| Low thermal expansion(LTE) superalloys can be used to make turbine cases,seals, rings and shrouds to enhance the efficiency of Gas turbine engines.By using LTE superalloys,the clearances between turbine or compressor blade tips and outer seals can be maintained tight over a long range of turbine operating temperatures,and thus fuel consumption can be reduced.To meet the requirements of high efficiency engines,it is necessary to investigate the alloy compositions and to develop a new kind of LTE superalloys with excellent resistance to oxidation.The effects of different alloying elements on the thermal expansion of superalloys are computed by multiple regression method.The results indicated that the additions of W,Mo, Cr,Al,Ti,Nb can increase the thermal expansion coefficients of the alloys.The additions of Co or Ni can reduce the coefficients,and Co is stronger than Ni in reducing the coefficients.Several alloys of different compositions are used to assess the credibility of this method.It's shown that the errors between the computed coefficients and measured ones are small.Several experimental superalloys are designed through the analysis of the current alloys and the calculated effecting coefficients.In consideration of the properties of this kind of alloys and the problems existed,emphasis is put on the thermal expansion properties and oxidation resistance in this investigation.Alloy 1# and 2# have the lowest coefficients,with the coefficients of 20℃～400℃are 11.08×10-6/℃and 12.3×10-6/℃respectively.The element Mo is found to have complex effects on the thermal expansion coefficients,and it's better to study this problem related to its existing form in the alloys.The compositons and structures of the cross section and the surface of the oxide scales were studied.The oxidation results indicate that all of the four alloys have excellent oxidation resistance at 850℃.The oxidation resistance of alloy 1# and alloy 2# is increased by the addition of Al,and that of alloy 3# and alloy 4# is increased by the addition of Cr. The oxide scales are usually consisted of three layers.The compositions of the middle layer are mainly the elements to be selectively oxidized(Cr,Al),but the inner and the outer layer are mainly the matrix elements(Fe,Co,Ni).The study of alloy 3# and alloy 4# indicates that the element Mo is detrimental to the forming of continuous protective scales.Both alloy 1# and alloy 2# have to precipitates,γ′andβ.The quantity ofβprecipitates in alloy 2# is larger than that in alloy 1#.Bothβandγ′precipitates have two different forms in the two alloys.The different microstructure features resulted in that the strength of alloy 1# is inferior to that of alloy 2# at room temperature,and equivalent to that of alloy 2# at 650℃,while the rupture life of alloy 1# is longer than that of alloy 2#.Hot Formability tests are conducted on alloy 1#.It's shown that the alloy has excellent hot formability,with the largest deformation about 60%achieved between 900℃and 1100℃.The study on the recrystallization structures indicates that optimum grain size can be obtained when the deformation temperature is between 900℃and 1050℃,within the deformation extent of 30%to 40%. |
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