The Local Structure Of Molten Ni-based Alloy: An Ab Initio Molecular Dynamics Study

Ni-based superalloys are widely used in the heat-resistant parts of aero-turbine engines such as turbine disks, turbine blades and turbine rear casings. They have to undergo liquid to solid transition once or several times before they become the final assemblies. The nucleation and dendrite growth are intrinsically influenced by the structure of the melt,so it is essential to make clear the nature of molten structure for understanding and subsequent controlling the solidification process. Due to the extreme complexity of the molten structure of Ni-based superalloy, efforts on the molten Ni or Ni-M binary alloy are necessary. The present paper focuses on the research of the local structures of molten Ni with molten Al as an auxiliary object and Ni_1-xM_x（M = Al, Ti, Ta, Nb, Cr, Mo, W and Re） by Ab initio molecular dynamics（AIMD） approach. The main findings are as follows.There are abundant 1551, 1541 and 1431 bonded-pairs, a certain amount of 1661 and 1441 bonded-pairs, a small amount of 1422, 1421 and 1321 bonded-pairs in molten Ni. The diversity of the coordination polyhedral short-range orders（SROs） in molten Ni is confirmed. Just a small amount of full icosahedral short-range orders（ISROs） are found in molten Ni. Very few FCC- and HCP-type orders are found in molten Ni. With decreasing temperature from 2123 to 1473 K, the coordination numbers（CNs） of molten Ni increase; the fractions of 1661, 1551 and 1441 bonded-pairs increase and the ones of other bonded-pairs decrease; the fraction of the more ordered coordination polyhedrons increases.There are also abundant 1551, 1541 and 1431 bonded-pairs in molten Al, similar to molten Ni. However, the amounts of 1422, 1421 and 1321 bonded-pairs in molten Al are very large. Molten Al is less ordered than molten Ni near their melting temperatures. The coordination polyhedral orders in molten Al are also diversified. There are only a small amount of full ISROs in it. The amounts of FCC- and HCP-type SROs are very few. With increasing temperature from 943 to 1523 K, the CNs of molten Al decrease nearly linearly. With increasing temperature, the fractions of 1661, 1651, 1551, 1541, 1441 and 1431 bonded-pairs decrease, while the ones of 1321, 1311, 1301, 1211 and 1201 bonded-pairs increase, molten Al becomes less ordered. The non-monotonous or nonlinear evolution for the amount of the specific SROs, with five and six 15xx+1431（sum of 15 xx and 1431） bonded-pairs packing around the central atom, leads the self-diffusion constants to obey different Arrhenius relationships at temperature ranges lower and higher than 1073 K.It has been confirmed that the interaction between the unlike atoms is stronger than the one between the same kind of atoms in molten Ni_1-xM_x（M = Al, Ti, Ta, Nb, Mo and W） through the analysis of the PCFs and the electron densities of states. The interaction between Ni and Ni is stronger than the one between Ni and Cr in molten Ni1-xCrx. In molten Ni1-xRex, the interaction between Re and Re is close to the one between Ni and Re, both of them are stronger than the one between Ni and Ni.The competition of the interaction between the unlike atoms and the one between the same kind of atoms induces the existence of chemical orders in these melts. With increasing the solute concentration from zero to 0.25, the Cargill-Spaepen chemical order parameters, ηNiM, of molten Ni_1-xM_x（M = Al, Ti, Ta and Nb） increase continuously; the ones of molten Ni1-xMox and Ni1-xWx increase first and then decrease. The chemical orders in molten Ni1-xCrx and Ni1-xRex are always very weak, no mater how varied the solute concentration is.Solute atoms try to avoid bonding to each other and prefer dispersing in the melt so as to form Ni-M bonds as many as possible in molten Ni_1-xM_x（M = Al, Ti, Ta and Nb）. With increasing the solute concentration, zinc-blende-structure like Ni-M networks are formed gradually in these melts, which induces the emergence of prepeak in the partial SFs and total SFs at the q value range 1.0².2 ?-1.The bonded-pairs of 1551, 1541 and 1431 are rich in molten Ni_1-xM_x. There are also relatively abundant 1661 and 1441 bonded-pairs in these melts. The fractions of 1661, 1551 and 1441 bonded-pairs with Ni-Ni and Ni-M root-pairs decrease with increasing solute concentration, generally. The polyhedral SROs in molten Ni_1-xM_x are diversified. Just a small amount of full ISROs are found. The fractions of FCC- and HCP-type SROs in these melts are very small.Liquid-liquid structure transition（LLST） in molten Ni0.852Al0.148 is found by AIMD simulation at the temperature range 1723²073K. The LLST is also comfirmed by differential thermal analysis experiment. With increasing temperature from 1923 to 1948 K, ZAlNi, partial CN, decreases suddenly, meanwhile ZAlAl increases suddenly, the chemical order parameter ηNiAl decreases suddenly. The obvious enthalpy change（LLDH ? 578 Jmol-1） and weak entropy change（LLDS ? 0.3 Jmol-1K-1） indicate the LLST of molten Ni0.852Al0.148 being a first order phase transition.With increasing temperature, ZNiNi and ZNiM of molten Ni0.87Nb0.13 and Ni0.852W0.148 decrease, ZNbNb and ZWW increase, ηNiNb and ηNiW decrease, continuously. LLST is not found in molten Ni0.87Nb0.13 and Ni0.852W0.148 at the temperature ranges studied in this paper.