Thermodynamic Analysis And Alloy Design Of Co-Al-W Based Superalloys

Compared with Ni-based superalloys, conventional Co-based superalloys exhibit better resistance to hot corrosion and oxidation. However, conventional Co-based superalloys have not so many applications as that of Ni-based superalloys, as their high-temperature strength is obviously lower than that of Ni-based superalloys. It is well known that Ni-based superalloys are strengthened by γ’ precipitating in the disordered fcc matrix phase y, where y’stands for Ni3Al with the L12structure. However, there is no similar γ’ phase found in conventional Co-based superalloys, and the strength of conventional Co-based superalloys depends only on solution strengthening and the precipitation of carbides. Recently a stable ternary γ’-Co3(Al, W) phase with L12structure has been found in a Co-Al-W alloy, which can be stable above950℃. It has been reported that the Co-Al-W alloys have a good high-temperature strength and exhibit an anomalous temperature dependence of flow stress. The development of new-type Co-based superalloys with γ+γ’ two-phase microstructure becomes to be a research focus in the field of high-temperature superalloys.As we know, alloying of superalloys is very complex. In the Co-Al-W based superalloys, Ni can both dissolve in the matrix y phase as a solution stregthening element, and substitute for Co in the γ’ phase to enhance the thermodynamic stability of the γ’ phase. On the other hand, Cr is an effective alloying element in superalloys which can improve the resistance to corrosion and oxidation. Therefore both Ni and Cr are very important alloying elements in the Co-Al-W alloy. Study on the phase equilibria in multi-component alloys of Co-Al-W-Ni-Cr and to establish a thermodynamic description of a multi-component y’phase are the foundation to study the alloying of Co-Al-W alloys. In this work, thermodynamic analyses of all the constituent ternary systems included in the Co-Al-W-Ni-Cr system were carried out by CALPHAD technique, and then a thermodynamic database for Co-Al-W based superalloys was developed. Based on this, the characteristic of alloying in Co-Al-W based superalloys were studied, which can be regarded as useful information for Co-Al-W based superalloys design. The main results are as follows. A thermodynamic analysis of the Co-Ni-Cr system was carried out. The stability of (yCo, Ni) was described well based on the experimental data from800-1300℃and the solubility of Cr up to40at.%in the (yCo, Ni) was analysed by our thermodynamic calculation. The σ phase was modeled by a three-sublattice model (Co, Ni)8(Cr)4(Co, Cr, Ni)18. The results show that the a phase has a high thermodynamic stability in the Co-Ni-Cr system. The large solid solubility of Ni in the σ phase as well as its variation with temperatures were well reproduced. The liquid appeares in the Ni-Cr side at about1400℃in the Co-Ni-Cr system according to the calculation.The Co-Cr-W ternary system was critically assessed. It was found that there is a large solubility of Cr in the (yCo), while the solubility of W in the (yCo) decreases with an increase of Cr content. According to the crystal structure information of μ phase, a three-sublattice model of (Co, Cr, W)7(W)2(Co, Cr, W)4was established. Cr can replace W in the third sublattice strongly which brings about a solubility of Cr in the μ. phase up to50at.%. The σ phase was modeled by (Co, W)8(Cr, W)4(Co, Cr, W)18and W can stablize the a phase efficiently. The model of (Co, W)27(Cr, W)14(Co, Cr, W)12was used to describe a ternary intermetallic compound R phase. The calculation shows that the R phase can be stable between1022and1440℃. The corresponding invariant reactions include:an eutectoid reaction of R(?)μ+(γCo)+σ at1022℃and a peritectic reaction of L+μ+σ(?)R at about1440℃.A thermodynamic analysis of the Co-Al-Cr system was done, with a focus on the A2/B2order-disorder transition and the corresponding two-phase separation within the BCC phase. A modified sub-lattice model was used to discribe the disordered A2phase and the contribution caused by the B2ordering. The calculated order-disorder transition temperatures were in good agreements with the experimental data, which indicates that the thermodyanmic model and model parameters used in this work are reasonable in dividing the Gibbs energy of the BCC phase into the A2part and the contribution of the B2ordering part. The composition curves of the two-phase separation as well as their variations with temperatures were successfully described. It has been proved that it is the Gibbs energy contribution of the B2ordering, which brings spinodal points to the Gibbs energy curve of the BCC phase, that causes the two-phase separation. The calculation also shows that an invariant reaction of CoAl+σ(?)(γCo)+(αCr) occurs at1012℃in the Co-rich region.Based on the thermodynamic database developed in this work, the phase equilibria in the multi-component alloys of Co-Al-W-Ni-Cr system were calculated. The stability of the γ’ phase can be enhanced by the addition of Ni. The homogeneity range of the γ’ phase can be enlarged with the addition of Ni content, which shows that Ni can improve the working temperature of the Co-Al-W based superalloys. The distribution behavior of alloying elements between γ and γ’ phases and their effects on the alloy strengthening were discussed. In Co-Al-W-Ni-Cr alloys, the Al/W ratio has an effect on the distribution coefficient of Ni between γ and γ’ phases as denoted as Kγ’/γNi. Kγ’/γNi has a maximum value of1.24when Al/W ratio is2.60.