Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

International audienceA coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intragrain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

A coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intra-grain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach

The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals

Additively manufactured (AM) metals exhibit highly complex microstructures, particularly with respect to grain morphology which typically features heterogeneous grain size distribution, anomalous and anisotropic grain shapes, and the so-called columnar grains. In general, the conventional morphological descriptors are not suitable to represent complex and anisotropic grain morphology of AM microstructures. The principal aspect of microstructural grain morphology is the state of grain boundary spacing or grain size whose effect on the mechanical response is known to be crucial. In this paper, we formally introduce the notions of axial grain size and grain size anisotropy as robust morphological descriptors which can concisely represent highly complex grain morphologies. We instantiated a discrete sample of polycrystalline aggregate as a representative volume element (RVE) which has random crystallographic orientation and misorientation distributions. However, the instantiated RVE incorporates the typical morphological features of AM microstructures including distinctive grain size heterogeneity and anisotropic grain size owing to its pronounced columnar grain morphology. We ensured that any anisotropy arising in the macroscopic mechanical response of the instantiated sample is mainly associated with its underlying anisotropic grain size. The RVE was then used for meso-scale full-field crystal plasticity simulations corresponding to uniaxial tensile deformation along different axes via a spectral solver and a physics-based crystal plasticity constitutive model. Through the numerical analyses, we were able to isolate the contribution of anisotropic grain size to the anisotropy in the mechanical response of polycrystalline aggregates, particularly those with the characteristic complex grain morphology of AM metals. Such a contribution can be described by an inverse square relation

Additive manufacturing of interstitial-strengthened high entropy alloy: Scanning strategy dependent anisotropic mechanical properties

A non-equiatomic interstitial-strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), is manufactured by laser powder-bed fusion (LPBF) with stripe and chessboard scanning strategies. The present study highlights the correlation between the laser scanning strategies with resulting microstructure, textures, and anisotropic mechanical properties in as-built iHEA. The results show that the LPBF processed iHEA exhibits an excellent strength-ductility synergy due to the combined deformation mechanisms of dislocation slip, martensite phase transformation- and nano twinning-induced plasticity. The samples printed by the stripe scanning strategy show more evident mechanical anisotropy than that of the chessboard-scanned samples. The difference in the degree of mechanical anisotropy is mainly attributed to the heterogeneous grain morphology and crystallographic texture resulted from different scanning strategies

Experimental characterization and crystal plasticity modeling of mechanical properties and microstructure evolution of additively manufactured Inconel 718 superalloy

In this thesis, the mechanical behavior of the additively manufactured (AM) IN718 nickel-based superalloy and their correlations with the evolution of microstructure are studied comprehensively. The effects of manufacturing parameters, build orientations, and post processing procedures, i.e. standard heat treatment and hot isostatic pressing (HIP), on various mechanical properties including monotonic compression and tension strength, low cyclic fatigue performance, high cyclic fatigue behaviour, and fatigue crack growth behavior are investigated. Due to the high temperature applications of the IN718 alloy, elevated temperature properties are examined as well. Electron Backscattered Diffraction (EBSD) technique is employed to measure the initial and deformed textures. In addition, an elasto-plastic self-consistent polycrystal plasticity model is developed to interpret the deformation behavior of the alloy in room temperature and high temperatures. The model incorporates the contributions of solid solution, precipitates shearing, and grain size and shape effects into the initial slip resistance. For activating the slip systems, the non-Schmid effects and backstress are implemented in the model. The crystal plasticity model is capable of simulating the monotonic and large-strain load reversal cycles of the material with pole figure difference (PFD) values no more than 0.2

Additive manufactured high entropy alloys:A review of the microstructure and properties

High entropy alloys (HEAs) are promising multi-component alloys with unique combination of novel microstructures and excellent properties. However, there are still certain limitations in the fabrication of HEAs by conventional methods. Additive manufactured HEAs exhibit optimized microstructures and improved properties, and there is a significantly increasing trend on the application of additive manufacturing (AM) techniques in producing HEAs in recent years. This review summarizes the additive manufactured HEAs in terms of microstructure characteristics, mechanical and some functional properties reported so far, and provides readers with a fundamental understanding of this research field. We first briefly review the application of AM methods and the applied HEAs systems, then the microstructure including the relative density, residual stress, grain structure, texture and dislocation networks, element distribution, precipitations and the influence of post-treatment on the microstructural evolution, next the mechanical properties consisting of hardness, tensile properties, compressive properties, cryogenic and high-temperature properties, fatigue properties, creep behavior, post-treatment effect and the strengthening mechanisms analysis. Thereafter, emerging functional properties of additive manufactured HEAs, namely the corrosion resistance, oxidation behaviors, magnetic properties as well as hydrogen storage properties are discussed, respectively. Finally, the current challenges and future work are proposed based on the current research status of this topic

Small-Scale Characterization Of Additive Manufactured Ti-6al-4v Alloy Through Instrumented Indentation

Ti-6Al-4V alloy has been favored by the transportation applications in the automotive and aerospace industries due to its good combination of excellent physical and mechanical properties. Ti alloys are naturally suited to additive manufacturing (AM) method, a layer wise manufacturing technique, since conventional manufacturing method of Ti alloys are quiet challenging. However, cooling rate and thermal processing history of AM Ti-6Al-4V alloy are quite different in comparison to conventionally fabricated Ti-6Al-4V alloy which leads to undesirable microstructures in the AM Ti-6Al-4V alloy with respect to large columnar prior β grains being found to grow potentially across the entire height from bottom layer to top layer. Therefore, it is required to assess the microstructure-process-structure-property-performance relationship of the additive manufactured Ti-6Al-4V alloy to assess whether it could meet the demands of engineering design considerations. The samples studied in this research were prepared using laser powder bed fusion (L-PBF) method, a well-developed AM process to print Ti-6Al-4V alloy in different scan direction and scan size. Instrumented indentation testing technique, a robust, reliable, convenient, and non-destructive characterization method to study small-scale mechanical properties in metals and alloys at ambient and elevated temperatures, was used to assess ambient-temperature indentation creep of AM Ti-6Al-4V alloy. To examine depth-sensing indentation creep behavior of Ti-6Al-4V alloy at ambient temperature, a dual-stage scheme (loading followed by a constant load-holding and unloading) at different peak loads of 250 mN, 350 mN, and 450 mN with holding time of 400 s was performed. Creep parameters i.e. creep rate, creep stress exponent, and indentation size effect were analyzed and compared with conventional findings, according to the Oliver and Pharr method, at different additive manufacturing scan directions and scan sizes. The effect of post heat treatment (i.e. aging and solutionizing with different cooling rates) on the microstructure and micromechanical properties of a Ti-6Al-4V alloy processed by laser powder bed fusion (L-PBF) technique is studied. Heat treatment cycles employed in this study include solutionizing at 950 °C (for 1 h) followed by three different cooling rates (water quench, air cooling, and furnace cooling). A separate set of samples were also used toward artificial aging (solutionizing followed by water quenching and artificial aging). To assess small-scale properties of as-printed/ heat treated materials, instrumented nanoindentation testing technique as a robust, convenient, and non-destructive approach is employed. The martensitic α and ά in as -printed Ti-6Al-4V alloy grows in lamellar structure in epitaxial way upon various heat treatments below β- transus temperature. With the relatively steep cooling rate, the β phase recrystallization transforms into a compact secondary basket-weave α phase since the primary α-phase develops and connects each other with different orientations. xvi Microstructural quantitative analyses (i.e. optical microscopy and scanning electron microscopy) were performed as well to assess processing parameter-microstructure-property correlations in the additively manufacture Ti-6Al-4V alloy. These studies were done in parallel to the two main tasks of this project to be able to elaborate the mechanical measurements with microstructural evidences. Also, the obtained results were compared against traditionally processed Ti-6Al-4V

Mechanics of Heterogeneous Metallic Materials

Overcoming the strength-ductility tradeoff is a widely pursued goal in the materials community. In recent years, design, fabrication, and optimization of heterogeneous microstructures have been extensively explored to achieve exceptional combinations of strength and ductility. However, there is currently a critical lack of the mechanics understanding of heterogeneous microstructures. In general, structural heterogeneities generate mechanical heterogeneities that are manifested as spatially non-uniform back stresses and forward stresses. These long-range, directional internal stresses can result in enhanced yield strength, work hardening, and tensile ductility. To understand the effects of heterogeneous microstructures and associated internal stresses on mechanical properties, this thesis is focused on development of novel constitutive and atomistic models for several emergent heterogeneous material systems, including additively manufactured metal alloys, gradient nanotwinned metals, nanocrystalline thin films, and nanodispersion-strengthened composites. Overall, the thesis research provides a new framework to bridge the structural heterogeneities and mechanical heterogeneities in several heterogeneous material systems through new constitutive models of strain gradient plasticity, internal-stress-dependent crystal plasticity, and dual-phase crystal plasticity. Atomistic simulations uncover the critical deformation processes that are strength/rate-controlling. Coupled with novel material processing, characterization, and testing, the modeling and simulation results offer quantitative predictions and mechanistic insights toward the design of heterogeneous metallic materials with improved combinations of strength and ductility.Ph.D

On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail