Average Misorientation Angle Research Articles

Study of microstructural evolution and strength–toughness mechanism of heavy-wall induction bend pipe

The induction bending and subsequent tempering process has recently been proven to be an optimal technological route for the manufacture of bend pipe with a better combination of strength and toughness. In this work, high temperature quenching followed by tempering treatment from 500°C to 700°C has been applied to heavy-wall bend pipe steel produced by the ultra fast cooling technology. The evolutions of microstructure and dislocation were characterized by means of an optical microscope, the positron annihilation technique (PAT), SEM, TEM, XRD and EBSD. Microstructure observations showed that fine and homogenous M/A islands as well as dislocation packages in a quasi-polygonal ferrite (QPF) matrix after tempering at 600–650°C generated an optimal combination of strength and toughness. At higher tempering temperature, the yield strength decreased dramatically; however, the impact toughness still remained at a high value of more than 300J. Dislocation analysis by means of TEM, EBSD and PAT suggested that the decrease and pile-up of dislocation could provide better toughness and tempering stability. EBSD analysis indicated that the average misorientation angle enlarged and the effective grain size diminished with the tempering temperature increasing, and these caused more energy cost during the microcrack propagation process with subsequent improvement in impact toughness. Microcracks mainly originated from the interfaces between ferrite matrix and M/A islands. The ring-type join-up of microcracks and the large number of branches formed during the propagation process effectively improved the toughness. All these results benefit the cost-effective commercialization of heavy-wall induction bend pipe with high performance.

Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (∆T max) between various locations within samples during quenching. When ∆T max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ∆T max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.

Tailoring Microstructure and Mechanical Properties of 6063 Aluminium Alloy for Lightweight Structural Parts

In this work, an attempt has been made to improve the mechanical strength of 6063 aluminium alloy and thus its lightness via combination of severe plastic deformation (grain size refinement) and heat treatment (precipitation hardening). 6063 aluminium alloy was chosen as the best material for lightweight structures, where mass reduction is important, because of its high extrudability. Samples were hydrostatically extruded (HE) in supersaturated condition and subsequently subjected to an aging process. HE brings about significant grain size refinement well below 1 micron. The influence of aging parameters such as time and temperature on mechanical properties evolution of extruded material was determined. The microstructure of ultrafine grained (UFG) alloy was investigated using transmission electron microscopy. The average grain diameter and grain boundary misorientation angles (using Kikuchi lines) were measured. Mechanical properties were examined in microhardness and tensile tests. The results have shown that it is possible to combine grain boundary and precipitation strengthening and obtain ultrahigh strength in 6xxx series alloys. Additionally, heat treatment of UFG samples causes an increase in ductility measured in tensile tests, which is rather poor in severely deformed materials. To prove advantages of UFG aged samples for lightweight applications, finite element modelling was performed to compare the mass of chair elements made of coarse and ultrafine grained material. Simulations were made for the same stresses applied. It has been shown that if the chair was made of UFG aluminium alloy the mass reduction would be approximately 30 %.

Mechanical, microstructure and texture characterization of API X65 steel

American Petroleum Institute (API) grade X65 steel plate was characterized for its microstructure, texture and mechanical properties. The microstructure was found to be mainly composed of polygonal ferrite grains coexisting with few quasi polygonal ferrite grains. The average grain size with a critical misorientation of 15° was found to be about 3.3μm. The average misorientation angle was about 36° with a fraction of high angle grain boundaries of 90%. Texture was studied and revealed a high orientation density function values around the γ-fiber. Tensile tests yielded a strength coefficient of 711MPa, a strain hardening exponent of 0.087 and a tensile yield to ultimate stress ratio of 0.85. Constitutive modeling for the room temperature experimental stress–strain responses in compression was also conducted and the materials constants for the Johnson–Cook model are reported. The correlations from the model were seen to be in close proximity with the observed experimental data. Fracture toughness was measured on a compact tension samples along three directions, as per specifications mentioned in ASTM E399-09 standard. The KI values were not for the plane strain condition as the thickness requirement was not met.

Severe plastic deformation of commercial purity aluminum by rotary swaging: Microstructure evolution and mechanical properties

The microstructure evolution and change in mechanical properties of commercially pure aluminum (Al 1050) were investigated during severe plastic deformation by rotary swaging (RS) at ambient temperature. Optical microscopy (OM) and scanning electron microscopy (SEM) using electron back scatter diffraction (EBSD) were utilized to document the evolution of the microstructure. Hardness and tensile tests were conducted to characterize mechanical properties. Rotary swaging was found to lead to a marked decrease in grain size. The microstructure consisted of cell structure of low angle grain boundaries (LAGBs) within bigger grains. After heavy swaging to true deformation degree of φ=3, the microstructure was quite uniform normal to flow direction in terms of cell size, average misorientation angle and content of low angle grain boundaries. Extremely fine dynamically recrystallized grains, heterogeneously nucleated at existing grain boundaries were evident on the section parallel to the flow direction. As compared to the as-received condition, the yield stress and ultimate tensile strength of the material was strongly increased by a factor of 8 and 2.3, respectively. Furthermore, the elongation to fracture was drastically reduced as was the uniform strain indicating a marked reduction in work hardening capability.

Assessment of Plastic Deformation Induced by Indentation on a Large Grain on Inconel 600 Using Synchrotron Polychromatic X-ray Microdiffraction

A synchrotron-based three-dimensional (3-D) Laue technique called Differential Aperture X-ray microscopy (DAXM) was used to investigate plastic deformation of Inconel 600 induced by indentation. The DAXM technique is capable of probing up to 60 to 100 μm into the Ni alloy, with micron resolution. A conical indenter was used to generate an indent on a large surface grain on Inconel 600. The DAXM Laue images from the uppermost grain exhibited pronounced streaking and splitting of Laue spots, which increased as the grain boundary was approached. Splitting of the Laue patterns correlates with subgrain or dislocation cell formation. A significant amount of dislocation cell formation was found within the 1-μm voxels probed by DAXM. A change in total angle of 8.79 deg was determined for the uppermost grain from the diffraction data from depths 4 to 28 μm, with an average misorientation angle of 2.15 deg between the dislocation cells. The next grain having a different crystallographic orientation continued to rotate in the same direction as the uppermost grain due to the large plastic deformation.

Thermal Behavior of Nickel Deformed to Ultra-High Strain by High Pressure Torsion

Polycrystalline Ni (99.5 %) has been deformed to an ultra-high strain ofεvM=100 (εvM, von Mises strain) by high pressure torsion (HPT) at room temperature. The deformed sample is nanostructured with an average boundary spacing of 90 nm, a high density of dislocations of >1015m-2and a large fraction of high angle boundaries (>15o) 68% as determined by transmission electron microscopy and 80% as determined by electron backscatter diffraction. The thermal behavior of this nanostructued sample has been investigated by isochronal annealing for 1h at temperatures from 100 to 600°C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness have been determined. Based on microstructural parameters the stored energy in the deformed state has been estimated to be 24 MPa. The isochronal annealing leads to a hardness drop in three stages: a relatively small decrease at low temperatures (recovery) followed by a rapid decrease at intermediate temperatures (discontinuous recrystallization) and a slow decrease at high temperatures (grain growth). Due to the presence of a small amount of impurity elements, the recovery and recrystallization are strongly retarded in comparison with Ni of high purity (99.967%). This finding emphasizes the importance of alloying in delaying the process of recovery and recrystallization, which enables a tailoring of the microstructure and properties through an optimized annealing treatment.

Effect of over-aging on the microstructural evolution in an Al–Cu–Mg–Ag alloy during ECAP at 300 °C

This report explores over-aging as it relates to microstructural evolution during equal channel angular pressing (ECAP) at 300°C using a novel Al–Cu–Mg–Ag alloy doped with zirconium and scandium. Material characterization was conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray analysis in which microstructural evolution in a quenched alloy (QA) material was compared to that in an over-aged alloy (OA) material. It was shown that over-aging facilitates grain refinement where, at ɛ∼12, a fraction of the high angle boundaries (HABs) and average misorientation angles are ∼0.72 and 30°, respectively. In contrast, a partially recrystallized structure evolves in the QA sample even after ɛ∼12. Intense plastic straining affects precipitation. In the QA, ECAP leads to the precipitation in Ω-phase with a plate-like shape, initially. Further strain leads to a shearing offset associated with a localized dislocation glide followed by dissolution of Ω-phase and precipitation of the θ-phase (Al2Cu) and S-phase (Al2CuMg), both of which share an equaxed shape. In the OA, the dissolution of the Sc enriched θ-phase consisting of relatively coarse particles is followed by precipitation of nanometer-sized scale dispersoids of a Sc depleted θ-phase on deformation-induced boundaries. In addition, Ag and Mg-rich particles being β′(MgAg) and U(AlMgAg) phases precipitate during ECAP, in both states of the Al–Cu–Mg–Ag alloy explored herein.

Dependence of the Electron Beam Energy and Types of Surface to Determine EBSD Indexing Reliability in Yttria-Stabilized Zirconia

Electron backscatter diffraction (EBSD) is a powerful technique for surface microstructure analysis. EBSD analysis of cubic yttria-stabilized zirconia (YSZ) is demonstrated. The statistics related to EBSD indexing reliability shows that the probability of accurate grain orientation detection increased significantly when the electron beam energy was increased from 10 to 30 kV. As a result of better sampling with increased interaction volume, a disparity between local and average grain misorientation angle also exhibited the dependence of the electron beam energy to determine the accuracy of grain orientation. To study EBSD indexing reliability as a function of surface roughness and overlayer formation, rapid EBSD measurement tests were performed on (a) YSZ surfaces ion-polished at ion beam energies of 65 nA at 30 kV and 1 nA at 30 kV and (b) carbon-coated versus uncoated YSZ surfaces. The EBSD results at both 10 and 30 kV electron beam energies indicate that EBSD indexing reliability is negatively affected by higher ion beam milling current and amorphous overlayer formation.

Enhancement of static and fatigue strength of 1050 Al processed by equal-channel angular pressing using two routes

In the present investigation, annealed billets of commercially pure Al (1050) with coarse-grained microstructure of 0.6 mm were equal channel angular pressing (ECAP) processed using two routes A and BC up to four passes. The change in the processing route results in the change of the shear plane, and consequently the change in the formed microstructure and texture. The average grain size was in the order of 1 μm with average misorientation angle of 15° after four passes of route BC. Texture study was conducted on the shear plane for two and four-pass processed samples. A study of mechanical behavior for the ECAP specimens, using tensile and compression specimens, showed the enhanced strength with asymmetry behavior between tension and compression. Fatigue strength under load control (HCF) was also performed and the S–N curves were established as a function of number of passes and processing route. Improved fatigue strength was noticed with increasing the number of ECAP passes in harmony with the increase in tensile strength. The fractography of fatigue tested specimens was also performed and showed the formation of cleavage planes and dimples suggesting (localized) micro-plasticity at the deformation bands at the plastic zone of the crack tip. The degree of micro-plasticity increases with the applied stress amplitude.

Finite Element Modelling of the Effects of Average Grain Size and Misorientation Angle on the Deformation

This paper comprises an investigation using finite element analysis to study the behaviour of nanocrystalline grain structures during Equal Channel Angular Press (ECAP) processing of metals. The effects of average grain size and misorientation angle on the deformation are examined in order to see how microstructural features might explain the observed increase in strength of nanocrsytalline metals. While this approach forms a convenient starting as it offers a simple way of including grain size effects and grain misorientation to which we could add additional phenomena through developing the material model used to describe the anisotropy and techniques that would automatically re-mesh the refined grain structure produced under severe plastic deformation. From this, it can be concluded that these additional techniques incorporated into the finite element model produced effects that correspond to observed behaviour in real polycrystals.

Mechanical properties, microstructure and micro-texture evolution for 1050AA deformed by equal channel angular pressing (ECAP) and post ECAP plane strain compression using two loading schemes

Electron back scattered diffraction (EBSD) was used to document the microstructure and microtexture developed due to cross deformation of commercial purity 1050 aluminum alloy. The material was first deformed in an equal channel angular pressing die (ECAP) to one up to four passes, via route B c and then deformed in plane strain compression (PSC), in two different directions, to two axial true plastic strain values of 0.5 and 1.0. The study provides a documentation of the evolution of microstructure parameters namely; cell size, average misorientation angle, fraction of submicron cell size and fraction of high angle grain boundaries. These microstructure parameters were investigated on the plane normal to the loading direction in PSC (plane constituted by the rolling and transverse directions). These microstructure parameters were compared to those achieved due to the ECAP process only. The ideal rolling texture orientations are depicted and crystal orientation maps were generated. The spatial distribution of grains having these orientations are revealed through these maps. The fraction of the main texture components for a 15° spread around the specified orientations was experimentally calculated and a quantitative idea on the evolution of microtexture was also presented. Deformation path change was shown to be an effective tool for manipulating texture as well as microstructure.

Microstructural features, texture and strengthening mechanisms of nanostructured AA6063 alloy processed by powder metallurgy

Nanostructured AA6063 (NS-Al) powder with an average grain size of ∼100 nm was synthesized by high-energy attrition milling of gas-atomized AA6063 powder followed by hot extrusion. The microstructural features of the consolidated specimen were studied by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques and compared with those of coarse-grained AA6063 (CG-Al) produced by hot powder extrusion of gas-atomized powder (without using mechanical milling). The consolidated NS-Al alloy consisted of elongated ultrafine grains (aspect ratio of ∼2.9) and equiaxed nanostructured grains. A high fraction (∼78%) of high-angle grain boundaries with average misorientation angle of 33° was noticed. Microtexture evaluation by plotting pole-figures and orientation distribution function (ODF) analysis showed Copper and P texture components for both the consolidated Al alloys. Tensile test at room temperature and microhardness measurement revealed that a significant improvement in the strength of AA6063 alloy is obtained through refinement of the grain structure. The strengthening mechanisms are discussed based on the dislocation-based models. The role of high-angle and low-angle grain boundaries on the strengthening mechanisms is discussed.

Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

The performance of superconducting radio-frequency (SRF) resonant cavities made of bulk niobium is limited by nonlinear localized effects. Surface analysis of regions of higher power dissipation is thus of intense interest. Such areas (referred to as ``hotspots'') were identified in a large-grain single-cell cavity that had been buffered-chemical polished and dissected for examination by high resolution electron microscopy, electron backscattered diffraction microscopy (EBSD), and optical microscopy. Pits with clearly discernible crystal facets were observed in both ``hotspot'' and ``coldspot'' specimens. The pits were found in-grain, at bicrystal boundaries, and on tricrystal junctions. They are interpreted as etch pits induced by crystal defects (e.g. dislocations). All coldspots examined had a qualitatively lower density of etch pits or relatively smooth tricrystal boundary junctions. EBSD mapping revealed the crystal orientation surrounding the pits. Locations with high pit density are correlated with higher mean values of the local average misorientation angle distributions, indicating a higher geometrically necessary dislocation content. In addition, a survey of the samples by energy dispersive x-ray analysis did not show any significant contamination of the samples' surface. The local magnetic field enhancement produced by the sharp-edge features observed on the samples is not sufficient to explain the observed degradation of the cavity quality factor, which starts at peak surface magnetic field as low as 20 mT.

Development of Ultra-Fine Grained Structure in an Al-5.4%Mg-0.5%Mn Alloy Processed by ECAP

Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of ~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 m. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.

Texture evolution in FCC metals from initially different misorientation distributions under shear deformation

We applied the crystal plasticity finite element method (CPFEM) based on a rate sensitivity model to examine the subgrain texture evolution of FCC metals under shear deformation. We used two kinds of microstructure models with the same orientation distribution function (ODF) but different spatial arrangements or misorientation distributions (MDs). One contained a great high frequency around low misorientation angles and the other a great high frequency near high misorientation angles. Different misorientation angles among neighboring crystals caused different interactions among them, particularly at the subgrain scale. The difference in MD affected the evolution of texture and average misorientation angles during deformation. The average misorientation angles of the subgrain boundaries increased with shear strain.

Fabrication of Ultrafine Grained Copper Alloy by 3-Layers Accumulative Roll-Bonding Process

The 3-layers accumulative roll bonding process (ARB) has been attempted to increase the strength of copper alloy (Cu-0.02wt.%P) by refining grain size. The 3-layers accumulative roll bonding was conducted up to 7 cycles at room temperature without lubrication. Microstructural evolution of the copper alloy with the number of the 3-layers ARB cycles was investigated by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD). The average grain size has been refined from 20 μm before ARB to 170 nm after 7 cycles of 3-layers ARB. More than 70% of ultrafine grains formed by 3-layers ARB were composed of high angle grain boundaries. The average misorientation angle of ultrafine grains was 30.7 degrees in the center of the specimen. Tensile strength after 7 cycles of 3-layers ARB was 605 MPa, which is about 3.2 times higher than the initial value.

Microstructure and texture evolution of particle-containing AA3104 alloy cold rolled to large strains

The evolution of the microstructure in a particle-containing AA3104 aluminium alloy cold rolled to large strains has been investigated using the electron channelling contrast imaging and electron backscattered diffraction techniques. It is found that the microstructure evolves from cell-block structure at low strains into a lamellar structure composed of lamellar boundaries (LBs) and interconnecting cell boundaries at high strains. The orientation dependence of deformation microstructure is also observed in this particle-containing alloy even at high strains. Different particles have different effects on the microstructure evolution. It is found that there is no clear effect of the fine dispersoids on the evolution of lamellar structure even at large strains, while significant distortion of LBs structure occurs around the coarse constituent particles. For both types of boundaries, the average boundary spacing decreases and the average misorientation angle increases with increasing strain. No saturation of these structure parameters is reached. The texture evolves in a typical manner with finely distributed deformation texture components composed of small volume elements of the ideal texture components (brass, S and copper) separated by LBs. It is found that the particles have no strong effect on the texture evolution.

Distribution in angular mismatch between crystallites in diamond films grown in microwave plasma

High resolution electron backscattered diffraction (EBSD) has been used for analysis of grain size, texture and stress distribution on growth side of free-standing polycrystalline diamond films of different grade. The undoped and moderate boron-doped films of 0.3–0.5 mm thickness were grown by microwave plasma CVD. The highest number of stressed domains, mostly located at grain boundaries, and the largest average grain misorientation angle ( θ ≈ 6°) have been found for B-doped film. Highly defected and highly [001] oriented “black” diamond exhibited much more rear stress domains, this being ascribed to angular mismatch as small as θ = 0.5° in that film. The samples of “white” diamond showed somewhat intermediate pictures, with stress observed both in bulk and on grain boundaries. Evolution of texture (columnar growth) and stress distribution with film thickness has been observed with EBSD study of film cross-sections.

Thermal behavior of Ni (99.967% and 99.5% purity) deformed to an ultra-high strain by high pressure torsion

Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of ε vM = 100 ( ε vM , von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ∼100 nm, a high density of dislocations and a large fraction of high angle boundaries (>15°) of 0.68–0.74, as determined by transmission electron microscopy, and 0.8–0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1 h at temperatures from 100 to 600 °C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14 MPa and 24 MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment.