The unprecedented increase in component design space has led to significant focus on fusion-based additive manufacturing (AM) technologies. The new design possibilities integrate features and ...functionalities that are not supported by conventional manufacturing, and simultaneously achieve unitization of components. However, fusion-based AM suffers from a lack of diverse alloys, as conventional high-strength Al alloys are prone to hot cracking and other defect formation during printing. New alloy design approaches are being pursued to overcome these limitations. Current strategies for design of printable Al alloys for laser-powder bed fusion (L-PBF) are primarily experimental and revolve around grain refinement and eutectic solidification (ES). Each of these strategies targets hot cracking at only a specific stage of solidification. Consequently, the processing window of the alloy shrinks and fine-tuning of the alloy microstructure becomes difficult, thus prohibiting the activation of multiple deformation mechanisms. On the other hand, strategies that integrate microstructural refinement (MR) and ES attack the problem during multiple stages of solidification. Such MR+ES integrated alloy design strategies allow widening of alloy-processing-window (printability) and activation of multiple deformation mechanisms such as back-stress strengthening and work-hardening, thus producing alloys with excellent synergy of printability and performance.
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•Design of Al alloys for laser-powder bed fusion (L-PBF) can lead to fundamental change in the microstructure evolution.•Proposed alloy design strategies lead to synergy between and printability and performance.•Hierarchical and heterogeneous microstructures have potential to overcome the strength-ductility tradeoff.•Alloy design incorporating the specific attributes of an AM technology is crucial to maximizing its potential.
AlxCoCrFeNi is a prominent high entropy alloy system with varying crystal structure from FCC to BCC depending on aluminum content. The mechanical behavior of Al0.7CoCrFeNi with dual phase FCC+BCC ...microstructure has been compared with that of single phase FCC Al0.3CoCrFeNi. Both quasi-static and dynamic strain rate regimes were investigated. Hypo-eutectic Al0.7CoCrFeNi showed much higher strength due to fine lamellar microstructure with a large number of FCC-BCC interphase boundaries. But this also leads to lower strain rate sensitivity due to the long-range nature of these interfaces, overcoming them is indifferent with temperature elevation to assist slip, thus making them athermal barriers. Both these precipitation hardenable alloys were aged to induce precipitation of ordered L12 in the FCC phase. This coherent nano-scale L12 precipitate caused a significant increase in the yield strength of both single-phase and dual phase structures while reducing the strain rate sensitivity (SRS) only slightly. L12 precipitation in FCC matrix greatly enhanced twinning during dynamic deformation. Large-scale deformation twins were observed in coarse Al0.3CoCrFeNi FCC and FCC + L12 microstructures. The scale of deformation twins was much smaller in the dual phase Al0.7CoCrFeNi whose refined lamellae width retarded twinning. The lamellar structures, nevertheless, had higher work hardening due to their higher dislocation density storage capability.
The paradigm shift of alloying approach that led to high entropy alloys (HEAs) is now well established. Although the initial years were dominated by equiatomic approach, recent years have seen ...expansion in non-equiatomic compositional space that can be termed as complex concentrated alloys (CCAs). These HEAs/CCAs provide opportunities for tunable performance by manipulating deformation mechanisms. Understanding has advanced to the point that certain aspects of core effects (entropy of mixing, lattice distortion, sluggish diffusion, and cocktail effect) can be critically examined. In addition, new aspects of metastability engineering and emergence of a wide range of processing strategies has put this field on an exponential growth path. In this review, we categorize the compositional and microstructural approaches that exhibit potential for a combination of shear induced phase transformation and twinning, thereby expanding beyond the slip based mechanisms. The emerging HEAs give greater flexibility for tailoring transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), which have guided design of next-generation steels over the last 20 years to a new level. For TRIP HEAs, the ductility can be extended to as high as 50% while maintaining a strength exceeding 1 GPa. On the other hand, hierarchical microstructural engineering in AlxCoCrFeNi alloys can lead to over 2 GPa strength and >10% ductility. Observations of evolving c/a ratio in HCP phase of certain HEAs hint at possibility of new micromechanisms. While crack tip twin-bridging has been shown as a key mechanism to extend the toughness, concurrent phase transformation at the crack tip has been shown to push the fatigue endurance limit. Tunability of deformation mechanisms in HEAs is unprecedented as compared to the conventional metallic materials, particularly in compositions that exhibit shear induced transformation. The opportunities can be further enhanced by integrating the compositional and microstructure domains, and these aspects are highlighted in this review. The microstructural tailoring can take advantage of high enthalpy states in metastable HEAs with low stacking fault energy values of < 40 mJ m−2. The range of microstructural engineering in HEAs include, heterogeneous grain structure, duplex and triplex microstructures with intermetallic phases, twinning engineered microstructure, coherent boundary engineered microstructure, and dual-phase and triple-phase microstructure with solid solution phases.
Fatigue is a major cause of failure and requires dedicated investigation to validate the integrity of potential structural materials. As high and medium entropy alloys (HEAs and MEAs) are garnering ...significant attention from scientific communities, it is important to test the structural integrity of these novel alloys in cyclic loading environment. This study analyzed both monotonic and cyclic loading properties of a novel medium entropy alloy (MEA) CoCr1.3Ni in the ultrafine-grained (UFG) and fine grain (FG) conditions. The UFG MEA performs better than the FG MEA during cyclic loading. Microstructural evolution after cyclic loading via electron back-scattered diffraction shows microstructural stability of both FG and UFG microstructures. Transmission electron microscopy suggests formation of stacking faults and fine-scale HCP transformation during cyclic loading.
Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a ...candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.
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Friction stir welding under ambient cooling conditions, using tool rotation rate of 800rpm and welding speed of 200mm per minute was carried out on 2195 and 2199 Al-Li alloys. Underwater welding was ...also performed on both the alloys using a pool on the top of workpiece. Post weld heat treatment for various durations was conducted to study the effect of alloy chemistry on aging kinetics in various metallurgical zones developed during welding. Vickers microhardness, differential scanning calorimetry, and transmission electron microscopy results showed that low dislocation density after recrystallization in weld nugget results in delayed aging kinetics for both the alloys. Ag containing alloy 2195 showed faster aging response compared to Ag-free alloy 2199. Coarse precipitates at grain boundary and in the grain interior were found responsible for loss of non-recoverable strength in heat affected zone for both the alloys.
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•Underwater friction stir welding reduced the strength loss in heat affected zone and thermo-mechanically affected zone.•Low dislocation density was responsible for the delayed aging kinetics in case of alloy 2195.•Low dislocation density and absence of Ag was responsible for the extremely sluggish aging kinetics in case of alloy 2199.•Partial dissolution of T1 precipitates was responsible for the low strength in heat affected zone (HAZ) of 2195.•Full dissolution of Al3Li and partial dissolution of T1 phase were responsible for reduction in strength in HAZ of 2199.
Recent studies indicate that eutectic high-entropy alloys can simultaneously possess high strength and high ductility, which have potential industrial applications. The present study focuses on Al
...CoCrFeNi, a lamellar dual-phase (fcc + B2) precipitation-strengthenable eutectic high entropy alloy. This alloy exhibits an fcc + B2 (B2 with bcc nano-precipitates) microstructure resulting in a combination of the soft and ductile fcc phase together with hard B2 phase. Low temperature annealing leads to the precipitation of ordered L1
intermetallic precipitates within the fcc resulting in enhanced strength. The strengthening contribution due to fine scale L1
is modeled using Orowan dislocation bowing and by-pass mechanism. The alloy was tested under quasi-static (strain-rate = 10
s
) tensile loading and dynamic (strain-rate = 10
s
) compressive loading. Due to the fine lamellar microstructure with a large number of fcc-bcc interfaces, the alloy show relatively high flow-stresses, ~1400 MPa under quasi-static loading and in excess of 1800 MPa under dynamic loading. Interestingly, the coherent nano-scale L1
precipitate caused a significant rise in the yield strength, without affecting the strain rate sensitivity (SRS) significantly. These lamellar structures had higher work hardening due to their capability for easily storing higher dislocation densities. The back-stresses from the coherent L1
precipitate were insufficient to cause improvement in twin nucleation, owing to elevated twinning stress under quasi-static testing. However, under dynamic testing high density of twins were observed.
Starting from the thermodynamics to thermal and mechanical properties, high entropy alloys (HEAs) always deviate from the behavior of conventional materials and stamp its uniqueness among the alloy ...systems. In this study, tensile deformation mechanisms of HEA was investigated. A simple system, an Al0.1CoCrFeNi HEA, with a single crystal structure (face-centered cubic, FCC), coarser grains (CG), and low dislocation density was chosen to exclusively study the effect of intrinsic lattice on the HEA deformation mechanisms and kinetics. Monotonic tests were done at the strain rate of 10−3s−1, and all the transient tests were started at the initial strain rate of 10−3s−1. Strain-jump tests were carried out at strain rates of 10−5s−1 and 10−3s−1. Repeated stress relaxation tests were performed along the stress–strain curve to calculate the physical activation volume. Surprisingly, a large rate sensitivity of the flow stress and low activation volume of dislocations were observed, which are unparalleled, as compared to conventional CG FCC metals and alloys. The observed trend has been explained in terms of the lattice distortion and dislocation-energy framework. As opposed to the constant dislocation line energy and Peierls potential energy (amplitude,ΔE) in conventional metals and alloys, both the line energy and Peierls potential undergo continuous variations in the case of HEAs. These energy fluctuations have greatly affected the dislocation mobility and can be distinctly noted from the activation volume of dislocations.
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•Two different ultrafine grained (UFG) microstructure were obtained using friction stir welding.•Effect of grain size reduction within the UFG regime on fatigue properties was ...studied.•Comparison of fatigue properties among UFG microstructure and fine grain microstructure of same composition was done.•Fractographic study to understand the crack initiation and propagation of all the three microstructure was done.
The main objective of the present study was to examine the effect of grain size on the fatigue properties of 5024 Al alloy. Two ultrafine-grained (UFG) and one fine-grain microstructures were produced via friction stir processing (FSP). Fatigue properties were investigated using a fully reversible bending fatigue testing machine. Microstructural evolution during fatigue was analyzed by carrying out interrupted fatigue testing with subsequent orientation imaging microscopy. Emphasis was given to grain size on the evolution of deformed structure, intergranular dislocation density, and overall S-N curve. A comparison with literature demonstrates the importance of the processing route to obtain the UFG microstructure. This difference is highlighted through an analysis of Basquin equation parameters. The results indicate that FSP can be utilized as an effective route to obtain stable UFG microstructure. A fractographic study was done to understand the effect of microstructure on fatigue crack initiation and propagation rates. Crack growth rates in stage-II has been compared for the three different grain sizes.
Abstract
Smart alloying and microstructural engineering mitigate challenges associated with laser-powder bed fusion additive manufacturing (L-PBFAM). A novel Al–Ni–Ti–Zr alloy utilized grain ...refinement by heterogeneous nucleation and eutectic solidification to achieve superior performance-printability synergy. Conventional mechanical testing cannot delineate complex micromechanics of such alloys. This study combined multiscale nanomechanical and microstructural mapping to illustrate mechanical signatures associated with hierarchical heat distribution and rapid solidification of L-PBFAM. The disproportionate hardening effect imparted by Al
3
(Ti,Zr) precipitates in the pool boundaries and the semi-solid zone was successfully demonstrated. Nanomechanical response associated with heterogeneity in particle volume fraction and coherency across melt pool was interpreted from nanoindentation force–displacement curves. The hardness map effectively delineated the weakest and strongest sections in the pool with microscopic accuracy. The presented approach serves as a high throughput methodology to establish the chemistry-processing-microstructure-properties correlation of newly designed alloys for L-PBFAM.
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