Families Of Grains Research Articles

Study on location-related thermal cycles and microstructure variation of additively manufactured inconel 718

The complicated thermal history of wire + arc additive manufacturing (WAAM) will affect the microstructure variation and mechanical properties of the as-deposited material. Numerial models of Inconel 718 in WAAM deposition were established and the location-related thermal history and temperature distribution were analyzed. A hybrid method of WAAM and cold rolling was investigated and its effect on the microstructure distribution and texture was investigated compared to that in as-deposited condition. The results show that WAAM deposition features repeated thermal cycles, high heating rate and low cooling rate. The trough values of the thermal cycles first increases and then decreases, while the peak temperature always decreases with the proceeding of the deposition process. The as-deposited samples show columnar dendrites and its average dendrite arm spacing increases with the increased build height due to the location-related heat accumulation. The strongly textured columnar dendrites with preferred <001> orientation transform to equiaxed grains with random orientation in heat-affected zone after cold rolling process, and its average size decreases with the increased rolling force. The as-deposited samples show the strongest intensity of 7.609 for the {100} family of grains oriented along the transverse direction; while it decreases to 3.629 and 2.057 for the cold rolled conditions with the force of 50 kN and 75 kN, respectively. The relationship between thermal history and the microstructure distribution was discussed. The mechanism of hybrid WAAM and cold rolling method consisted of spatially and temporally heterogeneous work hardening and recrystallization.

Abnormal grain growth in ultrafine grained Ni under high-cycle loading

Abnormal grain growth can occur in polycrystalline materials with only a fraction of grains growing drastically to consume other grains. Here we report abnormal grain growth in ultrafine grained metal in a rarely explored high-cycle loading regime at ambient temperature. Abnormal grain growth is observed in electroplated Ni microbeams with average initial grain sizes less than 640 nm under a large number of loading cycles (up to 109) with low strain amplitudes (< 0.3%). Such abnormal grain growth occurs predominantly in the family of grains whose <100> orientation is along the tensile/compressive loading direction. Micromechanics analysis suggests that the elastic anisotropy of grains dictates the thermodynamic driving force of abnormal grain growth, such that the lowest strain energy density of the <100> oriented grain family dominates grain growth. This work unveils a unique type of abnormal grain growth that may be harnessed to tailor grain microstructures in materials.

Microscale residual stresses in additively manufactured stainless steel

Additively manufactured (AM) metallic materials commonly possess substantial microscale internal stresses that manifest as intergranular and intragranular residual stresses. However, the impact of these residual stresses on the mechanical behaviour of AM materials remains unexplored. Here we combine in situ synchrotron X-ray diffraction experiments and computational modelling to quantify the lattice strains in different families of grains with specific orientations and associated intergranular residual stresses in an AM 316L stainless steel under uniaxial tension. We measure pronounced tension–compression asymmetries in yield strength and work hardening for as-printed stainless steel, and show they are associated with back stresses originating from heterogeneous dislocation distributions and resultant intragranular residual stresses. We further report that heat treatment relieves microscale residual stresses, thereby reducing the tension–compression asymmetries and altering work-hardening behaviour. This work establishes the mechanistic connections between the microscale residual stresses and mechanical behaviour of AM stainless steel.

PARTITIONING TUNGSTEN BETWEEN MATRIX PRECURSORS AND CHONDRULE PRECURSORS THROUGH RELATIVE SETTLING

ABSTRACT Recent studies of chondrites have found a tungsten isotopic anomaly between chondrules and matrix. Given the refractory nature of tungsten, this implies that W was carried into the solar nebula by at least two distinct families of pre-solar grains. The observed chondrule/matrix split requires that the distinct families were kept separate during the dust coagulation process, and that the two families of grain interacted with the chondrule formation mechanism differently. We take the co-existence of different families of solids in the same general orbital region at the chondrule-precursor size as given, and explore the requirements for them to have interacted with the chondrule formation process at significantly different rates. We show that this sorting of families of solids into chondrule- and matrix-destined dust had to have been at least as powerful a sorting mechanism as the relative settling of aerodynamically distinct grains at least two scale heights above the midplane. The requirement that the chondrule formation mechanism was correlated in some fashion with a dust-grain sorting mechanism argues strongly for spatially localized chondrule formation mechanisms such as turbulent dissipation in non-thermally ionized disk surface layers, and argues against volume-filling mechanisms such as planetesimal bow shocks.

A study of deformation twinning in a titanium alloy by X-ray diffraction contrast tomography

A specimen of Ti–4Al deformed in-situ to about 0.7% compressive strain using neutron diffraction, and showing early stages of twinning, has been investigated in 3D using synchrotron X-ray diffraction contrast tomography (DCT). A cylindrical volume of 900 μm × 400 μm diameter was observed using DCT, containing about 400 grains of which almost 60 grains were identified to have twinned predominantly by {101¯2} 〈1¯011〉 011 tensile twinning. To consider possible twin nucleation criteria, non-twinned grains of similar orientation to the twinned grains were compared against the family of twinned grains. Such comparison highlights that the twinned grain family has a grain size distribution shifted to a higher mean value than the corresponding family of grains that has not twinned. An initial neighbourhood analysis did not reveal any significant differences of the two grain families. However, complex twin chains and clusters were identified forming a slightly imperfect network demonstrating the importance of the 3D analysis. Analysis of the parent grain orientations within those chains/clusters using the Luster-Morris parameter revealed a significantly higher propensity of prismatic 〈a〉 slip transfer compared to the neighbourhood of the non-twinned grain family while no difference was observed for the likelihood of twin shear transfer. The findings suggest that grain chains/clusters with high ability of prismatic 〈a〉 slip activity and slip transfer does promote formation and clustering of twins, which is likely associated with the build up of tensile intergranular strain along the 〈c〉 axis perpendicular to the loading direction recently suggested by crystal plasticity modelling.

Prediction of microstructure evolution during multi-stand shape rolling of nickel-base superalloys

AbstractIn this paper, a comprehensive numerical approach to predict the microstructure of nickel-base superalloys during multi-stand shape rolling is presented. This approach takes into account the severe deformation that occurs during each pass and also the possible reheating between passes. In predicting the grain size at the end of the rolling process, microstructural events such as dynamic recrystallization (DRX), metadynamic recrystallization (MDRX), and static grain growth are captured at every deformation step for superalloys. Empirical relationships between the average grain size from various microstructural processes and the macroscopic variables such as temperature (T) and effective strain ( ε ̄ ) and strain rate ( ε ̄ ̇ ) form the basis for the current work. These empirical relationships are based on Avrami equations. The macroscopic variables are calculated using a finite element analysis package wherein the material being rolled is modeled as a non-Newtonian fluid with viscosity that depends on the effective strain rate, strain, and temperature. A two-dimensional transient thermal analysis is carried out between passes that can capture the MDRX and/or static grain growth during the microstructural evolution. The presented microstructure prediction algorithm continuously updates two families of grains, namely, the recrystallized family and strained family at the start of deformation in any given pass. In addition, the algorithm calculates various subgroups within these two families at every deformation step within a pass. As the material undergoes deformation between the rolls, recrystallization equations are invoked depending on critical strain and strain rate conditions that are characteristics of superalloys. This approach predicts the microstructural evolution based on recrystallization kinetics and static grain growth only. The methodology was successfully applied to predict the microstructure evolution during the multi-pass rolling of nickel-base superalloys. The predicted results for Alloy 718 for a 4-stand rolling followed by air cooling and for a 16-stand rolling followed by a combination of air and water cooling are also compared with experimental observations.

Application of a Dislocation Density-Based Constitutive Model to Al-Alloyed TWIP Steel

High Mn steels exhibit an exceptional combination of high strength and large ductility owing to their high strain-hardening rate during deformation. The addition of Al is needed to improve the mechanical performance of TWIP steel by means of the control of the stacking fault energy. In this study, a constitutive modeling approach, which can describe the strain-hardening behavior and the effect of Al on the mechanical properties, was used. In order to understand the deformation behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels, a comparative study of the microstructural evolution was conducted by means of transmission electron microscopy and electron backscatter diffraction. The microstructure analysis focused on dislocations, stacking faults, and mechanical twins as these are the defects controlling the strain-hardening behavior of TWIP steels. A comparison of the strain-hardening behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels was made in terms of a dislocation density-based constitutive model that goes back to the Kubin–Estrin model. The densities of mobile and forest dislocations are coupled in order to account for the interaction between the two dislocation populations during straining. The model was used to estimate the contribution of dynamic strain aging to the flow stress. As deformation twinning occurred only in a subset of the grains, the grain population was subdivided into twinned grains and twin-free grains. Different constitutive equations were used for the two families of grains. The analysis revealed that (i) the grain size and dynamic recovery effects determine the strain-hardening behavior of the twin-free grains, (ii) the deformation twins, which act as effective barriers to dislocation motion, are the predominant elements of the microstructure that governs the strain hardening of the twinned grains, and (iii) the DSA contribution to strain hardening of TWIP steel is only minor.

Indium tin oxide coatings properties as a function of the deposition atmosphere

Indium tin oxide coatings were deposited by magnetron sputtering physical vapour deposition under different atmospheres. Microstructural, electrical, and optical properties were measured, finding a correlation among properties and process parameters. Texture analyses carried out by X-ray diffraction showed that films microstructure depended by the oxygen content in the deposition vessel: high values of the oxygen content (e.g., 5%) caused the film to grow along the <111> orientation; under pure Ar, the grains grew along the <100> orientation. Intermediate values of the oxygen content caused the growth of two families of grains, respectively oriented along the <111> and the <100> directions.

X-ray line profiles analysis of plastically deformed metals

Abstract The theoretical basis of X-ray line profile analysis and its application to microstructural characterization of plastically deformed metallic alloys is presented. The microstructure is described in terms of coherent domain size, planar fault density, dislocation density and a dislocation arrangement parameter. Two evaluation methods are introduced: the momentum method and the extended Convolutional Multiple Whole Profile fit procedure. Their use is exemplified on plastically deformed single crystals, single grains residing in the bulk of a polycrystal and family of grains making up texture components. The selected examples show the potential of X-ray line profile analysis applied to diffraction patterns recorded with laboratory or synchrotron sources.

Self-consistent modelling of the plastic deformation of f.c.c. polycrystals and its implications for diffraction measurements of internal stresses

Using a self consistent scheme we model the development of elastic lattice strains during uniaxial loading for selected families of grains with specific orientations. These lattice strains vary dramatically for the different grain orientations, and most families of grains show a high degree of non-linearity at the start of the plastic regime. The 311 reflection does, however, respond almost linearly to loading, and therefore it constitutes a suitable reflection for characterization of macroscopic stresses and strains by diffraction for the given conditions. As a consequence of the high degree of non-linearity in the lattice strain response during loading highly anisotropic intergranular residual lattice strains develop during unloading. The evaluation of the model predictions by neutron diffraction is exemplified by selected results from in-situ loading experiments performed on austenitic stainless steel specimens. As a necessary condition for the proper understanding of the results we have included a description of the slip pattern resulting from the model applied and its relation to the slip patterns derived from the upper-bound Taylor model and the lower-bound Sachs model.

About the shape of eutectic grains solidifying in a thermal gradient

In eutectic alloys solidified in a thermal gradient, it has been observed that the final shape of the grains nucleated in the bulk of the liquid is more elongated in the direction opposite to that of the heat flow. This experimental result appears to be in contradiction with that expected for the growth of an isolated grain, since the portion of the interface located in the downstream heat flow direction is the most undercooled and thus has the highest growth rate. However, when considering a family of grains which continuously nucleate in the bulk of the liquid, it is shown that the impingement of the grains limits their growth in the downstream heat flow direction and thus explains their final shape. In order to investigate this phenomenon, the differential equation which governs the growth of an isolated eutectic grain in a thermal gradient has been derived and solved analytically for the two extreme positions of the interface along the heat flow direction. Using these relationships, the asymmetry factor of the grains has been deduced as a function of the solidification parameters. The overall shape of an isolated grain has also been predicted using numerical integration. Finally, these results are integrated into a stochastic model of grain structure formation and the simulated microstructure is compared with experimental micrographs previously obtained for hypereutectic aluminium-silicon alloys remelted by laser.

Characteristics and mass distribution of extraterrestrial dust from the Greenland ice cap

Extraterrestrial grains with sizes ≳50 µm can now be extracted in large numbers from terrestrial sediments but their origins are still uncertain. At least 99% of these grains have been thought to be spherules1,2 resulting from the melting of their parent bodies in the atmosphere, which destroyed their mineralogical composition. Here we present analyses of extraterrestrial grains extracted from six samples of cryoconite3 (black dust) collected from the melt zone of the Greenland ice cap. In addition to families of grains never reported before, we have found a surprisingly high abundance of unmelted chondritic fragments, in which the nonvolatile component of the parent bodies has been well preserved. The mass distribution of the grains is very similar to that of the micrometeorite flux at 1 AU (ref. 4), indicating that most of the grains are micrometeorites, and not ablation products of larger meteorites. These results yield new constraints on the ablation of such Greenland micrometeorites, as well as guidelines for using them as glaciological and geological markers.

Yield loci for polycrystalline metals without texture

Calculations of the yielding behaviour of polycrystals are usually performed with the hypothesis that there is an identical critical resolved shear stress (CRSS) value on all the slip systems. The influences of different initial CRSS values and of different CRSS values induced by a prestrain on the yield surface are investigated for materials with a random grain orientation distribution. Calculations are made for a family of grains with particular orientations, submitted to a shear strain. The results are extended to more general cases and compared with experiments.

The Fabric of Polycrystalline Ice Deformed in Simple Shear: Experiments in Torsion, Natural Deformation and Geometrical Interpretation

Three cylinders of artificial ice have been deformed in torsion at about –10℃ up to finite shear strains γ of 0.6, 0.95 and 2. The initial random lattice orientation rapidly evolves into a bimodal distribution of the basal slip planes as already observed by Kamb (1972) and Duval (1981) for low‐strains experiments near the melting point. For the γ = 0.6 and 0.95 experiments, one family of grains (&gt; 50%) corresponds to basal planes tending to parallel the imposed shear plane; the basal planes of the other family make a broader maximum at about 60° from the shear plane. The direction of minimum concentration between the two populations approximately corresponds to the flattening plane or to the elongation direction of the strain ellipsoid. With increasing strain (γ = 2) the second submaximum vanishes and only the principal maximum parallel to the shear plane remains. This evolution is conformable with the data of Hudleston (1977) in a natural shear zone in glacial ice; it also compares remarkably well with Etchecopar′s (1977) geometrical computer model of simple shear in the same range of γ values. Single slip on the basal plane with no preferential slip direction in that plane can explain the analogy between fabrics in ice deformed in plane strain and fabrics obtained from the two‐dimensional computer model.The bimodal distribution reflects predominant slip on the basal plane; the progressively increasing heterogeneous strain enhances internal distorsion, rigid body rotation and recrystallization of grains unfavorably oriented for further slip, leading to the unimodal distribution. The adequacy of fabric analyses to infer the strain regime and the sense of shear in plastically deformed rocks is strengthened.

On Behavior of Granular Materials in Simple Shear

Simple shear deformation under vertical pressure of granular materials which consist of cohesionless rigid grains, is considered. Based on the mechanics of the relative motion of the grains or families of grains at the microlevel, and by considering the corresponding rate of frictional losses, a simple energy equation is obtained, on the basis of which the phenomena of initial densification, subsequent dilatancy for a dense sample, and the net amount of densification which accompanies cyclic shearing, are explained.