ductility have been obtained by varying the milling time, in other words, varying grain sizes. Due to technological restrictions and the pyrophoric nature of Mg, it is a tedious task to carry out microstructural study using TEM. The following tasks are recommended for the analysis on the influence of grain boundary, reinforcement and second phase particles, dislocation, processing history, structural flaws to comprehend the deformation behaviors of Mg composite. 1. Both pure Mg and composite samples showed similar deformation behaviors. Further creep tests and TEM microstructural examination can be performed on the pure Mg samples to confirm the diminishing effect of reinforcements (insitu formed second phase particles and/or ex-situ reinforcement particles) when the grain size becomes very small in nanometer range. 2. Grain boundary activities played an important role in the deformation process of nanostructured Mg composite. In-depth analysis of the nature of grain boundary, types of impurities, excess free volume should be carried out to study their influence on the grain boundary diffusivity. 3. For convincing microstructural evidence, an attempt should be made to facilitate the in-situ observation of grain boundary activities and its interaction with other structural defects and particles during deformation process. 4. Combined positron-lifetime and Fe-tracer diffusion measurement could provide the correlation between free volumes and atomic diffusion. The variation of the free volume pattern and the diffusion behavior with different milling durations can verify the role of excess volume at the grain boundaries in grain boundary diffusion and deformation process. 158 . nanostructured pure Mg which is synthesized and tested with the same parameters as the composite. The Mg composite and pure Mg consolidated from MMed powders inherently contain processing defects. Combined. Conclusions and recommendations 155 Chapter 7 Conclusions and recommendations 7. 1 Conclusions Physical, thermal and mechanical properties of nanostructured Mg- 5Al- 1AlN composite are studied in. tests and TEM microstructural examination can be performed on the pure Mg samples to confirm the diminishing effect of reinforcements (in- situ formed second phase particles and/ or ex -situ reinforcement