Rapid Solidification Microstructure Research Articles

Evolution mechanism of rapid solidification microstructure of an undercooled copper-based single phase alloy

Using undercooling technology combined with melt purification of glass and recirculation overheating, different undercoolings were successfully obtained for Cu60Ni38Co2 alloy samples. Applying high speed photography technique, the physical process of undercooled solidification was observed and relationship between undercooling and solidification rate was analyzed. Experimental results indicate the critical undercoolings exist in the structure evolution process. Electron backscatter diffraction (EBSD) analysis was applied to characterize the Cu60Ni38Co2 alloy sample with a maximum undercooling of 255K, revealing grain orientation and the corresponding orientation difference distribution. In addition, transmission electron microscopy (TEM) showed high density dislocation networks. Combining EBSD and TEM data analysis, an important conclusion was drawn: recrystallization is the main factor for grain refinement at high undercooling. This finding is of great significance for a deeper understanding of the structure evolution in the physical process of undercooled alloys.

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

Microstructural evolution of rapidly solidified hypoeutectic Al[sbnd]10Cu alloy during non-isothermal annealing transients induced by nano-second laser pulses

The evolution of characteristic nonequilibrium features presenting in morphologically distinct regions of rapid solidification (RS) microstructures in a hypoeutectic Al10Cu (atomic %) in response to non-isothermal annealing transients has been studied by transmission electron microscopy (TEM). The capabilities of the Movie-Mode Dynamic TEM (MM-DTEM) instrument were used to expose select regions of the RS microstructure to sequences of rapid heating and cooling transients induced by nanosecond laser pulses while permitting in-situ observation. Partial melting, microstructural scale coarsening, morphological changes of the nonequilibrium features in the multi-phase RS microstructure, and solid-state phase transformation were observed. Heterogeneous nucleation of nanoscale θ-Al2Cu phase involved metastable supersaturated α-Al and the θ'-Al2Cu phases, establishing different sets of orientation relationships for the stable θ-Al2Cu and α-Al phases. Replacement of banded morphology grains that formed under conditions driven farthest from equilibrium by an equiaxed nanocrystalline structure comprised of α-Al phase, the primary solidification product, and an intergranular network of Al2Cu crystals has been attributed to local remelting. The experimental approach explored here permitted discovery of mechanistic details of location-specific transformation pathways activated in the multi-phase RS microstructure of hypoeutectic AlCu during subsequent nonisothermal transients.

Constructing solidification microstructure selection map for AlSi10Mg alloy processed by laser powder bed fusion

The cellular structures, which consist of cells and eutectic networks at the cell boundaries, determine the mechanical properties of the Al–Si alloys fabricated by laser powder bed fusion (L-PBF). Therefore, it is scientifically interesting and technologically important to establish the relationship between cellular structures and processing parameters, which is applicable to all kinds of processing methods involving rapid directional solidification, such as L-PBF. In this work, a rapid solidification microstructure selection map (SMSM) of the Al–10Si system in temperature gradient (G)–solidification velocity (V) space is constructed by a synergistic calculation with dendritic and eutectic growth models compatible to the L-PBF process. Considering that the divorced eutectic reaction should occur as the width of the liquid film entrapped in the α-Al cell boundaries becomes smaller than the Al–Si eutectic spacing, the G–V domain suitable for acquiring the cellular structures decorated with divorced eutectic Si networks is determined on the SMSM. The microstructure evolution trends predicted by the SMSM are well consistent with those in the L-PBFed AlSi10Mg melt pools with different processing parameters. The length scales of actual microstructures observed by the experiments from this work and the literature are quantitatively compared to those estimated by the SMSM, and the experimental results are basically in accord with the SMSM. It is demonstrated that divorced eutectic Si networks prefer to form at the bottom of the melt pool, where G is large (> 5 × 106 K/m) and V is small (3 × 10−4–6 × 10−3 m/s) in the SMSM. The outcomes of this work may provide fundamental guidance for tailoring the microstructures of the L-PBFed Al–Si alloys for achieving optimal mechanical properties.

Probing effects of solute trapping on the mechanical properties of α-Al in rapidly solidified hypoeutectic Al-10at.%Cu after surface laser melting

After scanned laser surface remelting the resultant rapid solidification microstructures of multicomponent alloys exhibit morphological gradients, with scale refinement and non-equilibrium features, including non-equilibrium solute trapping, which are associated with property changes. Combinations of site-specific mechanical property measurements by nano-indentation and microstructural analyses by electron microscopy have been used to determine microstructure-property relationships for the characteristic features of the rapid solidification microstructure of a hypo-eutectic Al–Cu alloy containing 10 atom percent Cu. A microstructure gradient developed as the solidification rate increased monotonically during the re-solidification process. For a laser scan velocity of 3 mm/s most of the rapid solidification microstructure is comprised of micrometer scale refined dendritic cells of α-Al(Cu) and nanometer scale refined intercellular θ-Al2Cu phase or eutectic. The cellular α-Al(Cu) constituent consistently showed Cu-solute supersaturation and a 36 % hardness increase relative to the as-cast state. The eutectic microconstituent exhibited increases in hardness that correlated quantitatively with the increasing lamellar spacing refinement obtaining for increasing solidification rate. The possible contributions from precipitation, solute and grain-size strengthening mechanisms to the hardness increase in the α-Al(Cu) cells have been assessed. The hardness increase of the α-Al(Cu) cells was attributed to a combination of solute and grain size strengthening.

Transmission electron microscopy of the rapid solidification microstructure evolution and solidification interface velocity determination in hypereutectic Al-20at.%Cu after laser melting

The evolution of rapid solidification (RS) microstructure and solidification interface velocity has been studied experimentally by in-situ transmission electron microscopy and postmortem characterization for hypereutectic Al-20at.%Cu (37 wt.%Cu) after laser melting. Four morphologically distinct regions formed via growth modes changing from θ-Al2Cu phase dendrites to eutectic cell growth, possibly α-Al dendrite growth (α-cell), banded growth, and α-Al plane front growth. Tendency for faceting and low capacity for solute trapping limited the θ-phase dendrite growth to solidification interface velocity v < 0.07 m/s. Consequently, formation of pro-eutectic micro-constituent was suppressed, and RS microstructure formation was dominated by eutectic, α-cell, and banded morphology grains for the Al-20at.%Cu alloy. Eutectic growth operated for interface velocity of 0.1 m/s ≤ v < 0.3 m/s, with a transition from regular lamellar, 2-λ and 1-λ mode to a dense irregular morphology dominated by α-phase at v = 0.3 m/s. Interface temperature calculations indicated feasibility of α-cell growth mode for 0.3 m/s ≤ v < 0.7 m/s. Banded growth occurred for 0.7 m/s ≤ v < 1.3 m/s. Plane front α-phase growth was evident for interface velocities v ≥ 1.3 m/s. Previous work on rapid solidification microstructure development in hypereutectic Al-Cu alloys reported a regime of α-cell growth subsequently to eutectic and prior to transition to banded growth for Al-19at.%Cu (36 wt.%Cu), while for Al-22at.%Cu (40 wt.%Cu) eutectic growth transitioned directly to α-plane front growth without emergence of a banded regime. Based on the current study the disappearance of the banded growth regime occurs for a composition larger than 20at.%Cu.

High conductive copper alloys for additive manufacturing

For applications where high thermal and/or electrical conductivity combined with reasonably high strength is required, copper alloys may be used. Although many different alloys were already developed in the past, additive manufacturing like laser powder bed fusion (PBF-LB/M) opens up new possibilities for alloy development, mainly driven by the very high cooling rates. This allows for the usage of precipitation-hardened alloys with compositions exceeding the maximum solubility. The present work focuses on the investigation of a well-known CuCr1Zr alloy as well as CuZr alloys with 1 and 2 wt.% Zr. For a fast, resource-efficient screening and demonstration of feasibility, the investigated alloys were not printed from powder. Instead, solid sheets were partially re-melted in a PBF-LB/M machine to obtain a microstructure similar to the printed state. This rapid-solidification microstructure is investigated, and precipitates with a size 50 nm or even smaller are found. After subsequent aging heat treatments, the hardness of the alloys exceeds the maximum hardness achievable with conventional manufacturing methods (excluding work hardening). The investigations in this work revealed the great hardening potential of these alloys for usage in the PBF-LB/M process.

Influence of Molybdenum on Rapid Solidification Microstructure and Microsegregation in Primary Ferrite Solidified Stainless Steel

Influence of Molybdenum on Rapid Solidification Microstructure and Microsegregation in Primary Ferrite Solidified Stainless Steel

Co effect on rapid solidification microstructure transition of highly undercooled copper alloys

The present work systematically studied nonequilibrium solidification microstructure and its morphology evolution with undercooling of Cu60Ni40 binary alloy and Cu60Ni35Co5 ternary alloy, and the characteristic undercooling degree of alloy evolution was determined. Combining with BCT model, dendritic growth of undercooled Cu–Ni alloy was studied. On the basis of the above, a new ternary single-phase alloy was formed by adding minor Co element into Cu60Ni40 alloy. Subsequently, a comparative analysis was conducted on Cu60Ni40 and Cu60Ni35Co5 alloys, exploring the effect of Co element on the evolution of microstructure of deeply undercooled Cu–Ni alloys.

Effect of Sb/Bi atom substitution site on electronic transport properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; alloy

Mg&lt;sub&gt;2&lt;/sub&gt;(Si,Sn)-based thermoelectric materials, which are environmentally friendly and low-cost, have great development potential in a moderate temperature range. Electronic transport properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;1-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Sn&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; alloys can be optimized by doping elements. Doping is still one of the most effective methods of optimizing electronic transport performance, such as carrier concentration, mobility, and effective mass. The most effective doping elements are Sb and Bi. Much attention has been paid to the influence of element type and doping content. Different substitution sites will also greatly affect the electronic transport parameters. In this work, the defect formation energy value of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; alloy for substituting Sb atoms and Bi atoms for Sn sties and Si sites, respectively, are calculated by first-principles calculations. The influence on electronic transport parameters is systematically analyzed by combining the calculated results of band structures and density of states. Corresponding component Sb and Bi atoms doped Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; alloys are prepared by rapid solidification method, and microstructures, Seebeck coefficients, and electrical conductivities of the alloys are measured. Combined with the predicted results by solving the Boltzmann transport equation, electronic transport performances are compared and analyzed. The results indicate that both Sn and Si sites are equally susceptible to Sb and Bi doping, but the Si sites are preferentially substituted due to their lower ∆&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;f&lt;/sub&gt; values. Doped Bi atoms provide a higher electron concentration, and Sb atoms offer higher carrier effective mass. Thus, the maximum &lt;i&gt;σ&lt;/i&gt; value of the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.615&lt;/sub&gt;Bi&lt;sub&gt;0.01&lt;/sub&gt; alloy is 1620 S/cm. The maximum &lt;i&gt;S&lt;/i&gt; value of the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.365&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt;Sb&lt;sub&gt;0.01&lt;/sub&gt; alloy is –228 μV/K. Correspondingly, the highest &lt;i&gt;PF&lt;/i&gt; value for this alloy is 4.49 mW/(m·K) at &lt;i&gt;T&lt;/i&gt; = 800 K because of the dominant role of &lt;i&gt;S&lt;/i&gt; values. Although its power factor is slightly lower, the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.615&lt;/sub&gt;Sb&lt;sub&gt;0.01&lt;/sub&gt; alloy is expected to exhibit lower lattice thermal conductivity due to the lattice shrinkage caused by substituting Sb sites for Sn sites. The optimal doping concentration of the Bi-doped alloy is lower than that of the Sb-doped alloy. These results are expected to provide a significant reference for optimizing the experimental performance of Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn)-based alloys.

Effect of Different Sb/Bi Substituted Sites on Electronic Transport Properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; Alloy

Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn)-based thermoelectric materials, which are environmentally friendly and low-cost, have great development potential at moderate temperature range. Electronic transport properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;1-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Sn&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; alloys can be optimized by doping elements. Doping is still one of the most effective methods for optimizing electronic transport performance, such as carrier concentration, mobility, and effective mass. The most effective doping elements are Sb and Bi. A significant amount of attention has been focused on the influence of element type and doping content. Different substituted sites will also have a great impact on the electronic transport parameters. In this work, the defect formation energy values of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; alloys for Sb/Bi atoms substituting Sn/Si sites were calculated by first-principles calculations. The influence on electronic transport parameters was systematically analyzed combined with the calculated results of band structures and density of states. Corresponding component Sb/Bi doped Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt; alloys were prepared by rapid solidification method, and microstructures, Seebeck coefficients, and electrical conductivities of the alloys were measured. Combined with the predicted results by solving the Boltzmann transport equation, electronic transport performance was compared and analyzed. The results indicate that, both Sn and Si sites were equally susceptible to Sb and Bi doping, but the Si sites were preferentially substituted due to their lower ∆&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;f&lt;/sub&gt; values. Doped Bi atoms provided a higher electron concentration, and Sb atoms provided a higher carrier effective mass. Thus, the maximum &lt;i&gt;σ&lt;/i&gt; value of the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.615&lt;/sub&gt;Bi&lt;sub&gt;0.01&lt;/sub&gt; alloy was 1620 Scm&lt;sup&gt;-1&lt;/sup&gt;. The maximum &lt;i&gt;S&lt;/i&gt; value of the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.365&lt;/sub&gt;Sn&lt;sub&gt;0.625&lt;/sub&gt;Sb&lt;sub&gt;0.01&lt;/sub&gt; alloy was -228 μVK&lt;sup&gt;-1&lt;/sup&gt;. Correspondingly, the highest &lt;i&gt;PF&lt;/i&gt; value for this alloy was 4.49 mWm&lt;sup&gt;-1&lt;/sup&gt;K&lt;sup&gt;-1&lt;/sup&gt; at &lt;i&gt;T&lt;/i&gt;=800 K because the dominant role of &lt;i&gt;S&lt;/i&gt; values. Although its power factor was slightly lower, the Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.375&lt;/sub&gt;Sn&lt;sub&gt;0.615&lt;/sub&gt;Sb&lt;sub&gt;0.01&lt;/sub&gt; alloy was expected to exhibit lower lattice thermal conductivity due to the lattice shrinkage caused by Sb substituting Sn sites. The optimal doping concentration of the Bi-doped alloys was lower than that of the Sb-doped alloys. These results are expected to provide a significant reference for the experimental performance optimization of Mg&lt;sub&gt;2&lt;/sub&gt;(Si, Sn)-based alloys.

Studying rapid solidification microstructure evolution in hypoeutectic ternary Al(Cu-Ag) alloys by fast in-situ and post-mortem TEM experiments

Studying rapid solidification microstructure evolution in hypoeutectic ternary Al(Cu-Ag) alloys by fast in-situ and post-mortem TEM experiments

Rapid solidification microstructure evolution and grain refinement of deeply undercooled nickel alloys

High undercoolings of Ni75Cu25, Ni70Cu30 and Ni65Cu35 alloys were achieved by combined methods of cyclic superheating and molten glass purification. The average grain size of the undercooled alloys was decreasing with increasing Cu atomic percent. In the obtained undercooling ranges, it could be found that there were two grain refinement events at low undercoolings and high undercoolings. The grain refinement at low undercoolings was caused by dendrite remelting, while high-angle grain boundary and annealing twins indicated that recrystallization had occurred at high undercoolings. Applying TEM method, it was found that many dislocations and twins were present in the substructure at high undercoolings. The microhardness of Ni65Cu35 alloy with undercooling was sharply decreased at the critical undercooling confirmed that the stress accumulated in the microstructure at high undercoolings was consumed due to recrystallization process.

Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel

The solidified microstructure and carbide precipitation behavior in an S390 high-speed steel processed by electron beam melting (EBM) have been fully characterized. The as-EBM microstructure consists of discontinuous network of very fine primary carbides dispersed in auto-tempered martensite matrix together with a limited amount of retained austenite. The carbide network consists of M2C/M6C and MC carbides. Both the columnar and near-equiaxed grain structures were found in as-EBM microstructure and the presence of inter-dendritic eutectic carbides assisted in revealing the dendritic solidification nature. The top-layer microstructure observation confirmed that the columnar dendritic structured grains were located adjacent to the micro-melt pool boundary, indicating an epitaxial growth with the average growth direction parallel to the maximum thermal gradient. At the center of the micro-melt pool, the near-equiaxed grains were developed by dendritic growth parallel to the beam traveling direction. The carbide decomposition was revealed by scanning transmission electron microscopy and confirmed by transmission Kikuchi diffraction. The MC carbides (rich in V followed by W) nucleated at the interface between M2C (W, Fe, Mo, and Co in the order of significance) and the matrix and then grew from the outside inward, but their nucleation might occur from the M2C carbide itself. The thermal effect induced by the adjacent scan lines seems to trigger a solid-state phase transformation of MC → M2C + γ-Fe. The elemental migration was theoretically calculated and compared with the experimental results. The high hardness of ~ 65 HRC and good transverse rupture strength of ~ 2500 MPa in as-EBM S390 means that EBM processing can be used to fabricate highly alloyed tool steels. With the help of the post-processing heat treatment, the best Rockwell hardness of 73.1±0.2 HRC and transverse rupture strength of 3012±34 MPa can be obtained.

Coupling effect of undercooling and cooling on Ti–Al–V alloy solidification

Rapid solidification is one of the most significant studies for titanium alloys. In this paper, we investigated the rapid solidification of Ti–Al–V alloy micro-droplets by a drop tube. The generally ignored coupling effect of undercooling and cooling on the solidification was explored, which associated with inherent solute contents. According to the results of thermal analysis, as well as the calculation of undercooling and cooling rate, a valid model was proposed to discuss the β → α phase transition. It suggested that the microstructures of supersaturated Ti–Al–V alloys hardly preserved primary β phase in the rapid solidification. Moreover, the formation energy calculated from first principle complementally indicated that excess addition of vanadium was against to the stability of β phase. Accordingly, the rapid solidification paths and microstructure evolutions were summarized, which was explained by the calculation results. The final microstructures were all composed of α phase with various grain configurations, including lamellar and dendritic crystals, evolving with the decrease of droplet diameter. The excess solutes changed the way of dendrite growth from the regular dendrite to worm-like dendrite. Note that the competitive nucleation behavior between β phase and α phase was clarified in the rapid solidification. In addition, the mechanical properties of master alloys and solidified droplets were also studied.

Finite interface dissipation phase field modeling of Ni–Nb under additive manufacturing conditions

During laser powder bed fusion (L-PBF) parts undergo multiple rapid heating-cooling cycles, leading to complex microstructures with nonuniform properties. In the present work, a computational framework which weakly couples a finite element thermal model to a non-equilibrium PF model was developed to investigate the rapid solidification microstructure of a Ni–Nb alloy during L-PBF. The framework is utilized to predict the spatial variation of the morphology and size of cellular segregation structures as well as the differences in melt pool microstructures obtained under different process conditions. A solidification map demonstrating the variation of microstructural features as a function of the temperature gradient and growth rate is presented. A planar to cellular transition is predicted in the majority of keyhole mode melt pools, while a planar interface is predominant in conduction mode melt pools. The predicted morphology and size of the cellular segregation structure agree well with experimental measurements.

The significance of spatial length scales and solute segregation in strengthening rapid solidification microstructures of 316L stainless steel

Selective laser melting (SLM) can produce outstanding mechanical properties in 316L stainless steel. Nonetheless, the technique can lead to considerable variation in quality. This reflects an incomplete understanding and control of the process-structure-properties linkage. This paper demonstrates how length-scale informed micromechanical behavior can be linked to solidification microstructures and how these structures depend on SLM process conditions. This linkage is produced by sequential phase field and crystal plasticity simulations. Rapid solidification is described with a recent quantitative phase field model with solute trapping kinetics, where a range of process conditions are considered in terms of thermal gradients and pulling speeds. The predicted morphological transitions (dendritic-cellular-planar) are consistent with experiments, including segregation-free microstructures, which emerge in planar growth conditions. The predicted cell spacing vs. cooling rate data are also consistent with experiments. The simulated cellular structures produced through phase field modeling are then analyzed with a Cosserat crystal plasticity model with calibrated length-scale and hardening effects and with a solid solution strengthening description that depends on the local microsegregation. It is found that the length scale characteristics and solute segregation greatly influence the overall hardening behavior and affect plastic localization and the evolution of geometrically necessary dislocation (GND) type hardening. Our results suggest that the material strength of SLM 316L steel is more sensitive to cell spacing (microstructural length scale) than to the magnitude of solute segregation. Pulling speed (solidification velocity) is identified as the main process condition determining the material micromechanical behavior. Further analysis of idealized polycrystalline structures demonstrated that plastic incompatibilities and subgrain cell interactions with grain boundaries lead to notable strengthening. The presented sequential phase field-crystal plasticity modeling scheme is a proof-of-concept for systematically investigating and discovering new compositions, process conditions and microstructures for SLM.

The Significance of Spatial Length Scales and Solute Segregation in Strengthening Rapid Solidification Microstructures of 316L Stainless Steel

Selective laser melting (SLM) is a novel manufacturing technique for producing complex parts, with unique mechanical properties that result from rapidly solidified structures. For SLM of austenitic 316L stainless steel, the reported mechanical properties have a large scatter, which reflects an incomplete understanding and control of the process-structure-properties linkage. This paper demonstrates how material micromechanical behavior --- including length scale dependent yield strength and hardening --- is linked to rapid solidification microstructure, and how this structure emerges from typical SLM process conditions. This linkage is produced by concurrently coupling phase field simulations with crystal plasticity modeling. In particular, the evolution of rapidly solidified SLM microstructures is described with a quantitative phase field model with imposed solute trapping kinetics. A range of process conditions are considered by varying the thermal gradient and pulling speed driving solidification. The phase field model can predict morphological transitions (dendritic-cellular-planar), microsegregation, and emergent microstructure length scales, as function of process conditions. Both 2D and 3D phase field simulations are conducted, yielding microstructure morphologies and cell spacing Vs. cooling rate data that are in good agreement with the available experiments in the literature. Micromechanical response of domains characterized by cellular subgrain solidification structures, generated from phase simulations, are analyzed with a Cosserat crystal plasticity model which includes calibrated length-scale and hardening effects, and a realistic description of solid solution strengthening. It is shown that length scale characteristics (cell spacing and grain size) and solute segregation greatly influence the overall hardening behavior, and also affect plastic localization and evolution of geometrically necessary dislocation (GND) type hardening. We conclude with a study of the micromechanical behavior of a polycrystalline structure, focusing on the effects of cell spacing and grain size, and the relative orientation between crystalline lattices and cells. Our results suggest that the material strength is more sensitive to the cell spacing (microstructural length scale) than to the magnitude of solute segregation. The concurrently coupled phase field-crystal plasticity modeling scheme is presented as a proof-of-concept for systematically investigating and discovering new compositions, processes and microstructures for additive manufacturing of metals.

Direct observation of dendrite fragmentation in the solidification of undercooled melts

The fragmentation of dendrites immediately following the recalescence phase of growth during the solidification of undercooled melts has been invoked to explain various rapid solidification microstructures. Despite this, little direct evidence of such a fragmentation process usually survives in the as-solidified material. We report on the rapid solidification of the single phase, congruently melting intermetallic β-Ni3Ge. During equilibrium solidification this material solidifies to the chemically ordered L12 crystal structure. Conversely, during rapid solidification, disorder trapping results in solidification to a random fcc solid solution, thereby providing a means to distinguish the rapidly solidified structures. We present results which show a range of microstructures in which the dendrite fragmentation process has been captured in progress. Results from EBSD Euler mapping reveal that dendrite fragmentation is a potential, but not particularly efficient, route to grain refinement.

Microstructure and mechanical properties of Hastelloy X produced by HP-SLM (high power selective laser melting)

In order to increase the production rate during selective laser melting (SLM), a high power laser with a large beam diameter is used to build fully dense Hastelloy X parts. Compared to SLM with a low power and small diameter beam, the productivity was increased from 6 mm3/s to 16 mm3/s, i.e. 2.6 times faster. Besides the productivity benefit, the influence of the use of a high power laser on the rapid solidification microstructure and concomitant material properties is highlighted. The current paper compares the microstructure and tensile properties of Hastelloy X built with low and high power lasers. The use of a high power laser results in wider and shallower melt pools inducing an enhanced morphological and crystallographic texture along the building direction (BD). In addition, the increased heat input results in coarser sub-grains or high density dislocation walls for samples processed with a high power laser. Additionally, the influence of hot isostatic pressing (HIP) as a post-processing technique was evaluated. After HIP, the tensile fracture strain increased as compared to the strain in the as-built state and helped in obtaining competitive mechanical properties as compared to conventionally processed Hastelloy X parts.