Ti纳米粒子熔化与凝结的原子尺度模拟

Simulations of Ti nanoparticles upon heating and cooling on an atomic scale

本文采用基于嵌入原子势的分子动力学方法模拟研究了不同尺寸Ti纳米粒子在熔化与凝结过程中的原子堆积结构变化.温度变化过程中对Ti纳米粒子中原子平均能量、对分布函数、键对和比热容的计算结果表明,粒子尺寸和温度变化方式对粒子的结构转变具有重要影响.小尺寸Ti粒子更易于形成二十面体构型.随着Ti纳米粒子粒径的增大,室温下粒子趋于保持初始密排六方的堆积结构.升温过程中,大粒径的纳米粒子内出现HCP向BCC的部分结构转变,导致HCP和BCC结构共存现象.大粒径粒子的熔化与体相材料相似,具有一个熔化温度.熔融粒子降温时,纳米粒子内部原子发生熔融态→BCC→HCP堆积结构的转变,且凝结温度较熔化温度滞后.该原子尺度的模拟提供了可用经典理论估算Ti粒子熔化所需能量的临界尺寸.

Titanium(Ti) has many advantages including high specific strength, low density, and biocompatibility, and is an excellent option for biomedical implant applications. Traditionally manufacturing processes have great difficulties in processing the hexagonal a-Ti with complex geometries, which would be transformed into the BCC b-Ti at high temperatures. Additive manufacturing(AM) or metal three-dimensional(3D) printing has made it possible to accurately fabricate Ti products with complex morphology. As nanoparticles have been used in the AM processing, an interesting issue arises naturally to understand packing changes of Ti particles with nanometer size during heating and cooling. The information provides the possibility in understanding the processing-structure-property-performance relations in the AM processes with the intent of producing the desirable microstructural features, and thus achieving the mechanical properties comparable or even superior to the conventionally manufactured parts. Because of lacking appropriate experimental techniques, computational approach becomes a good option to obtain various static and dynamic properties of metals reliably, in bulk or surface configurations. On a nanoscale, as the number of atoms in one particle increases, the computational cost increases exponentially and the data complexity increases correspondingly. Molecular dynamics(MD) simulation is a well-established technique to characterize microscopic details in these systems involving combined behaviors of atom movements and locally structural rearrangements. In this paper we conduct the simulations within the framework of embedded atom method provided by Pasianot et al. to study packing transformations of Ti nanoparticles upon heating and cooling on an atomic scale. Based on the calculation of the potential energy per atom, pair distribution function, pair analysis, and the specific heat capacity, the results show that the particle size and temperature changes play key roles in the packing transformations. Small size particles preferentially form icosahedral geometries. As the particle size increases, particles can hold their HCP packing at room temperature. Upon heating, the structural transformation from HCP to BCC occurs in these large size particles,and there coexist the HCP structure and the BCC structure. At a high temperature, these particles present the melting behavior similar to that of the bulk phase. When the molten particles are cooled, the atoms in the particles undergo melting-BCC-HCP structural transition, and the freezing temperature lags behind the melting temperature. The simulations provide an estimate of the critical size, and are applicable to classical theory for melting the Ti particles.