Theeffectsof Ti, Hf On The Precipitate-strengthening Co-basedalloy

Co-Al-W superalloywas used as the target materials. the Optical Microscope(OM),Scanning Electron Microscope(SEM), X-ray Diffraction(XRD), Energy Dispersive Spectrum(EDS), Transmission Electron Microscope(TEM), hardness test system were employeed to study the effects of content of Hf and Tion the γ’- strengthend superalloy,specially how they influence the microstructure and mechanical properties.The results showed that:For the Co-Al-W tenary alloy, it was found that the optium W content was between9%-11%, where the hardness of the alloy and the lattice conatant of the precipitate were reasonable, and the mismatch between the matrix and the precipitate was minimum. With the increasing W content, the lattice constant of the matrix without aging notebaly rised.After aging, both of the lattice constants of the matrix and the precipitate increased, and the mismatch between the matrix and precipitate rised remarkably. The volume of the strengthening precipitate in the alloy increased with the increasing W content, based on the quantitative metallurgy analysis, However the size of the precipitate was larger, which may be harmful to the property. The DTA analysis results show that the tehermal stability of the strengthening precipitate degraded when W was over introduced. W was a strengthening precipitate former, the distribution coefficient between matrix/precipitate was approximately 0.6.For the quaternary Co-Al-W-Ti alloy, three mixed strong peaks(111),(200),(220)appeared in the XRD pattern, they are mixed by the matrix and the precipitate. The PFM function in origin software was employeed to differentiate the peaks, therefore the lattice constants of those two phases and the misfit between them was gained. Because of the Ti was introduced in the matrix, the lattice constant of the matrix rised noteably, which led to the left shift of the XRD peaks. The SEM was used to study the microstructure, it demonstrated that the ideal structure and the lattice mismatch appeaed when 3%-4% Ti was added in the alloy. TEM was used to study the microstructure, by the analysis of Selected Area Daffraction Patten(SADP) and EDS, there are some other phases formed in the alloys when the Ti content was over 3%. The DTA results demonstrate that the thermal degradation of the strengthening precipitation when Ti was over introduced. Ti was a γ ’ former, was enriched in precipitate, and Ti enhanced the hardness of the alloys.The alloys was very brittle when Hf was over adeded. Through the analysis of XRD,Hf was enriched in the precipitate, therefore it was beneficial to the pecripitate of the strengthening phases and its stability. However, the lattice mismatch rised remarkably when Hf was over doped. Furthermore, the volume of the strengthening phase droped if Hf was over introduced,the shape precipitate was irregular. But the room temperature hardness rised.Therefore too much Hf was harmful to the alloy, the optium Hf content in the alloy is between 3%-4%.