One step synthesis and deification of intermetalllics by Spark Plasma sintering [Sintesi e simultanea densificazione di intermetallici mediante sinterizzazione in corrente pulsata]

Spark Plasma Sintering (SPS) represents a very attractive technique for obtaining a wide variety of advanced materials of interest in the metallurgical field, such as intermetallics and cermet composites, including nanostructured ones. SPS basically consists in the simultaneous application of a pulsed DC and an uniaxial mechanical load through a powder compact (cf. Figure 1). Other than providing rapid Joule heating and likely enhancing mass transport through electromigration, the imposed pulsed high current is also reported to generate a plasma within the voids surrounding the powder particles, thus facilitating the removal of oxides surface layers that may hinder the sintering process. After briefly reviewing the most recent studies obtained by SPS in the metallurgical field, selected results related to the preparation in our laboratory of the two intermetallic systems TiNi and NbAls are presented in this work Specifically, both the chosen examples are related to the use of the SPS technique for obtaining the desired material by simultaneously performing synthesis and consolidation in one-step. Regarding the preparation of dense NiTi by SPS when starting from elemental powders, it is found that product composition is strongly dependent on current intensity and synthesis time. Specifically, increasing the current intensity (cf. Figures 2 and 3) and the synthesis time (cf. Figure 4) results in a decrease in the amounts of the secondary phases, i.e. Ni 3Ti, NiT 1 and Ni 4Ti 3. Moreover, in comparison with other powder metalurgy methods, like conventional sintering and hot-isostatic pressing, it is demonstrated that the SPS technique makes possible the synthesis of relatively pure (only 5.7-7.8 mol% of NiTi 2) and dense (99% relative density) products in a shorter synthesis time (20 min). However, the goal of obtaining single phase NiTi seems difficult to achieve. A further increase in the applied current, beyond the range investigated, i.e. 800-1500 A, leads to the melting of the product during the synthesis with consequent spill of the liquid phase out of the die due to the simultaneous application of the load. Moreover, no significant effect on product purity is observed by further increasing the synthesis time beyond the range investigated, 5-40 min. An analogous investigation is performed for the preparation of NbAl 3 dense products. However, in the case the synthesis and consolidation by SPS of the intermetallic phase is performed starting from activated elemental powders. Specifically, a starting mixture consisting of Nb + 3Al powders is first ball milled for different time intervals (t M = 0-35 h) and then processed by SPS. The effect of the milling time on the composition of SPS products is first investigated, when the pressure is maintained equal to 20 MPa and the dwell temperature set to 600°C. The relatively low temperature and pressure levels adopted were aimed to avoid the possible spill of molten aluminum (660°C melting point) from the die. The obtained results are shown in Figures 5 and 6. In particular, Figure 5 indicates that both Nb and Al peaks broaden with increasing milling time and, within the detection limit of XRD analysis, intermetallic phases are not formed at milling times equal or less than 20 h, while the first evidence of formation of NbAl 3 is observed at t M=35h. Moreover, it is seen from Figure 6 that, the enhanced reactivity of the solid reactants, as a consequence of the performed mechanochemical treatment, leads to an increase of the degree of conversion to the desired intermetallic phase reached during the SPS process as the milling time is augmented. Nevertheless, the best result (t M = 35 h) is charaderized by incomplete conversion and the obtained product was rather porous (about 70% of the theoretical density). Thus, with the aim to increase product conversion, the SPS process was conducted by adopting the peculiar temperature cycle shown in Figure 7. A final product consisting of a single NbAl 3 phase only was obtained, as shown in Figure 8. However, when the entire SPS process is performed using a mechanical pressure equal to 20 MPa, the corresponding end product was still porous (about 88% of the theoretical density). The consolidation level was significantly improved by properly changing the mechanical pressure, which was maintained constant to 20 MPa for 15 min and subsequently increased to 40 MPa during the second stage (10 min) of the SPS process (of. Figure 7). Thus, a near to fully dense (95-100% of the theoretical density) NbAl 3 product was obtained under such conditions. On the basis of the results reported in this work, SPS appears to be a very powerful and versatile tool for the preparation of several products of interest in the metallurgical field. In addition, the use of SPS is supported by the fact that power consumption is about one-third to one-fifth of that of conventional techniques, such as pressureless sintering, hot pressing and hot isostatic pressing.