Elsevier

Acta Materialia

Volume 195, 15 August 2020, Pages 341-357
Acta Materialia

Full length article
Interface velocity dependent solute trapping and phase selection during rapid solidification of laser melted hypo-eutectic Al-11at.%Cu alloy

https://doi.org/10.1016/j.actamat.2020.04.006Get rights and content

Abstract

Microstructure formation of a hypo-eutectic Al-11Cu (atom percent) alloy during solidification after laser melting has been studied by transmission electron microscopy (TEM). The evolution of the solid-liquid-interface velocity, VSL, during the solidification process has been determined from direct observation by in-situ TEM. This enabled correlating VSL with development of four distinct microstructure zones. Crystal growth mode transitions from planar to cellular, cellular to dendritic, dendritic to cellular, and cellular to planar, have been observed for the accelerating solid-liquid-interface. The transition from coupled two-phase growth to the single-phase growth occurred for VSL = Va = (0.80±0.05) m/s, where Va is the velocity of absolute stability, at the onset of banded morphology grain formation. The in-situ and post-mortem TEM uniquely permitted determination of the non-equilibrium solidus for the rapidly solidifying Al-11Cu alloy. Experimental evidence for solute clustering and chemical ordering tendencies at length scales on the order of ≤ 5nm has been detected in the single-phase regions of the banded grains. The structural features of the single-phase bands have been interpreted as signatures of ‘frozen in’ configurations present in the liquid boundary layer adjacent to the growing crystal, which has a width of about 3nm. The nano-scale spatiotemporal resolution experimental TEM studies performed here provided quantitative metrics, e.g. the solidification interface velocity dependent solute concentration of the α-Al phase and the near-atomic scale structure in the single-phase bands, that are uniquely suitable for comparison with theory and model predictions for solidification microstructure development in multicomponent alloys after laser melting.

Introduction

Precipitation-strengthened aluminum alloys (Al-2xxx, -6xxx and -7xxx) are used widely in applications where their modest melting points are not a limitation and their superior density specific property combinations are advantageous [1], [2], [3]. Solidification processing is typically the first step in their preparation and typically results in multi-phase microstructures. The solidification velocity and the alloy composition are key parameters in controlling the details of the solidification microstructure [4,5]. Compared to more conventional liquid-solid processing, such as sand- or die-casting, rapid solidification (RS) can induce unusually large levels of solute supersaturation, novel modifications in texture, and metastable phase formation during laser- and electron-beam facilitated welding and additive manufacturing [6], [7], [8]. To develop predictive descriptions of microstructure evolution during additive manufacturing, it is therefore essential to investigate the formation of the initial solidification microstructure at different solid-liquid interface velocities.

Constitutional and thermal effects result in undercooling of the solid-liquid interface during RS [9,10]. Hence, variations in the solidification microstructure can be related to the local solid-liquid interface velocity and the composition of the liquid alloy. Microstructure response functions, such as the solid-liquid interface velocity, VSL(T,XB), and the interface velocity modified partition coefficient, kV(VSL), which vary with the interfacial temperature and the alloy composition, have been used in theoretical treatments and computational models of RS microstructure formation [9,11]. For instance, solid-liquid interface velocity dependent solute trapping and the critical conditions for transition from coupled two-phase crystal growth to single-phase solidification have been calculated for binary Al-alloy systems, including Al-Cu [11,12] . It has been shown that the transition from a cellular morphology in the two-phase crystal growth regime to banded morphology microstructure formation occurs in multicomponent alloys during RS when VSL reaches the critical value, Va, the velocity of absolute stability [11]. Based on ex-situ laser experiments and calculations based on solidification theory, RS microstructure morphologies have been predicted for Al-Cu alloys at different solid-liquid interface velocities [4]. With increasing solidification velocity, as conditions at the solid-liquid interface are driven increasingly farther away from equilibrium, the morphology transitions from planar → cellular → dendritic → cellular → banded →  partitionless (micro-segregation free) structure [5,13,14]. However, direct evidence (in-situ) for the full sequence of these crystal growth transitions have not been reported for multi-component systems.

The development of the dynamic transmission electron microscope (DTEM) and the advent of the movie-mode operation (MM-DTEM) has facilitated the direct observation of growth mode changes during multi-component Al-alloy RS [15], [16], [17], [18]. Using hypo-eutectic binary Al-Cu alloys as model systems, e.g. Al-4Cu and Al-7Cu (atomic %), it has been shown that the RS microstructures formed in alloy thin films used in the in-situ studies with the MM-DTEM exhibit morphologies that are in general equivalent to those reported for bulk alloys [15,16]. In hypo-eutectic Al-4Cu an extremely rapid acceleration of the solid-liquid interface has been determined for the initial RS crystal growth of the primary α-Al cells [16]. Notably, within 3 µs after the onset of directional RS the solid-liquid interface reached VSL ≥ 0.8 m/s [16]. As a result, obvious evidence for the morphological transitions from planar → cellular → dendritic, which are predicted to occur at lower velocities as the initially stationary solid-liquid interface accelerates, was not found. Instead, an abrupt morphological transition from growth of primary α-Al cells to morphologically modified eutectic cells comprised of discontinuously distributed metastable Al2Cu θ'-phase within the α-Al matrix was reported. The transition from the two-phase coupled growth of the modified eutectic cells to banded morphology formation occurred for VSL = 1.7 m/s. The large solid-liquid interface velocity and accelerations combined with the relatively low Cu content of the Al-4Cu alloy used in prior MM-DTEM studies prevented systematic studies of the solute trapping and mechanistic details of the atomic level process associated with the crystal growth under the driven conditions during RS microstructure formation [15, 16]. In Al-Cu alloys the magnitude of Va, the critical velocity for transition from two-phase to single-phase growth at the onset of banded morphology solidification microstructure formation, decreases with increasing Cu fraction in the composition range from Al to Al2Cu [19]. Hence, here we performed in-situ MM-DTEM experiments of the solidification microstructure formation after laser irradiation induced melting in hypo-eutectic Al-11Cu (atomic %). Complementing post-mortem TEM/STEM studies of the resulting solidification microstructure facilitated locally resolved composition and structural analyses. The current paper presents experimental measurements of Cu-solute trapping, secondary phase selection and morphology during solidification microstructure evolution as a function of the solid-liquid interface velocity, VSL, for a hypo-eutectic Al-11Cu (atomic %) alloy. Based on the experimental measurements of VSL and the associated composition of the α-Al phase the interfacial temperature has been calculated using solidification theory. This permitted determination of the solidification interface velocity modified alloy solidus and liquidus for VSL ≤ Va. Furthermore, nanoscale resolved composition and structural analysis of the solidification microstructure by high-resolution TEM/STEM have been used to interrogate mechanistic details of the phase formation and solute partitioning at the extremes of coupled two-phase growth and during single-phase growth at interface velocities VSL ≥ Va, i.e., under conditions characteristic for the banded morphology evolution during Al-Cu alloy RS. Implications of the experimental results for solidification theory and modeling of the RS microstructure evolution are discussed.

Section snippets

Experimental procedures

Thin films of hypoeutectic Al-Cu alloy with nominal composition of 11 atomic percent (at.%) Cu and thickness of 80 nm were prepared by dual electron beam evaporation (Pascal Technologies Dual E-Beam Deposition System). The nominally 50 nm thick amorphous Si3Ni4 membranes of windowed TEM grids (500 μm x 500 μm, Ted Pella, Inc.) served as substrates. The electron beam depositions were performed with high-purity elemental Al and Cu targets in high vacuum (base pressure < 5 × 10−8 torr) at a

Microstructure of the as-deposited alloy thin film

Fig. 1a is a high angle annular dark field (HAADF) STEM image of the as-deposited Al-Cu alloy, which consists of grains in the size range of 25-35 nm. The characteristically bright and dark regions indicate the presence of two different types of phases (Fig. 1a). Composition mapping by EDXS consistently showed that the bright regions contain higher amounts of Cu, on average (29.90 ± 2.5) at.% Cu as compared to the dark regions with (2.3 ± 0.7) at.% Cu (Fig. 1(a), (b) and (e)). The average Cu

Solid-Liquid Interface Velocity Evolution and Microstructure Zones

The solidification interface velocity evolution determined by the in situ MM-DTEM experiments for the Al-11Cu alloy showed characteristics that are qualitatively in agreement with those reported for more dilute hypo-eutectic Al-Cu alloys (Fig. 2) [15,16]. Following the incubation period the solid-liquid interface migrated with monotonically increasing velocity via three distinct stages, each signified by different interface acceleration regimes (Labels Stage I, II and III, Fig. 2(b)). Namely,

Summary and conclusions

The microstructure formation during laser irradiation induced rapid solidification of hypoeutectic Al-11Cu alloy thin films has been studied with nanoscale spatiotemporal resolution by in-situ MM-DTEM experiments and associated post-mortem S/TEM and HREM imaging, diffraction and composition analyses. The rapid solidification microstructure of the multi-component Al-11Cu alloy exhibited four distinct zones equivalent to those observed in bulk forms of hypoeutectic Al-Cu alloys. The distinct

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Acknowledgments

The research activities at the University of Pittsburgh received support from the National Science Foundation, Division of Materials Research, Metals & Metallic Nanostructures program through Grant No. DMR 1607922. Work at Lawrence Livermore National Laboratory (LLNL) was performed under the auspices of the U.S. Department of Energy, by LLNL under Contract No. DE-AC52-07NA27344. Activities and personnel at LLNL were supported by the U.S. Department of Energy, Office of Science, Office of Basic

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