Crystallization of High-Performance Metallic Materials

Abstract

Crystallization includes liquid/solid and solid/solid phase transitions, important processes for improving engineering material performance, which have attracted significant attention in the community. The current Special Issue (SI) entitled ‘Crystallization of High-Performance Metallic Materials’ has collected twelve research papers focusing on different aspects of the crystallization of metallic materials, e.g., the solidification of steel, fatigue and fracture behaviors of magnesium composites, nucleation of intermetallic compounds in aluminum alloys, microstructure evolution in nickel-based super-alloys, etc. The summary of crystallization behaviors at different temperature ranges in different metallic materials contributes to the state of the art of engineering material development.

1. Introduction

Crystallization describes the general material process where a solid phase nucleates in a liquid or solid matrix. The atoms or molecules are highly organized into a structure known as a crystalline cluster. In the manufacturing field, the crystallization of metallic materials includes the formation and growth of a new solid phase during solidification [1], as well as the subsequent phase transformations [2]. Other processes, e.g., pyrolysis [3], could also be included in the broad concept of crystallization; however, they are not always mentioned within the scope. Regarding crystallization mechanisms, several fundamental aspects of thermodynamics and kinetics should be included [1,2,3].

The solidification process includes heat and mass transfer, and various reactions and morphology evolutions occur, e.g., dendrite growth, control of macro-segregation, and the columnar-to-equiaxed transition (CET) [4,5,6]. Understanding solidification will benefit the understanding of the casting process in the metallurgical industry, contributing to preventing defect formation, e.g., porosity, shrinkage [7,8,9], and non-metallic inclusions [10,11,12]. Subsequently, crystallization behaviors can also include the structural evolution of mold flux during continuous casting [13,14], post-microstructure evolutions, e.g., different types of ferrite formation [15], acicular ferrite nucleation from non-metallic inclusions [16,17], and bainite and martensitic transformation in solid-state metallic materials [18]. Last but not least, additive manufacturing (AM), as a novel and short-process technology, enables increased creativity and faster development. It has attracted much attention in the metallurgical community; crystallization in AM [19,20] is the focus for current and future research.

In the current SI, we intend to emphasize the crystallization behaviors in various high-performance metallic materials. Both liquid/solid and solid/solid transitions are considered. Furthermore, we include conventional engineering materials, e.g., steels, Ni-based superalloys, and Al alloys, as well as novel metallic materials, e.g., light-weight magnesium metal matrix composites, AM-built Ti alloys, and multicomponent alloys. State-of-the-art characterization methods (e.g., high-temperature confocal laser scanning microscopy, high-resolution microscopies) as well as modeling work (e.g., first-principles simulation, CALPHAD) regarding crystallization of metallic materials are included in this SI. Specifically, we discuss defect formations during the crystallization of different metallic materials, e.g., δ-ferrite formation and growth during solidification and segregation influenced by the cooling rate in duplex stainless steel, pore defect formation in additively manufactured TC21 Ti alloy and its influence on high-cycle fatigue behavior, etc. Additionally, the behaviors of intermetallic precipitates in solid-state high-performance alloys, e.g., the nucleation of L1₂-Al₃M (M = Sc, Er, Y, Zr) nanophase in advanced al alloys, are included. Finally, metallic alloys’ mechanical properties associated with their crystallization behaviors, e.g., high-temperature creep behaviors of peak-aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag) alloy, are highlighted. The current SI collects comprehensive research on crystallization behaviors in high-performance metallic materials, aiming to pave the way to understanding the correlation between process, structure, and properties in engineering materials.

2. An Overview of Published Articles

Manuscripts on various subjects related to crystallization behaviors were submitted for consideration for the current Special Issue (SI). After the peer-review process, twelve papers were finally accepted for publication. The contributions and their descriptions are listed in

Table 1. Summary of the contributions published in this Special Issue.

No. of Contribution	Research Area	Focus	Type of Research
1	Mechanical properties of AZ91 Mg alloy	Fatigue and fracture behaviors	Experimental study
2	Calculation of mechanical properties of Beryllium	Assessment of the interatomic potentials	Molecular dynamics simulation
3	Pore defects on the AM-built TC21 Titanium alloy	High-cycle fatigue behavior of Ti alloy	Experimental study
4	Nucleation of L12-Al3M nanoparticles in Al alloys	Critical radius and nucleation energy of different intermetallic compounds	First-principles simulation
5	Solidification of hyper-duplex stainless steels	Crystallization kinetics and microstructure characterization	In situ characterization
6	Creep behaviors of multicomponent Al alloy	Effect of Ag addition on creep behavior and the mechanisms	Experimental study
7	Microstructure and properties of Ni-based single-crystal superalloy	Mechanical properties and thermal stability of Ni alloy	Experimental study
8	Nanohole propagation in single-crystal nickel	Effect of interatomic potentials on the properties of Ni	Molecular dynamics simulation study
9	Mechanical property evolution of near-β Titanium alloy	High-temperature deformation behavior of Ti alloy	Experimental study
10	Thin-film fabrication using vacuum electrodeposition	Preparation and characterization of a Cu(In,Ga)Se2 thin film	Surface treatment study
11	Elastic constitutive relationship of metallic materials	Grain shape effect on the elastic properties of metals	Theoretical study
12	Effect of stress on properties of thin-film solar cell materials	Electronic structure and optical properties of Cu2ZnSnS4	First-principles and DFT calculations

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Contribution 1 investigated the fatigue and fracture behaviors of short carbon fiber-reinforced squeeze-cast AZ91 at different temperatures between 20 and 250 °C. In this work, mechanical properties were examined by tensile tests at the abovementioned test temperatures to find suitable fatigue testing stress and strain for stress- and strain-controlled tests. The obtained fatigue curves of stress against the number of cycles revealed that the fatigue strength of composite AZ91–carbon was approximately 55 MPa under high-cycle fatigue; additionally, the fatigue strength of the matrix alloy AZ91 was 37 MPa at 250 °C. This work finally revealed that the fracture types were mixed ductile/brittle contained fatigue serration, fiber fracture, and separation in the reinforced AZ91–carbon materials.

Contribution 2 assessed the interatomic potential of Beryllium to determine its mechanical properties. In this work, molecular dynamics simulations were used to calculate the mechanical properties of imperfect hexagonal close-packed (HCP)-type Beryllium. Through the simulation, three types of potentials, i.e., MEAM, Finnis–Sinclair, and Tersoff, were assessed. Furthermore, the volumetric change (VC) with pressure according to MEAM and Tersoff and the VC with temperature according to MEAM were consistent with the obtained experimental data. Finally, MEAM-type potential was found to deliver the most reasonable predictions of the targeted properties.

Contribution 3 investigated the effect of pore defects on the high-cycle fatigue behavior of TC21 Titanium alloy prepared by electron beam melting-type additive manufacturing (AM). The obtained stress–life cycle (S-N) curve of non-HIP specimens clearly showed a tendency to decrease in very-high-cycle regimes, and HIP treatment obviously improved fatigue properties. Finally, a fatigue indicator parameter model according to the pore defect characteristics investigated was established to predict the fatigue life of HIP and non-HIP samples.

Contribution 4 provided a first-principles and thermodynamic study to investigate the nucleation of L1₂-Al₃M (M = Sc, Er, Y, Zr) nanophases in Al alloys. The calculation results showed that the critical radius and nucleation energy of the L1₂-Al₃M phases decreased in the order Al₃Er, Al₃Y, Al₃Sc, and Al₃Zr.

Contribution 5 investigated the effect of cooling rate on the crystallization behavior of hyper-duplex stainless steel SAF™ 3207 HD (also named UNS S33207) during solidification. A combination of in situ observation using high-temperature confocal laser scanning microscopy (HT-CLSM) and differential scanning calorimetry (DSC) was used. The effect of the cooling rate, i.e., 4 and 150 °C/min, on the nucleation and growth behavior of δ-ferrite in S33207 during the solidification was investigated in situ. The results showed that S33207 steel’s solidification mode was a ferrite–austenite type. Liquid to δ-ferrite transformation occurred at a certain degree of undercooling, and merging occurred during the growth of the δ-ferrite-phase dendrites.

Contribution 6 reported Ag’s effect on the high-temperature creep behaviors of peak-aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag) multicomponent alloys. The high-temperature creep performances of the proposed alloy were significantly improved with Ag addition. Subsequently, constitutive relational models of the multicomponent alloy during high-temperature creep were built, and the activation energy could be calculated. The creep mechanism after Ag addition transitioned from lattice diffusion control to grain boundary diffusion control.

Contribution 7 provided a comprehensive study of the microstructure, mechanical properties, and thermal stability of Ni-based single-crystal superalloys with low specific weight (LSW). A multicomponent Ni-Co-Cr-Mo-Ta-Re-Al-Ti system was investigated. The alloys’ mechanical properties were examined by tensile tests, elongation tests, and thermal exposure tests. The results of this work provided scientific insights for developing Ni-based single-crystal superalloys with LSW properties.

Contribution 8 performed a molecular dynamics simulation on the effect of interatomic potential on the nature of nanohole propagation in single-crystal nickel. It showed the difference between the different styles of interatomic potentials characterizing nanohole propagation in single-crystal Ni. Furthermore, it provided a theoretical basis for the selection of interatomic potentials using the molecular dynamics simulation methodology.

Contribution 9 studied the high-temperature deformation behavior of near-β Titanium alloy (Ti-3Al-6Cr-5V-5Mo); the flow stress behavior and processing maps in an α + β-phase field and β-phase field were investigated. The experimental data obtained from hot compressing simulations at 700 to 820 °C were used to establish constitutive models. After the deformation test, the maximum number of dynamic recrystallization grains and the minimum average grain size could be obtained. The current results are consistent with the high-power dissipation coefficient region, which is predicted by the processing map.

Contribution 10 studied the vacuum electrodeposition of Cu(In,Ga)Se₂ (CIGS) thin films prepared under a 3 kPa vacuum. Furthermore, the vacuum electrodeposition mechanism of CIGS was investigated. Meanwhile, the route of Ga incorporation into the thin films could be controlled in a vacuum environment by inhibiting pH changes at the cathode region. A higher current density and a lower diffusion impedance and charge transfer impedance were used in the abovementioned preparation process.

Contribution 11 investigated the elastic constitutive relationship of metallic materials containing different grain shapes; a new expression of the elastic constitutive relationship of polycrystalline materials containing different grain shape effects was established. The experimental results showed that the grain shape parameter was consistent with the theoretical results of the material’s macroscopic mechanical properties.

Contribution 12 studied the electronic structure and optical properties of Cu₂ZnSnS₄; first-principles calculations based on density functional theory were applied for this study. Through this method, the band structure, density of states, and optical properties of Cu₂ZnSnS₄ under isotropic stress were calculated and analyzed. The results showed that Cu₂ZnSnS₄ is a direct band gap semiconductor under isotropic stress, and the lattice is tetragonal.

3. Summary

The current Special Issue (SI), Crystallization of High-Performance Metallic Materials, collects research contributions about solidification, casting, recrystallization during deformation, and mechanical property evolution of different engineering materials, e.g., stainless steels, Ni-based superalloys, Ti alloys, etc. Both experimental and simulation studies on crystallization topics were reported in different papers. Some research work on topics like the crystallization of slags (silicates) and mold flux has not been collected yet; we will continue to organize a Volume II SI on the same topic to collect more contributions on different topics of material crystallization.