Abstract

The microstructural parameters and high temperature properties of single crystal (SC) superalloys containing Re, e.g., PWA1484, were examined by our alloy design computer programme.This examination suggested that the lattice misfit change by Readdition is the major factor of the improvement in creep rupture strength in these alloys. The strongest single crystal superalloys available in the Ni-Co-Cr-Mo-W-Al-Ti-Nb-Ta-Hf-Re multi component system were searched for by the same computer programme. Alloys with high Mo and Ta concentrations, having large negative lattice misfit values, were calculated to have highest creep rupture strengths. The Re addition was not necessarily needed to design such ultra-high strength alloys. It was concluded that the control of lattice misfit is of vital importance in designing ultra-high strength SC superalloys.

1. Introduction

Alloy Design Computer Programme

Table 1. Regression equation for calculating creep rupture life at 1040 deg.C and 137MPa.

Results of Calculation

Table 3. Keys for the computer output shown in Table 2.

Fig.3 Relationship between calculated and experimental creep rupture lives.

l040 deg.C. These calculations agreed very well with experimental data as shown in Fig. 3.
It has been believed that the excellent temperature capabilities of Re containing superalloys should be attributed to the solid solution strengthening effect of Re in the gamma phase[7]. A micro-clustering of Re in the gamma phase has also been suspected to be the important factor of the improvement[9]. However our calculations described above clearly show that the lattice misfit change caused by Re addition is the major factor of the improvement.
Another calculation was made for a series of alloys having a variety in chemical concentrations in Mo, W and Ta, although concentrations of Cr, Al, Ti and Re were kept constant[10]. As also shown in Fig. 3, calculated creep rupture lives agreed very well with reported experimental data; the error is within a factor of 2. It was also found by the calculation that the strongest alloys, TUT95, TUT92C and TUT31D, had suitable lattice misfit values(0.05 to 0.16% at room temperature) and this is the main reason for the highest creep rupture strengths among the 10 alloys.
Thus the lattice misfit is found to be playing a predominant role on the strengthening of the practical and experimental single crystal superalloys. The effect of solid solution strengthening by refractory elements is also an important factor as seen in the coefficients in the regression equation in Table 1.

Fig.4 Effect of lattice misfit on the temperature capability of SC superalloys.

However, when we compare alloys fully solid solution strengthened by the refractory elements, difference in the solid solution strengthening level is not so large compared with the difference in the lattice misfit which changes significantly depending upon the combination and balance of the alloying elements.
In fact, not only Re but also Cr, Mo and W are preferentially partitioned to gamma phase to shift the lattice misfit towards negative direction. This means Re is not necessarily needed to design ultra-high strength SC superalloys. This has in fact already been verified by our previous work on TMS-63 and TMS-64[5]. Containing no Re, their temperature capabilities are higher than those of PWA1484 and CMSX-4 by 20 to 30 deg.C since they are automatically designed through the alloy design computer programme to have large negative lattice misfit values at high temperatures. Our recent X-ray measurement up to 1200 deg.C supported our calculation; the lattice misfit in TMS-63 was measured to be larger negative than that of CMSX-4 at high temperatures.
In the series of calculations described above, it was confirmed that our alloy design computer programme gives a very good estimation in both microstructure and high temperature properties. Using this programme, the strongest single crystal superalloys available in the whole gamma/gamma-prime region in Ni-Co-Cr-Mo-W-Al-Ti-Nb-Ta-Hf-Re multi component system were search for. The results of calculation under several selection conditions are summarised in Fig.4.
It is suggested in this figure that the temperature capability can be improved significantly by changing the lattice misfit towards larger negative direction, although there is a limitation in practice. If the lattice misfit is too large, the coherency is easily broken down during heat treatment, even during cooling after solutioning, and creep rupture strength is decreased[6]. Another problem which we found in alloys with too large lattice misfit is a discontinuous coarsening of gamma-prime during creep. We believe this structural change is driven by a large coherent or semi-coherent strain energy at the gamma/gamma-prime interface. Taking these into consideration, the limitation should be set for the lattice misfit at around -0.2S at room temperature. Thus the temperature capability of the strongest alloy with 5% Cr is 1100 deg.C which is higher than that of PWA1484 by 50 deg.C. Reducing Cr below 5S might also be worth considering, since our calculation predicts that it may be possible, for instance, to increase the temperature capability up to 1130C with alloys containing no Cr; this is higher than that of PWA1484 by 80 deg.C.

Conclusions

Acknowledgment

References