diffraction in as received conditions. All the intermetallic compounds were cleaned carefully and then milled at high speed for 1 h before the mixing with aluminium powder (it also was high speed milled during 1 h before the blending stage). Fig. 16.2 shows the Nb2Al intermetallic powders before (left column) and after milling (right column). By this way, very hard particles were milled by using high speed milling device (4000 rpm). Results of differential thermal analysis of the Aluminum, Glass Bubbles (GB) powders are shown in Fig. 16.3. In the same Figure images of the powders obtained by SEM are also presented. The critical temperature-transformation points of the different raw materials can be observed in the DTA curves: a high energy transformation is observed for Al powders around 650 C without mass loss, related to the melting point of the aluminum. For Glass Bubbles, it shows some reaction when heating from room temperature to 140 C, with around 5 % of mass addition. This can be related to some chemical reaction in the glass material and must be further investigated. SEM images show aluminum powders with irregular, elongated shape, with average dimensions ranging from 16 to 300 μm. Hollow glass spheres are perfectly rounded, presenting diameters from 2 to 30 μm with very low density (0.20 g/cm3). Figure 16.4 shows the results of quasi-static compression tests for all of the compositions investigated. They contain 30, 40 50 and 60 wt% Nb2Al intermetallic compounds respectively Evolution of the stress values depending on the deformation were compared with different parameters, for example, peak values (stress as MPa) are found very similar for the compositions containing 40, 50 and 60 wt% Nb2Al intermetallics (200–250 MPa) for three compositions but only the specimens called 30Nb2Al (containing 30 wt% Nb2Al) have given much lower values, around 175 MPa. The compositions discussed in Fig. 16.4 do not contain glass bubbles. Other test results carried out on the second series of the composites containing additionally 5 vol% glass bubbles. These two test series are summarized in Fig. 16.5 (because of the small scale of the axes, the compositions here indicated with shortened names, e.g. Nb30 means first series containing 30wt%Nb2Al and Nb30-5 is 30 wt% Nb2Al with 5 vol% glass bubble added, and so on). From the two sets of compression tests, it seems that the role of Glass Bubbles is relevant on the plasticity of the composites and they give better ductility if they are added in the matrix up to 5–10 vol%. Some of the test results not given here have shown that beyond these values (>10 vol% of Glass Bubbles) the effect is a decrease in ductility; the material becomes brittle at the higher percentages of this kind of additive. Figures 16.6, 16.7 and 16.8 shows all the details of the composites designed in this study with interface positions of matrix/reinforcements. The results showed that compacting and sintering at higher levels lead to the transformation of Nb2Al particles to thin layers of Al3Nb. It was also shown that the prolonged milling time to produce Al3Nb intermetallic and the prolonged ball milling procedure for mixing the powders, both, promote the diffusion process at reinforcement/matrix interface. In the same way, Fig. 16.8 indicate special zones taken from different compositions 30Nb2Al,50Nb2Al-5 and 60Nb2Al,and EDX analyses have been performed on the different regions of these special zones. Regarding to the Figs. 16.6, 16.7 and 16.8, Fig. 16.2 Nb2Al intermetallic powders before (left column) and after milling (right column) 16 Design of Al-Nb2Al Composites Through Powder Metallurgy 133
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