First-principles Study Of Intergranular Embrittlement In Ni-based Alloy

Due to increasing demand for clean, safe and cost-effective energy, the next generation ofinnovative nuclear energy systems known as Generation IV has been proposed as mostpromising energy supply for future energy needs. As the only liquid-fueled reactor selected in thesix Generation IV concepts, Molten Salt Reactor (MSR) has many unique characteristics, whichcan afford great advances. The technology base for MSR was established in Molten Salt ReactorExperiment (MSRE,1965-1969) of Oak Ridge National Laboratory (ORNL, USA). Hastelloy N,a Ni-based alloy developed specially as the structural material for MSR in ORNL, has theexcellent corrosion resistance against molten salts.Unfortunately, MSRE revealed that all the surfaces of Hastelloy N exposed to fuel salt hadintergranular cracks. Strong evidence suggested that tellurium (Te) is the major cause of thisintergranular embrittlement. Te is one of fission products dissolved in fuel salt in MSR. MSREoperated successfully for about4years, and surface cracks of Hastelloy N ranged from150to250μm at grain boundaries (GBs). The Te-induced intergranular embrittlement is irrelevant tocorrosion and irradiation. And it is one of the main problems in the structural materials of MSR,whose mechanism is still unclear.MSRE had found that Nb additions to Hastelloy N (1to2%) are very beneficial in reducingTe-induced intergranular cracking, but the Nb-modified Hastelloy N is still embrittled by Te to aless extent. MSRE had discovered that Ni-based alloy containing23%Cr resisted Te cracking,but it is questionable whether its corrosion rate would be acceptable in molten salt. Themechanisms of the effects of Nb and Cr on the Te-induced intergranular embrittlement are stillunknown. The rare-earths (REs) can be used as alloy additives which are deliberately added intothe Ni-based alloys to improve their performance, and MSRE had added La into Hastelloy N toimprove its resistance of Te attack, but the research on RE additive was inadequate in MSRE.MSRE had found that Te attack can be controlled by controlling the oxidation state of the moltensalt. However, the composition of the fuel salt is very complicated, and changing the oxidationstate of the molten salt may cause other problems.On the other hand, Te-induced intergranular embrittlement is related with irradiationembrittlement which is another major problem of Hastelloy N. MSRE found that Ti additiveappeared very beneficial for Hastelloy N to resist irradiation embrittlement. However, Ti is noteffective in reducing Te attack. Moreover, Ti can negate the beneficial effects of Nb on theresistance of Te attack. However, Nb improves the resistance of Hastelloy N to irradiation embrittlement to a less extent, and this effect of Nb is likely not useful much above650°C.State-of-the-art first-principles calculations with the help of the supercomputer are verypowerful tools to promote material science. First-principles calculations are suitable for theresearch on the structures at the atomic and electronic scales which are critical to understand themechanisms. On account of the research results in MSRE as stated above, in this doctoraldissertation first-principles calculations are employed to research the following subjects:intergranular embrittlement induced by Te in Ni-based alloy, the effect of Nb on the Te-inducedintergranular embrittlement, the effect of Cr on the Te-induced intergranular embrittlement, andthe effect of RE on Ni GB cohesion.In the research of intergranular embrittlement induced by Te, we confirm preferentialsubstitutional occupation of Te in the GB region. Te tends to form strong bonds with theneighboring Ni atoms. However, Te induces the GB expansion due to the mismatch in atom size,thus weakens the interfacial Ni-Ni bonds which are essential to the GB cohesion. The effect ofTe concentration on GB cohesion is also investigated.In the research of the effect of Nb on the Te-induced intergranular embrittlement, thecalculated adsorption energy suggests that Te atoms prefer diffusing along the GB to forming thesurface-reaction layer with Nb on the surface of Ni-based alloy. First-principles tensile testsshow that the Nb segregation can enhance the cohesion of GB. The strong Nb-Ni bonding canprevent the Te migration into the inside of the alloy along the GBs.In the research of the effect of Cr on the Te-induced intergranular embrittlement, thecalculated binding energy suggests that Cr does not have the tendency to segregate to the GB.We find that Cr atoms in certain atom sites of GB form Cr-Cr dimer, which may prevent Te fromdiffusing along the GBs. Only the preliminary research is done, and further studies are needed.In the research of the effects of REs on Ni GB cohesion, the strengthening/embrittlingenergy is calculated to find that all the REs can impair the GB cohesion, according to theRice-Wang model. Furthermore, the strengthening/embrittling energy is decomposed into themechanical and chemical components. The radius and charge of RE in GB, along with the chargedensity distribution, are analyzed to uncover the physical origins of these two components andtheir change tendencies.Furthermore, impurity atoms segregated in the GB can induce embrittlement, which has adirect effect on the mechanical properties of metal materials. P is generally classified as adetrimental impurity in Ni-based alloys which can cause the GB embrittlement. On the otherhand, it has been found that optimal content of P in Ni-based alloys can be very beneficial to the performance of Ni-based alloys. In this doctoral dissertation, first-principles calculations are alsoemployed to investigate the effect of P on Ni GB. We find that P forms strong and covalent-likebonding with nickel, which is beneficial to the GB cohesion. However, a too high phosphoruscontent can result in a thin and fragile zone in the GB, due to the repulsion between phosphorusatoms. Obviously, there exists an optimum concentration for phosphorus segregation, which isconsistent with observed segregation behaviors of phosphorus in the Ni GB.