Intermetallic Compounds in Co-Base Alloys-Phase Stability and Application to Superalloys
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1128-U06-06
Intermetallic Compounds in Co-Base Alloys−Phase Stability and Application to Superalloys K. Ishida Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-02 Aoba-yama, Sendai 980-8579, Japan
ABSTRACT The phase stability of intermetallic compounds of Co3X has been discussed with a focus on the γ’ phase of Co3Al, Co3W and Co3(Al, W) with the L12 structure. The critical temperatures of the γ’ phase of Co3Al, Co3W and Co3(Al, W) compounds are estimated to be about 870 °C , 980 °C and 1076 °C , respectively. The effect of alloying elements on the Co-Al-W -base alloys was found to be very similar to that of Ni-base superalloys, where Ti, Ta, Nb and V are the γ’ stabilizing elements, while Mn, Fe and Cr are the γ forming elements. The mechanical properties of Co-Al-W-base alloys were also found to be similar to those of Ni-base alloys. In particular, the flow strengths of Co-Al-W-base alloys at temperatures above 800 °C were comparable or higher than those of Ni-base superalloys, which implies that the Co-base superalloys strengthened by the γ’ phase have great potential as a new type of high-temperature alloys. INTRODUCTION Nickel-base superalloys have been widely used for high-temperature applications such as aircraft engines, industrial gas turbines, reactors, chemical industry, etc., where the intermetallic compound of Ni3Al with the L12 structure plays a key role in the contribution to the strength at higher temperature [1]. As shown in figure 1 (a) of the phase diagram of the Ni-Al binary system [2], the γ’ phase of the Ni3Al compound appears stably up to 1395 °C , while no Co3Al compound is formed, as shown in figure. 1 (b). As discussed on the GCP (Geometrically Close-Packed) phases which form A3B of Co-base alloys [3], two types of GCP phase have been reported in Co3X : Co3Ti with the L12 structure [4,5] and ordered fcc Co3Ta [6,7]. Although the effect of various alloying elements on the stability and morphology of the γ’ Co3Ti phase has been investigated, the usefulness of the γ’ phase is restricted to temperatures below 750 °C . In the
Figure 1
Phase diagrams of (a) Ni-Al and (b) Co-Al binary systems.
case of Co3Ta, the ordered fcc phase is metastable and ready converts to the hexagonal closed-packed structure of Co3Ta [8,9]. Therefore, GCP phases, Co3X with the L12 structure, do not form easily in Co-base alloys, so other ordered phases such as hcp Co3W and Co3Mo are expected as strengthening phases. On the other hand, CoAl compound with the B2 structure is very stable as well as in the case of NiAl phase as shown in figure 1, which suggests that the enthalpy of formation of the metastable Co3Al γ’ phase is not a small negative value. In fact, Ohtani et al. [10] have recently reported that the enthalpy of formation of metastable Co3Al with the L12 structure is – 19900 J/mol by the first principle calculation. In this paper, the phase stability of intermetallic compounds in Co-base alloys is discussed with a focus on the Co3X phase compared with those of Ni-base
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