镍基合金熔体局域结构的从头算分子动力学研究

发布时间：2019-06-05 20:21

【摘要】：镍基高温合金广泛应用于航空发动机涡轮盘、涡轮叶片以及后机匣等热端部件。这些部件被加工成最终产品前都至少要经历一次凝固过程。熔体作为凝固组织的母态,在本质上影响着形核以及枝晶的生长,从而影响凝固组织的特征,进一步影响零部件的力学性能和使用寿命。正确认识镍基高温合金熔体的结构对发展凝固理论以及调控镍基高温合金凝固组织有着非常明显的积极意义。由于镍基高温合金中除了基体元素Ni以外,仍有多种合金化元素如Al、Ti、Ta、Nb、Cr、Mo、W和Re等,合金化元素之间复杂的相互作用导致镍基高温合金熔体结构的研究与分析相当困难,研究相对简单的纯金属Ni以及二元镍基合金的熔体结构无疑是一种较好的选择。本文采用从头算分子动力学方法研究了纯Ni(辅之以纯Al)和二元合金Ni_(1-x)M_x(M=Al、Ti、Ta、Nb、Cr、Mo、W和Re)熔体的局域结构。主要结论如下:Ni熔体中存在丰富的1551、1541和1431键对,较多的1661、1441键对和少量的1422、1421、1321等键对。Ni熔体中的配位多面体序呈多样性。在Ni熔体中发现了完整的二十面体序,但该类短程序在Ni熔体中并不占主导地位。Ni熔体的FCC和HCP短程序含量极少。随着温度从2123K降低至1473K,Ni熔体的配位数增大,熔体中的1661、1551和1441键对含量增加而其余的键对减少,同时有序程度相对较高的配位多面体序含量增加。与Ni熔体类似,Al熔体中也存在丰富的1551、1541和1431键对,但是Al熔体中1422、1421和1321键对含量相对较多。在各自熔点附近,Al熔体比Ni熔体有序程度低。Al熔体中的配位多面体序也呈多样性。Al熔体中存在少量完整的二十面体序,FCC和HCP短程序极少。在943~1523K温度区间,随着温度升高,Al熔体的配位数基本上线性地减小。随着温度升高,Al熔体中的1661、1651、1551、1541、1441和1431键对的含量逐渐降低,1321、1311、1301、1211和1201键对的含量逐渐增多,熔体变得更加无序。Al熔体的自扩散系数在943~1073K和1073~1523K两个温度区间满足不同的Arrhenius关系,这主要是中心原子周围存在5个和6个15xx+1431键对的配位多面体序含量在1073K附近随温度的非单调或非线性的演变导致的。Ni_(1-x)M_x(M=Al、Ti、Ta、Nb、Mo和W)熔体中Ni-M相互作用强于Ni-Ni与M-M相互作用;Ni1-xCrx熔体中Ni-Ni相互作用强于Ni-Cr相互作用;Ni1-xRex熔体中Re-Re相互作用与Ni-Re相互作用强度接近,强于Ni-Ni相互作用。在Ni_(1-x)M_x熔体中Ni-Ni、Ni-M和M-M三类相互作用的竞争导致熔体中表现出不同的化学序。随着溶质浓度增大至0.25,Ni_(1-x)M_x(M=Al、Ti、Ta和Nb)熔体的Cargill-Spaepen化学序参数ηNi M持续地增大,Ni1-xMox和Ni1-xWx熔体的ηNi M先增大后减小,Ni1-xCrx和Ni1-xRex熔体ηNi M值一直较小。Ni_(1-x)M_x(M=Al、Ti、Ta和Nb)熔体中的溶质原子尽量分散在熔体中以便于形成尽量多的Ni-M键。随着溶质浓度增大至0.25,这些Ni-M键可形成类似闪锌矿结构的Ni-M网络。该网络导致熔体的偏结构因子与总结构因子在低q值区(1.0~2.2?-1)出现预峰。在所研究的八种Ni_(1-x)M_x熔体中都存在丰富的1551、1541和1431键对,以及较多的1661和1441键对。这些熔体中以Ni-Ni和Ni-M为根键的1661、1551和1441键对含量大体上随着溶质浓度的增大而减少。这些熔体中的配位多面体序都呈多样性。这些熔体中存在少量完整的二十面体短程序,极少的FCC和HCP短程序。Ni0.852Al0.148熔体在1723~2073K温度区间发生了液-液结构转变。差热分析实验证实了该熔体的液-液结构转变。当温度从1923K升高至1948K,Ni0.852Al0.148熔体的偏配位数ZAl Ni突然大幅减小,ZAl Al突然大幅增大,化学序参数ηNi Al突然大幅减小。Ni0.852Al0.148熔体的液-液结构转变存在明显的焓变(LL(35)H?578 Jmol-1)和熵变(LL(35)S?0.3 Jmol-1K-1),属于一级相变。随着温度升高,大体上Ni0.87Nb0.13和Ni0.852W0.148熔体中的ZNi Ni和ZNi M连续地减小;ZNb Nb和ZWW连续地增大;化学序参数ηNi Nb和ηNi W连续地减小。在本文所研究的温度区间内,Ni0.87Nb0.13和Ni0.852W0.148熔体中没有发现类似于Ni0.852Al0.148熔体的液-液结构转变。
[Abstract]:The nickel-based high-temperature alloy is widely used in the hot end parts such as the aero-engine turbine disk, the turbine blade and the rear case. These components are subjected to a solidification process at least once before being processed into the final product. As the mother state of the solidified structure, the melt influences the nucleation and the growth of the dendrites, thus affecting the characteristics of the solidified structure, and further affecting the mechanical properties and the service life of the components. The correct understanding of the structure of the nickel-based high-temperature alloy melt has very significant positive significance to the development of the solidification theory and the control of the solidification structure of the nickel-based high-temperature alloy. in addition to that matrix element Ni in the nickel-based high-temperature alloy, a variety of alloying elements, such as Al, Ti, Ta, Nb, Cr, Mo, W and Re, etc., are still present, and the complex interaction between the alloying elements results in a considerable difficulty in the research and analysis of the melt structure of the nickel-based high-temperature alloy, The research of the relatively simple pure metal Ni and the melt structure of the binary nickel-based alloy is no doubt a good choice. The local structure of pure Ni (supported by pure Al) and binary alloy Ni _ (1-x) M _ x (M = Al, Ti, Ta, Nb, Cr, Mo, W and Re) is studied by means of ab initio molecular dynamics. The main conclusions are as follows: There are abundant 1551,1541 and 1431 key pairs in the Ni melt, and more 1661,1441 bond pairs and a small number of key pairs 1422,1421 and 1321. The order of the coordination polyhedra in the Ni melt is diverse. The complete icosahedral order is found in the Ni melt, but the short process is not dominant in the Ni melt. The content of the FCC and HCP short procedures for the Ni melt is very low. With the decrease of the temperature from 2123 K to 1473 K, the coordination number of the Ni melt is increased, the content of 1661,1551 and 1441 in the melt is increased, while the remaining bond pairs decrease while the order degree of the coordination polyhedra with relatively high degree of order is increased. Similar to the Ni melt, there are abundant 1551,1541 and 1431 bond pairs in the Al melt, but the content of 1422,1421 and 1321 in the Al melt is relatively high. In that vicinity of the respective melting point, the Al melt is less ordered than the Ni melt. The order of the coordination polyhedra in the Al melt is also different. There is a small amount of complete icosahedral order in the Al melt, and the FCC and HCP short procedures are very few. The coordination number of the Al melt decreases linearly with the increase of the temperature in the 943-1523K temperature range. As the temperature increases, the content of the bond pairs 1661,1651,1551,1541,1441 and 1431 in the Al melt is gradually reduced, and the content of the key pairs of 1321,1311,1301,1211 and 1201 is gradually increased, and the melt becomes more disordered. The self-diffusion coefficient of the Al melt satisfies the different Arrhenius relationship between 943-1073K and 1073-1523K, which is mainly caused by the non-monotonic or non-linear evolution of the coordination polyhedrin with 5 and 6 15xx + 1431 bond pairs around the central atom in the vicinity of 1073K. The interaction of Ni-M in the melt of Ni _ (1-x) M _ x (M = Al, Ti, Ta, Nb, Mo and W) is stronger than that of Ni-Ni and M-M; the interaction of Ni-Ni in the Ni1-xCrx melt is stronger than that of Ni-Cr; the interaction of Re-Re in the Ni1-xRex melt is close to that of the Ni--Re, and is stronger than that of the Ni--Ni interaction. The interaction of Ni-Ni, Ni-M and M-M in the Ni _ (1-x) M _ x melt results in different chemical sequences in the melt. As the solute concentration is increased to 0.25, the Cargill-Sphaepen chemical sequence parameters of Ni _ (1-x) M _ x (M = Al, Ti, Ta, and Nb) melt continue to increase, and the Ni1-xMox and Ni1-xWx melt have a smaller Ni-M value, and the Ni1-xCrx and Ni1-xRex melt--Ni-M values have been small. The solute atoms in the melt of Ni _ (1-x) M _ x (M = Al, Ti, Ta, and Nb) are dispersed as far as possible in the melt to facilitate the formation of as many Ni-M bonds as possible. These Ni-M bonds may form a wurtzite structure-like Ni-M network as the concentration of the solute increases to 0.25. The network results in a pre-peak between the partial structural factor of the melt and the total structural factor in the low q-value region (1.0-2.2? -1). There are abundant 1551,1541 and 1431 bond pairs in the investigated eight Ni _ (1-x) M _ x melt, as well as more of 1661 and 1441 bond pairs. The content of the bonds 1661,1551, and 1441 of Ni-Ni and Ni-M as the root bonds in these melts is substantially reduced as the concentration of the solute increases. The order of the coordination polyhedra in these melts is diverse. A small number of complete icosahedral short procedures exist in these melts, with minimal FCC and HCP short procedures. The transition of liquid-liquid structure occurred in the temperature range of 1723-2073K by Ni0.82Al0.148 melt. The change of liquid-liquid structure of the melt was confirmed by differential thermal analysis. When the temperature increased from 1923 K to 1948 K, the partial coordination number ZAl Ni of the melt of Ni0.852Al0.148 was abruptly decreased, and the Al-Al of ZAl was suddenly increased, and the chemical sequence parameter, such as Ni-Al, was abruptly decreased. The transition of liquid-liquid structure of Ni0. 852Al0.148 melt has a significant potential change (LL (35) H?578 Jmol-1) and entropy change (LL (35) S? 0.3 Jmol-1K-1), which is a grade-change. As the temperature increases, the ZNiNi and ZNiM in the melt of the general Ni0.87Nb0.13 and Ni0. 852W0.148 melt are continuously reduced; the ZNb and ZWW are continuously increased; the chemical sequence parameters are continuously reduced by the Ni Nb and the ZNiW. The transition of the liquid-liquid structure similar to the melt of Ni0.852Al0.148 was not found in the melt of Ni0.87Nb0.13 and Ni0.852W0.148 in the temperature range studied in this paper.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG132.3;TG111.4

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