Microstructure Evolution in Zn Embrittlement Nickel-Based Single Crystal Superalloys with Different Holding Time
Superalloys are primarilyused for aerospace petrochemical nuclear industry shipbuilding,because of their excellentcorrosion resistivity and mechanical strength.The extensive use of superalloys has accumulated more and more scraps.However,the recovery technology of superalloys craps in China is immature a large number of superalloy scraps are not fully utilized,resulting in huge resource waste and environmental pollution.Therefore,it is urgent to developenergy saving environment-friendly and efficient recovery technology of superalloy scraps to alleviate the crisis of metal mineral resources energy and environment.Thus,the composition of superalloys is becoming increasingly complicated.However,the growing market demand is driving an ever-increasing necessity for Ni-based superalloys for jet and turbine engines.The widespread application of Ni-based superalloys has led to the generation of large quantities of scrap containing both processing scrap and waste products.At present,there is no method for appropriately reusing superalloy scraps,which mostly accumulate or are sold with foundational components.Therefore,it is necessary to develop a simple method for recovering precious metals from superalloy scrap.Although superalloys can be dissolved by chemical and electrochemical methods,the very small contact area of block superalloy scrap and the formation of a dense passivation film results in an extremely low dissolution efficiency.This in turn makes the dissolution of the blocksuperalloy scrap an extremely time and energy-consuming process.Breaking down superalloy scrap into small pieces is one of the simplest,and most efficient ways to increase the contact surface area.However,the high strength and toughness of superalloy scrap make crushing difficult.The first challenge is therefore superalloy embrittlement.Compared with superalloy dissolution,there has been a lack of research on embrittlement.Therefore,it is necessary to systematically study the impact of environmental changes on the embrittlement process and microstructural evolution reveal their inherent,relationship,analyze the embrittlement mechanism of superalloys in molten Zn and provide a theoretical basis for the industrial application of this technology.The study showed that active metals such as Ca,Mg,Al and Zn,could react with the matrix element nickel in superalloys,potentially destroying the microstructure and causing phase transformations that lead to embrittlement.However,the boiling points of Ca,Mg and Al were higher than that of Zn,and the phase transition temperature for the embrittlement of superalloys was also higher.Furthermore,it was found that the vapor pressure of Zn and other main components of superalloys(Ni,Al,Co,Cr,Ta and Re)varied as a function of temperature.The vapor pressure of Zn at its boiling point of 1180 K(907 C)was about 7 orders of magnitude higher than that of other alloy elements in superalloys.Therefore,Zn-Ni alloys could be easily distilled and selectively removed by heating at around 1180K(907 ℃).Additionally,zinc oxidation could be prevented when distillation was carried out under an inert atmosphere,and Zn could be recycled after Ni extraction.High-purity Ni could be obtained without generating toxic waste or waste liquids by analyzing these characteristics of Zn.Therefore,using Zn as the reaction agent for Ni in superalloys ensured that the reaction between Zn and Ni in the superalloy destroyed the original superalloy microstructure and effectively mitigates the loss of metal elements caused by the reaction of Zn with other metals in the superalloy,resulting in remarkable operability and economy.Currently,superalloy scraps are mainly recovered by hydrometallurgy processes.However,the excellent corrosion resistance of the superalloy made it easy to form passivation film during the chemical or electrochemical dissolution process,which hindered the dissolution of superalloy and reduced the dissolution rate of superalloy.In addition,the high hardness and high mechanical strength of the superalloy made it difficult to crush the superalloy.Hence,the contact area with the leaching solution was small,and the dissolution rate was low,which seriously affected the recovery efficiency of the superalloy.Therefore,how to efficiently and quickly dissolve superalloys had become one of the key processes for recycling superalloy scraps in the hydrometallurgy process.This work focused on the grinding waste of nickel based single crystal high-temperature alloy(DD5)with a reaction temperature of 900 ℃.The Ni-based single crystal high-temperature alloy was reacted with Zn for 2,4,8 and 10 h using vacuum sealing technology,respectively.The microstructure and phase composition of the reaction samples were observed through scanning electron microscopy(SEM),X-ray diffraction(XRD)and electronprobe microanalysis(EPMA),elucidating the process of tissue evolution.The results showed that Zn reacted with Ni in the superalloy to form Ni5Zn21 intermetallic compound,and also formed part of Zn(Ni)solid solution.With the extension of reaction time,Zn(Ni)solid solution transformed into more Ni5Zn21 intermetallic compound.The study also found that W,Re and Ta enriched and formed WRexTax compound after the superalloy was treated with Zn.In addition,the results also showed that Ni,Al and Co,Ni,Co and Cr formed NiAlxCoy and NiCoCr intermetallic alloys respectively in a shortreaction time,but when the reaction time was long enough NiAlxCoy decreased significantly,and disappeared gradually while NiCoCr gradually transformed into CoCr.
Ni-based single crystal superalloyorganizational evolutionholding timeZn embrittlementelement distribution
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