Elevated temperature ductility dip in an additively manufactured Al-Cu-Ce alloy

Abstract

The deformation and failure mechanisms of Al-9Cu-6Ce (wt%) based alloys fabricated with laser powder bed fusion were investigated from room temperature to 400°C. The yield and ultimate tensile strengths decreased monotonically with increase in temperature, but the tensile elongation dipped unexpectedly at elevated temperatures and exhibited a minimum at 300°C. The dip in tensile elongation occurred with a concomitant dip in strain-rate sensitivity (SRS) of deformation. The as-fabricated alloy microstructure was heterogeneous, and the heat affected zone (HAZ) underneath the melt pool boundary was prone to strain localization. At 300 °C, the reduced SRS promoted the progression of strain localization in the HAZ leading to failure initiation and the dip in tensile elongation. A higher SRS or strain-hardening rate at other temperatures improved the tensile elongation by slowing the progression of strain localization in the HAZ such that failure initiated by other mechanisms elsewhere in the microstructure. Notably, the tensile elongation was limited by the defect structure only in a narrow temperature range (150 - 200 °C) while at other temperatures it was limited by the inherent microstructural features. This investigation exemplifies unexpected deformation and failure mechanisms possible in heterogeneous microstructures that result from additive manufacturing.

Introduction

Additive manufacturing (AM) is gaining interest as a method to produce Al alloys that afford light-weighting solutions in structural applications [1], [2], [3], [4]. With the emergence of AM, there has been the need to design new Al alloys that are amenable to processing by various AM techniques [5]. AM techniques can produce unique alloy microstructures that are distinct from the microstructures produced by conventional materials processing techniques. The AM microstructures are often characterized as hierarchical and heterogeneous across different alloy systems [6], [7], [8], [9], including Al alloys [3,[10], [11], [12], [13], [14], [15]]. The hierarchy in AM microstructures can span across length scales that differ by several orders of magnitude. For instance, this hierarchy may include sub-millimeter sized melt pools, micron sized grains, sub-micron solidification cells, and nanoscale precipitates. Heterogeneity can occur in AM microstructures due to spatiotemporal variation in the solidification parameters namely, the thermal gradient (G) and solidification velocity (R) during solidification of a melt pool [16,17]. The thermal gradient decreases and solidification velocity increases as solidification proceeds from the edge of a melt pool to its center, a scenario that is repeated during additive processing and subsequently creates a patterned heterogeneity in the solidification microstructure [18,19]. At the same time, the solidified layer is thermally cycled during fabrication of the subsequent layer, which further adds to the microstructural heterogeneity. This thermally cycled zone is analogous to the heat affected zone (HAZ) in the microstructure of weldments.

Such hierarchical and heterogeneous microstructures regulate the deformation mechanisms that finally impact the mechanical properties and failure mechanisms of AM materials. Wang et al. [9] elucidated in the case of hierarchical microstructure of an AM 316L stainless steel, the role of sub-micron solidification cells in regulating the deformation mechanisms that led to a simultaneous enhancement of strength and ductility at room temperature (R.T.). Chen et al. [10] demonstrated the hierarchical microstructure in AM AlSi10Mg alloy comprising micron sized grains, sub-micron cellular structure, and a nanoscale Al-Si eutectic structure in the cellular walls. The high strength and strain hardening at R.T. in this alloy was said to be a result of Orowan strengthening from the nanoscale Al-Si eutectic cellular walls. AM Al alloys containing L1₂ trialuminide formers such as Zr and Sc can produce a heterogeneous microstructure with bimodal grain size distribution such that finer equiaxed grains are present near the melt pool boundary (MPB) and large columnar grains in the melt pool interior (MPI) [20], [21], [22], [23]. The solidification velocity at the MPB is low enough to allow primary solidification of the L1₂ phase on which α-Al grains nucleate heterogeneously to produce a fine equiaxed grain structure [21,23]. The solidification velocity is high enough in the MPI to trap the solutes (X) and inhibit primary L1₂ (Al₃X) formation and, as a result, a columnar grain structure is promoted. During deformation, plasticity initiates first in the columnar grains due to their lower strength than the finer equiaxed grains [22]. This strain partitioning produces a back stress that increases the strain-hardening rate by accumulation of a high density of geometrically necessary dislocations (GNDs) at the interface of columnar and equiaxed grains. A high strength-ductility combination is achieved at R.T. as a result of the increased strain-hardening ability [22,24].

The failure mechanism in AM AlSi10Mg is influenced by the microstructural heterogeneity which occurs in terms of the morphology and interconnectivity of Si particles [25], [26], [27]. While Si particles in the MPIs formed a highly interconnected cellular network, the thermal cycling in the HAZ broke this interconnectivity and spheroidized the particles. The breakdown of interconnectivity and spheroidization softened the HAZ and made that region susceptible to strain localization. As a result, failure occurred along the HAZs in AlSi10Mg at R.T. [25], [26], [27]. We recently reported on the heterogeneous microstructure of an AM Al-Ce-Mn alloy in which the Al₂₀Mn₂Ce phase was present in two different morphologies [28]. Al₂₀Mn₂Ce solidified as the primary phase with a rosette morphology at the MPB and as a cooperative lamellar eutectic with α-Al in the MPI. The MPB was softer than the MPI due to larger inter-particle spacing of the rosette structure compared to the eutectic structure and therefore, was susceptible to strain localization. Under tensile loading, the failure initiated in the MPB by fracture of Al₂₀Mn₂Ce rosettes presumably due to strain localization. Subsequently, the MPBs decorated with these rosettes provided an easy path for failure propagation leading to a low tensile elongation at R.T. [28]. The above examples highlight the impact of microstructural hierarchy and heterogeneity on the mechanical properties and failure mechanisms of AM alloys at R.T. but the corresponding impact on elevated temperature mechanical behavior is not well understood.

Near-eutectic Al-Ce based alloys are promising materials for high temperature applications [16,[28], [29], [30], [31], [32], [33]]. The negligible solid-solubility of Ce in α-Al matrix gives superior coarsening resistance to Ce-rich intermetallic particles. Al-Ce based alloys derive a significant portion of their strength from the distribution of the intermetallic particles [28,31]. The high cooling rates in AM can be leveraged to achieve superior strength at elevated temperatures by refinement of the solidification microstructure. The addition of alloying elements can provide further solid-solution and precipitation strengthening to Al-Ce based alloys [[28], [29], [30], 33,34]. However, these additions can be accompanied by their own challenges. The strength of Al-Ce alloys is significantly improved by Mn addition but it can lead to severe embrittlement of the alloy at high concentrations [28]. Mg is a potent solid-solution strengthener in Al-Ce alloys but can render AM processing of the alloy challenging due to issues with vaporization [35]. Cu is another potential candidate that can be used to strengthen an Al-Ce alloy. Manca et al. [30] fabricated an Al-Cu-Ce alloy with a high R.T. yield strength of 257 MPa, but the alloy exhibited a low tensile elongation of ∽ 1%. Although, the initial reports on Al-Ce based alloys are encouraging, there is a need to develop new Al-Ce based alloys that are processable and offer good combination of mechanical properties.

In this work, we report on the heterogeneous microstructure and tensile properties of a new near-eutectic ternary Al-9Cu-6Ce (wt%) alloy fabricated using laser powder bed fusion (LPBF) additive manufacturing. The alloy exhibited a ductility dip at elevated temperatures with a minimum at 300°C. A similar behavior was observed in two other alloy variants including Al-9Cu-6Ce-1Zr and Al-9Cu-6Ce-1Zr-0.45Mn alloys. The phenomenon of ductility dip also called ‘intermediate temperature embrittlement’ is known to occur in several systems including Al [36], [37], [38], Ni [39], [40], [41], Cu [42], and Fe [43,44] based alloys. A new mechanism of ductility dip is proposed here that is unique to AM microstructures and distinct from the previously reported mechanisms. This ductility dip is explained to occur as a result of a unique interaction between the microstructural heterogeneity and low strain-rate sensitivity (SRS) at elevated temperatures. The identified failure mechanisms in the heterogeneous microstructure of the Al-Cu-Ce alloy will assist the future design of favorable AM alloy microstructures.