Investigation of Effect Mechanism of Hafnium and Zirconium in High Temperature Materials

Hafnium and zirconium can promote the formation of (γ+γ′) eutectic,MC(2) carbides,M2SC sulfides and Ni5M phases in superalloys,change script MC and M3B2 into blocky shape and delay the initiation and propagation of cracks by cleaning the free state sulfur at grain boundary and interdendritic regions and strengthening their cohesion.Both of elements Hf and Zr increase the stress rupture strength and ductility of cast superalloys at intermediate temperature and also improve the tensile strength and ductility at room temperature.Hf and Zr strict the precipitation of secondary carbides,such as M23C6 and M6C,therefore stabilize the microstructure of alloys during long-term exposure at elevated temperature.However,Hf and Zr lower the incipient melting temperature of superalloys.The melting of Ni5Hf or Ni5Zr phases may be considered as one of the main factors affecting incipient melting.By means of a pretreatment at 1150℃/8h,Ni5Hf phase can be eliminated in two ways: The reaction Ni5Hf+ γ(C) →MC(2)+ γ or solid solution.Hafnium may narrow down the range between temperature lost the interdendritic capillary feeding action and solidus,as well as may decrease the liquid content necessary for linking the interdendritic pools in the late solidification.The Hf-rich melts in interdendritic zone have superior fluidity,wettability and skin effect.These are the factors in increasing castability and weldability of Hf-containing superalloys and in decreasing tendency of hot tear.The high chemical activity of skin Hf-rich liquid promotes the formation of a thin layer of Hf2O on the surface of castings.Hf and Zr are the melting point depressant in the interlayer alloys used for brazing.It may be considered that dendrites formed in the early solidification are brazed by interdendritic Hfor Zr-rich melts.Eventually,the Ni-18.6Co-4.5Cr-4.7W-25.6Hf and Ni-10Co-8Cr-4W-13Zr alloys were developed according to the composition of Hf-or Zr-rich melts,and the Si-or B-free-bonding for single crystal superalloys could be realized.Studying the Hf-rich melts also leads to the development of unidirectionally solidified lamellar Ni3Al/Ni7Hf2 eutectic.The suitable compositions for these eutectic in situ composites are Ni-5.8Al-32Hf and Ni-4Al-26Hf-8Cr-4W.The results showed that the lamellar Ni3Al/Ni7Hf2 eutectic aligned parallel to the direction of solidification were prepared with temparature gradient G=250℃·cm-1 and solidification rate R=5μm·s-1 for the ternary alloy,and G=350℃·cm-1 and R=1μm·s1 for multicomponent alloy.