Investigations of MX and γ′/γ″ precipitates in the nickel-based superalloy 718 produced by electron beam melting

Samples with a composition similar to the nickel-based superalloy Inconel alloy 718 were produced by electron beam melting of prealloyed powder and investigated with respect to type and composition of the strengthening precipitates. The matrix consists of γ grains orientated in nearly the same direction, almost like a single crystal. Coarse precipitates (<2 μm), mostly of the (Ti,Nb)(C,N,B) type with B1 structure, are aligned along the growth direction. TEM and APFIM investigations of the γ matrix revealed very fine γ″ precipitates of around 5–10 nm in size. Additionally, at small angle grain boundaries, coarser γ″ precipitates of 50–100 nm in size have been observed. The 0 01 γ//0 0 1 γ″ and {1 0 0} γ//{1 0 0} γ″ orientation relationship between γ and γ″, known from literature [M. Sundararaman, P. Mukhopadhyay, Mater. Charact. 31 (1993) 191–196], was confirmed. Some γ′ precipitates of 2–5 nm in size were observed by means of FIM.     

The effect of heat treatment on the microstructure stability of modified 316LN stainless steel

As a conduit-sheath material for Cu–Nb–Sn wires, chemically modified 316LN steel is subjected to the same reaction heat treatment (100 h at 700 °C) used in transforming the wires into superconducting composite wires. In spite of the long annealing time at 700 °C, there was little or no change in the strength of the steel. A systematic study of the material annealed for 1, 10, 20, 50 and 100 h using orientation imaging microscopy (OIM) showed that with the exception of grain boundary precipitation at t = 100 h, the grain size and grain boundary character were stable. Our results show that twin boundaries (Σ3, Σ9, and Σ27) accounted for about 50% of the total boundaries in all the material conditions studied, suggestive that the density of twins had reached a limit. The stability of the material in spite of the prolonged heat treatment was attributed to the attainment of this maximum twin density in the as-received condition. In view of the high percentage of the twin boundaries in the microstructure, a comprehensive Hall–Petch relationship, which incorporates the contribution of the chemistry, grain size as well as twin boundaries to strengthening was developed. This upper bound theoretical strength compared favorably with the experimental value at 4 K.     

Quantitative analysis of {332}〈113〉 twinning in a Ti-15Mo alloy by in situ scanning electron microscopy

We have performed quantitative analysis of {332}〈113〉 twinning in a β-Ti-15Mo (wt.%) alloy by in situ scanning electron microscopy and electron backscattering diffraction (EBSD). Microstructure-twinning relations were evaluated by statistical analysis of the evolving twin structure upon deformation at room temperature. Our analysis reveals that at the early stages of deformation (ε < 1.5 to 2.0%), primary twinning is mainly determined by the applied macroscopic stress resolved on the twin system. Most of the primary twins (~70–80% of the analyzed twins) follow Schmid’s law with respect to the macroscopic stress, and most of the growth twins (~ 85% of the analyzed twins) correspond to the higher stressed variant. In the grain size range studied here (40–120 μm), we find that several twin parameters such as number of twins per grain and number of twins per grain boundary area exhibit grain size dependence. We ascribe these effects to the grain size dependence of twin nucleation stress and apparent critical resolved shear stress for twinning, respectively.     

Modeling the dependence of strength on grain sizes in nanocrystalline materials

Abstract

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall–Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall–Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ^?1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.     

Analyses of deformation twinning in the extruded magnesium alloy AZ31 after compressive and cyclic loading

The influence of different loading conditions on the microstructural development of extruded magnesium alloy AZ31 was investigated by optical microscopy and electron backscattered diffraction. Extruded magnesium profiles exhibit a significant asymmetry in the mechanical properties, due to the low activation energy of the extension twinning system \( \left\{ {10\overline{1} 2} \right\}\langle {10\overline{1} 1}\rangle,\) when compressing along the extrusion direction. For the analyses of this twinning system, compression tests with different applied strains 0.4 ≤ ε ≤ 11% were performed for two extrusion products exhibiting different microstructures. The main deformation mechanisms during cyclic loading are the formation of extension twins during compression and the detwinning during subsequent tensile loading. The strain-controlled fatigue tests were carried out with applied strain amplitudes 0.3 ≤ ε_A ≤ 5%. The tests were stopped at characteristic numbers of cycles N in the tensile or compression maximum of the hysteresis loop. The microstructural investigations deliver information about the type of twinning and the size, shape, local distribution, and volume fraction of twins as a function of the plastic deformation. These results will be discussed with regard to the microstructure of the initial state material and to the applied load.     

Mg-0.4Zn镁合金挤压板拉伸变形组织的演变

用电子背散射衍射(EBSD)技术结合原位拉伸,研究了在0％～20％应变条件下,Mg-0.4％Zn二元镁合金晶界、织构和裂纹的变化.结果 表明,在拉伸应变从0％增加到20％的过程中,随着应变量的增大材料微观组织中的孪晶逐渐增加.孪晶的类型以{10-12}拉伸孪晶为主;这种孪生使材料的组织织构类型发生了显著的变化,随着应变...     

Lattice rotation and stability of 22.5° ND rotated cube orientation in cold rolled polycrystalline AA 5182 aluminum alloy

AA 5182 aluminum alloy with a strong cube texture was cold rolled to different reductions at an angle of 22.5° to the prior rolling direction. The texture evolution at this new rolling direction was investigated by X-ray diffraction. The rotation paths and stability of the 22.5° ND rotated cube orientation were determined based on the variation in the three-dimensional orientation distribution function (ODF) with rolling reduction. The results show that most of the grains with the 22.5° ND rotated cube orientation are directly rotated to the β fiber along different rotation paths, but there are a few grains moving through the cube orientation to the β fiber. The {0 0 1}<1 1 0> oriented grains possess the lowest stability during rolling, and the stability increases as the initial orientation changes from the {0 0 1}<1 1 0> orientation to the {0 0 1}<1 0 0> orientation along the ₁ axis.     

Finite element simulations of notch tip fields in magnesium single crystals

Recent experiments using three point bend specimens of Mg single crystals have revealed that tensile twins of \(\{10\bar{1}2\}\) -type form profusely near a notch tip and enhance the fracture toughness through large plastic dissipation. In this work, 3D finite element simulations of these experiments are carried out using a crystal plasticity framework which includes slip and twinning to gain insights on the mechanics of fracture. The predicted load–displacement curves, slip and tensile twinning activities from finite element analysis corroborate well with the experimental observations. The numerical results are used to explore the 3D nature of the crack tip stress, plastic slip and twin volume fraction distributions near the notch root. The occurrence of tensile twinning is rationalized from the variation of normal stress ahead of the notch tip. Further, deflection of the crack path at twin–twin intersections observed in the experiments is examined from an energy standpoint by modeling discrete twins close to the notch root.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

Modeling and visualization of polycrystalline thin film growth

A novel polycrystalline thin film growth simulator, FACET, has been developed. FACET is a multi-scale model with two major components: an atomic level one-dimensional kinetic lattice Monte Carlo (1D KLMC) model and a real time feature scale two-dimensional facet nucleation and growth model.

The 1D KLMC model has been developed to calculate inter-facet diffusion rates. By inputting the diffusion activation energies, the model will calculate the inter-facet atomic flux between {1 0 0}, {1 1 0}, and {1 1 1} facets of FCC materials at any temperature. The results of the 1D KLMC model have been verified by comparison with a full three-dimensional kinetic lattice Monte Carlo (3D KLMC) model.

The feature scale polycrystalline thin film nucleation and growth model is based on describing grains in terms of two-dimensional faceted surfaces and grain boundaries. The profile of the nuclei are described by crystallographically appropriate facets. The position and orientation of the nuclei can be randomly selected or preferred textures can be created. Growth rates are determined from different deposition fluxes and surface diffusion effects. Quantitative microstructural characterization tools, including roughness analysis, average grain size analysis, and orientation distribution analysis, were incorporated into the model, which allows the users to design, conduct and analyze the virtual experiments within one integrated graphical user interface. Users can also visualize the nucleation and growth process of the film and obtain the final film microstructure. The effects of thickness, temperature, and deposition flux on thin film microstructures have been studied by FACET.     

Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 films deposited on SiO2 by molecular beam deposition

The evolution of microstructure and texture of molecular beam deposited Si_0.7Ge_0.3 films on SiO₂ at the deposition temperature range of 400–700°C was investigated by X-ray diffraction and transmission electron microscopy. At deposition temperatures between 400 and below 500°C, the films were directly deposited as a mixed-phase on SiO₂ and have a inversely cone-shaped structure. In this temperature range deposited as a mixed-phase, the grain size increases as the temperature increases, so that the grains not only grow up by deposition, but also laterally grow by the solid phase crystallization, furthermore, the texture is changed from a {110} texture to mixed {311} and {110} textures. At 500°C, the film was deposited as only a crystalline phase and has a columnar structure with a strong {110} texture. In the temperature range of 500–700°C, as the temperature increases, the {311} and {111} textures develop whereas the {110} texture reduces. The film deposited at 700°C has a random orientation and structure.     

Twinning characteristics in textured AZ31 alloy under impact loading along specified direction

A plate of hot extruded AZ31 alloy with some large grains was selected for impact tests. The occurrence of twinning was investigated. It was found that when impacted along the normal direction (ND) of the plate, {10-11}-{10-12} double twinning is commonly observed in large grains, and {10-12} tension twinning can also be activated although their c-axis is close to the compression direction. In small grains, {10-12} tension twins are favored in which case the extension strain component is parallel to the c-axis. In addition, twins with < 11-20> 75° misorientation to the parent matrix were also observed.     

A study of twinning in polyethylene

Both {1 1 0} and {3 1 0} deformation twinning modes have been predicted for polyethylene. The existence of the former twinning mode has been confirmed in this work by non-photographic X-ray methods. When comparing and explaining bulk and single crystal twinning behaviour in terms of the influence of fold surface geometry, certain discrepancies arise. These may be resolved by noting that {1 1 0} twins occur at relatively low stresses and that {3 1 0} twins are formed under conditions of high strain during which the crystalline morphology is changed from lamellar to microfibrillar.     

Formation of nanocrystalline structure of Ti–6Al–4V alloy by cyclic hydrogenation–dehydrogenation treatment

Ti–6Al–4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10–20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of As₂O₃ as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm⁻² for 3 h in 1 N H₂SO_4(aq) by adding 0.1 g L⁻¹ As₂O₃; (2) β transformation carried out at 850 °C for 1 h in air furnace, followed by a furnace cooling to 590 °C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 °C and 1.0 × 10⁻⁶ Torr for 10 h; (4) repeated the same processes once more.     

Design of texture for improved formability of high-strength steel

The effect of crystallographic textures on the formability of BCC steel sheets has been studied by using crystalline plasticity finite element analysis (FEA) and experiments. It was confirmed that one of the important reasons why the conventional high-strength steel sheet has poor formability was due to lack of {111} fiber texture components —γ-fiber texture—. In this paper, a texture adjusted design method is proposed to improve the formability of conventional high-strength steel sheets. First, an artificial γ-fiber texture is defined in terms of a rotationally symmetric Gaussian distribution of deviation angles, which has a certain scatter width along the given γ-fiber skeleton line in Euler space. The analytic textures are designed by introducing the artificial γ-fiber texture into the conventional high-strength steel model. The blending coefficient corresponding to the {111}/{001} volume fraction ratio is selected as the design parameter. Then, an optimum crystallographic texture of steel sheet is found through the limit dome height (LDH) formability tests by employing as objective function, which is evaluated by a maximum thinning ratio of the deformed sheet. Further, it is demonstrated that the sheet with the optimum texture shows the best straining in VDI benchmark stamping tests.     

The influence of crystallographic orientation on crack tip displacements of microstructurally small, kinked crack crossing the grain boundary

The paper presents an analysis of the effects of grain orientations on a short, kinked surface crack in a 316L stainless steel. The kinking of the crack is assumed to take place at the boundary between two neighbouring grains. The analysis is based on a plane-strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains, loaded monotonically in uniaxial tension to a maximum load of 0.96R_p0.2 (240 MPa). The influence that a random grain structure imposes on a Stage I crack is assessed by calculating the crack tip opening (CTOD) displacements for bicrystal as well as for polycrystal models, considering different crystallographic orientations. Since a Stage I crack is assumed, the crack is always placed in a slip plane. Results from a bicrystal case show that the maximal CTODs are directly related to the stiffness of the grain containing the crack extension. Anisotropic elasticity and crystal plasticity both contribute to this grain stiffness, resulting in maximal CTOD when Schmid factors are the highest on two slip planes. Such crystallographic orientation results in a soft elasto-plastic response. Anisotropic elasticity can additionally increase the softness of a grain at certain crystallographic orientations. Minimal anisotropic elasticity at the crystallographic orientations with the highest Schmid factors causes the CTOD to be maximized. Presuming that the crack will preferably follow the slip plane where the crack tip opening displacement is highest, we show that the crystallographic orientation can affect the CTOD values by a factor of up to 7.7. For a given grain orientation the maximum CTOD is attained when the crack extension deflection into a second grain is between −75.141° and 34°. For the polycrystal case we show that grains beyond the first two crack-containing grains change the CTOD by a factor of up to 3.3 and that the largest CTODs are obtained when placing the crack into a slip plane with crack extension that results in a crack extension being more perpendicular to the external load.     

Microstructural and mechanical properties evolution of magnesium AZ61 alloy processed through a combination of extrusion and thermomechanical processes

In this work, the microstructures and tensile properties of a commercial magnesium alloy “AZ61” processed by a combination of hot extrusion and thermomechanical processing (TMP) were investigated. The TMP was consisting of two or three hot rolling steps with large reductions per pass, thus allowing significant grain refinement. The microstructural evolution has been studied by means of optical and scanning electron microscopes, as well as X-ray diffraction analysis. The as-cast material is extruded in the form of a cylinder with initial diameter of 250 mm to a final diameter of 110 mm (80% reduction in cross-sectional area). Then hot rolling regimes were performed at 300 °C with different percentage of strain per pass. Tensile and hardness tests were performed in the samples (as-cast, extruded, and rolled) at room temperature in order to evaluate the mechanical properties of the material. The results of experiments demonstrated that fine grain size might be achieved in magnesium alloy AZ61 by using a two-step processing route involving an initial extrusion step followed by thermomechanical processing with large reduction in thickness per pass. This two-step process, designed to achieve average grain sizes of 10–20 μm.     

Distinguishing micro-scale from macro-scale tensile flow stress of sheet metals in microforming

This study describes the effect of grain size on the flow stress of sheet metal under simple tension in microforming. A simple model of the tensile flow stress of sheet metal is firstly developed based on the Hall–Petch equation. Experimental results verify the accuracy of the developed model, which is a function of T/D (sheet thickness/grain size). A critical condition (T/D)c that distinguishes micro-scale from macro-scale tensile flow stress is subsequently proposed based on Li's theory of dislocation with density type. The trend of the predicted (T/D)c with varying grain size is similar to the experimental finding that the (T/D)c decreases as the grain size increases. Therefore, the developed model can elucidate to understand the tensile flow stress of sheet metal in microforming.     

Surface wrinkling of the twinning induced plasticity steel during the tensile and torsion tests

‘Heterogeneous twinning’ is defined as plastic deformation due to the formation and progress of twins resulting in surface wrinkles on the deforming part when the initial grain size is relatively large compared to the typical size of the part. In the case of a Twinning Induced Plasticity (TWIP) steel with an initial grain size of ∼160 m, the heterogeneous twinning generated visible wrinkles, an orange peel effect, under medium uni-axial strains. The heterogeneous twinning did not occur in the material subjected to high shear strains. The complications resulting from this phenomenon on strain hardening characterization of the TWIP steels using two commonly used mechanical tests, tensile and torsion are discussed along with some experimental aspects of heterogeneous twinning.     

Effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE)

Three different mold structures were designed by changing the parameters of mold cavity to study the effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE) with 2 interim dies. Microstructure and texture evolution of AZ31 magnesium alloy after CVCDE were studied by electron backscatter diffraction (EBSD). Mechanical properties were determined by uniaxial tensile tests along extrusion direction (ED) at room temperature. Due to the differences of local strains among the three schemes, the microstructure of Scheme 1 was the most uniform and the average grain size of Scheme 1 was the smallest. Meanwhile, tensile strength and elongation of Scheme 1 were the highest. Different textures had been formed in the three schemes. Lots of extension twins {10–12} (86°< 1–210 >) occurred in the products of the three schemes. The main deformation modes of Scheme 1 and Scheme 2 were slip and twinning. However, slip was dominant in Scheme 3. The deformation modes provided an essential basic for the design of CVCDE mold structure with more interim dies.