Crystallization mechanism of Zr₅₅Cu₃₀Al₁₀Ni₅ metallic glass in an extended range of heating rates

Highlights

• The crystallization of Zr55 BMGs during non-isothermal heating process is analyzed.

• The crystallization morphologies are different after slow and rapid heating.

• The numerical model is established to describe the fast crystallization during SLM.

• The activation energy of dendritic and nanocrystallization is separately calculated.

Abstract

The crystallization mechanisms of Zr₅₅Cu₃₀Al₁₀Ni₅ (Zr55) amorphous alloys from slow continuous heating to rapid laser heating are investigated. The multiphase nanocrystalline structures such as CuZr₂-type, NiZr₂-type and Al₂Zr₃-type phases with various compositions are observed at heating rates lower than 50 K/s, while the CuZr₂-type/ZrCu-type eutectic dendrites are produced at heating rates higher than 50 K/s. A numerical model is established to calculate the onset crystallization temperatures as a function of the heating rates. This model indicates that in addition to rapid dendritic growth from a certain number of quenched-in nuclei, the non-Arrhenius behavior of high diffusion coefficient in the supercooled liquid region contribute to the serious crystallization during rapid heating process. The activation energy of dendritic crystallization is obviously smaller than that of multiphase nanocrystallization, which is due to a much smaller scale of diffusion distance and larger atomic diffusivity during dendritic growth.

Introduction

Bulk metallic glasses (BMGs) have been considered as potential structural materials in engineering fields due to their unique properties associated with disordering atomic structure [[1], [2], [3]]. However, the widespread applications of BMGs are constrained by the critical size limitation of the as-cast BMGs prepared by conventional copper mold casting method [4]. In recent years, selective laser melting (SLM) and laser direct manufacturing (LDM) techniques have attracted tremendous attentions in fabricating BMGs owning to their advantages on manufacturing the complex BMGs components without size limitation [[5], [6], [7]]. For instance, SLM technique has been introduced to fabricate various BMGs and BMGs composites [[8], [9], [10], [11]].

During SLM of Zr₅₅Cu₃₀Al₁₀Ni₅ (Zr55) metallic glass, the amorphous remelted zone (RZ) of the already-deposited layer kept the amorphous state [12], while a certain degree of crystallization still occurred in the heat affected zone (HAZ). The repeated reheating annealing/tempering treatment on the already-deposited amorphous layers during subsequent deposition led to the crystallization of HAZ. It had been demonstrated that the micro-scale coarse dendrites on the top of HAZ were formed upon rapid heating process [13]. In contrast, the nanocrystals were usually observed following non-isothermal and isothermal crystallization of BMGs upon slow heating process by using DSC [[14], [15], [16]]. Thus, the crystallization behaviors of BMGs during the rapid heating process and slow heating process are obviously different. H. Sun et al. revealed that the crystallization of HAZ was dominated by the rapid spherulitic growth from pre-existing nuclei, and they constructed a numerical model to calculate the spherulitic growth rate during LDM of a ZrCuNiAlNb metallic glass [17]. Here, the kinetic coefficient for crystal growth u_kin was obtained from the measurements of the viscosity η if the Stokes-Einstein (S-E) relation was invoked [18]. However, the S-E relation which links the viscosity η and the effective diffusion coefficient D broke down in the supercooled liquids for some cases, such as a Ni-Ti [19], Cu–Zr and Ni–Zr glass-forming alloys [20]. Thus, it is still unclear whether the calculation of dendritic growth rate during SLM depends on the S-E relationship. The numerical model used to describe the fast crystallization of HAZ needs to be reestablished.

In general, the RZ of the already-deposited layer will experience the reciprocating non-isothermal annealing treatment during SLM, and the temperature values corresponding to the maximum nucleation rate and maximum growth rate of crystals are different. Therefore, the crystallization mechanism of BMGs is considered to be strongly dependent on thermal history. In this work, the crystallization characteristics of Zr55 BMGs under a wide range of non-isothermal heating process at the heating rates from lower than 30 K/s to about 10⁴ K/s are discussed. The formation mechanism of dendrite in the HAZ is theoretically analyzed, which will lay a theoretical foundation for accurately control the crystallization of HAZ during SLM of Zr55 BMGs.