Kernel Average Misorientation Distribution Research Articles

Unravelling anisotropic deformation behaviour of Ti-6Al-4V ELI fabricated by powder bed fusion additive manufacturing

The uniaxial compression tests with respect to the different loading directions (i.e., parallel, and perpendicular to the building direction) were performed to investigate the anisotropic deformation mechanism of Ti–6Al–4 V ELI (extra low interstitial) alloy fabricated by laser powder bed fusion (L-PBF). The tests showed anisotropic mechanical properties; the sample loaded perpendicular to the building direction (TDx) has a higher fracture strain and lower yield strength, but a superior hardening effect than those of the sample loaded along the building direction (BD). Electron backscatter diffraction (EBSD) and digital image correlation (DIC) analyses were conducted used at regions of interest (ROI) in as-built and deformed samples. The kernel average misorientation (KAM) analysis revealed that a trend of dislocation evolution is strikingly different according to the loading directions; uniform KAM distribution is observed in the BD sample, whilst localization at prior β grain boundaries is found in the TDx sample. In addition, it was identified via a microscopic strain distribution map that the unique layered microstructure (i.e., layer band) and prior β grain boundary are more sensitive to compressive loading than the surrounding matrix. The layer band and prior β grain boundary were formed with their directivity during the additive manufacturing process; thus, their morphology is different depending on the loading direction. As a result, the layer band and prior β grain boundary play a key role in anisotropic behaviour, more precisely, the anisotropic deformation behaviour originates from the morphology of the layer band and β grain boundary, which is changed with respect to the loading direction.

Brazing manufacturing technology of plate-fin heat exchanger for solid oxide fuel cells

The plate-fin heat exchanger (PFHE) is the core equipment used to achieve the high efficiency of solid oxide fuel cell (SOFC), however, the issues of high-performance brazed joints in the manufacturing of the PFHE have been a challenge due to the poor mechanical properties. This study proposes a brazing manufacturing technology of isothermal solidification with the optimized post bonding heat treatment strategy, to synergistically improve the strength-ductility property and homogenize microstructure of brazed joints, aiming at guaranteeing the high energy efficiency of SOFC. The results show that brazing at 1065 °C for 25 min achieves the complete isothermal solidification and an intermetallic-free joint centerline with numerous borides generating in the diffusion-affected zone. Solution treatment then dissolves large quantities of acicular and blocky borides. The uniformity of grain size and kernel average misorientation distribution is also improved due to recrystallization, which becomes more pronounced after solution aging treatment. In addition, solution aging treatment results in an improvement in the ultimate tensile strength of the brazed joint, which is more prominent than that after solution treatment. However, the increase in elongation after solution aging treatment is smaller than after solution treatment, while still much higher than the as-brazed joint due to the dissolution of boride precipitates and growth of twin boundaries. The results demonstrate that the proposed brazing manufacturing technology not only homogenizes microstructure, but also significantly improves strength and ductility, further promoting the long-life operation of SOFC.

Crystalline characteristics of a dual-phase precipitation hardening stainless steel in quenched solid solution and aging treatments

This study investigated the development of the microstructures and aging reactions in a new dual-phase (allotriomorphic δ + γ) 17–4 precipitation-hardened (PH) stainless steel (SS) alloy. The 17–4 PH alloy, a martensitic stainless steel containing Cu (3–5 wt.%), was subjected to solid solution treatment followed by quenching using oil, H2O, or liquid N2. The microstructures of the steel specimens were primarily composed of lath martensite and allotriomorphic δ-ferrite. X-ray diffraction analysis indicated that the structures consisted entirely of bcc phase peaks. During aging at 560 and 600 °C, the local microstrain was released because of the tempering of the lath martensite and the formation of reverted austenite, resulting in a shift in the kernel's average misorientation distribution toward lower angles. The misorientation associated with the martensite boundaries ranged from 10° to 20° and from 47° to 57° in the packets. In the blocks, it ranged from 50° to 60°. The F1-ferrite approximately followed the Kurdjumov–Sachs orientation relationship with A1-austenite and A2-austenite.

Crack Behavior and Crystallographic Feature for TMCP690 Steel under Fatigue Loading

For a new type of thermomechanical controlled process steel, when stress intensity factor was about 32 MPa m1/2, several slip bands with different orientations occurred in cyclic plastic zone. Some slip bands had strong interaction with dominant crack, which led to crack bifurcation and aggregation. After that, electron back-scattered diffraction technique was employed to evaluate the cyclic plasticity effect on crystallographic features of kernel average misorientation (KAM), strain distribution and fraction of different categories of grains for fatigue loading sample in crack tip zone. Before fatigue loading, KAM distribution map revealed relatively homogeneous plastic deformation and few grains displayed distortion based on strain distribution map. After fatigue loading, KAM showed higher cyclic plasticity in the middle, and lower value on both sides. Strain network structure formed. Meanwhile, when fatigue loading was executed, proportion of recrystallized grains had a little variation from 3.15 to 5.58%. Ratio of substructured grains increased to 19.63%, and deformed grains reduced to 74.79%.

EBSD analysis of cyclic load effect on final misorientation distribution of post-mortem low alloy steel: A new method for fatigue crack tip driving force prediction

The effect of applied cyclic load, including the maximum plastic strain εmaxp and the plastic strain amplitude εap on final misorientation level of post-mortem low alloy steel after low cycle fatigue (LCF) test, was analyzed by Electron Back-Scattered Diffraction (EBSD) in this research at first. Two misorientation parameters widely used in EBSD analysis, Kernel Average Misorientation (KAM) and Grain Reference Orientation Deviation (GROD), are compared in terms of accumulated plasticity characterization after LCF failure, where a bilinear function is found between final KAM and applied cyclic load εmaxp &εap. Then the cyclic load effect on final misorientation distribution of post-mortem low alloy steel after fatigue crack propagation (FCP) test was further analyzed by EBSD based on the above bilinear function between final KAM and εmaxp &εap, as well as the equivalent ε¯maxp &Δε¯p distribution functions given by HRR field. −1/2 power law is established to describe final KAM distribution near crack path. The area SKAM between final KAM distribution curve and undeformed KAM0 base line within a certain range, rather than SGROD, is proposed as a better measurement of fatigue crack tip driving force ΔK in constant load ratio R condition. Meanwhile, the influence of load ratio R on final misorientation distribution is also discussed.

Measuring Plasticity with Orientation Contrast Microscopy in Aluminium 6061-T4

Orientation contrast microscopy (i.e., electron backscattered diffraction, EBSD) was employed to monitor the plastic strain in loaded tensile samples of aluminium alloy Al6061 in T4 condition. The kernel average misorientation (KAM) is known to be an appropriate parameter in orientation contrast microscopy which has the potential to characterise the plastic strain by monitoring the local misorientations. This technique was applied here to gauge the extent of the plastic zone around a fatigue crack. To establish the magnitude of strain (which can be identified by the KAM parameter), a series of tensile samples were strained in the range of 1% to 25%. KAM maps were compared, and the average misorientations were related to the tensile strain values. The KAM distribution functions for all the strained samples were derived from a large scanned area. In addition, Vickers microhardness tests were conducted for these series of samples. This allowed the comparison of the mesoscopic plastic strain measured by Vickers microhardness with the micro plastic strain locally obtained by KAM. Noise was observed in the average KAM values up to a plastic strain of 1.5%. For the plastic strain exceeding 1.5%, noise no longer dominates the KAM map, and a positive—though not linear—correspondence between plastic strain and KAM was observed. The observed plastic zone at the tip of the fatigue crack by micro-Vickers hardness measurements was about an order of magnitude higher than the plastic zone observed on the KAM maps. In view of the calibration of KAM values on the tensile samples, it could be concluded that in the larger area of the plastic zone, the strain did not exceed the critical value of 1.5%.

Determination of microstructure evolution in metallic components introduced by Chain-die forming

Abstract Chain-die forming is a promising manufacturing technology that is emerging as an alternative to roll forming and stamping in fabricating Advanced High-Strength Steel (AHSS) products. The microstructure evolution is a critical factor for evaluating the severity of a fabrication process, especially the plastic deformation. This paper mainly attempts to understand the microstructural changes involved with the Chain-die forming process. Scanning Electron Microscopic (SEM) observation and Electron Backscatter Diffraction (EBSD) analysis were adopted to have a deep understanding on microstructure evolution. Microscopic results of the investigation show that there is almost no variation of grains, with respect to Kernel Average Misorientation (KAM) value and rotation of the grains, in the undeformed areas. Rotation of the grains is observed between inner edge and outer surface regions. The microscopic analysis also revealed that the KAM distribution shifts to higher values at the highly deformed region.

Crystal Plasticity Finite Element Method Simulations for a Polycrystalline Ni Micro-Specimen Deformed in Tension

A micro-tensile test system equipped with in situ monitoring of the in-plane displacements of a surface and an electron backscattered diffraction-based serial-sectioning technique were used to study the deformation (up to 2.4 pct axial plastic strain in tension) of a polycrystalline nickel micro-specimen. The experimental data include the global engineering stress-engineering strain curve, the local mesoscopic in-plane displacement and strain fields, the three-dimensional microstructure of the micro-specimen reconstructed after the tensile test, and the kernel-average misorientation distribution. The crystal plasticity finite element method using elasto-viscoplastic constitutive formulations was used to simulate the global and local deformation responses of the micro-specimen. Three different boundary conditions (BCs) were applied in simulation in order to study the effects of the lateral displacement (perpendicular to the loading direction) of the top and bottom faces of the specimen gage section. The simulation results were compared to the experimental results. The comparison between experiment and simulation results is discussed, based upon their implications for understanding the deformation of micro-specimens and the causes associated with uncertainties embedded in both experimental and numerical approaches. Also, the sensitivity of BCs to near-field and far-field responses of the micro-specimen was systematically studied. Results show that the experimental methodology used in the present study allows for limited but meaningful comparisons to crystal plasticity finite element simulations of the micro-specimen under the small plastic deformation.

TRIP aided deformation of a near-Ni-free, Mn–N bearing duplex stainless steel

Abstract A near-Ni-free, Mn–N bearing duplex stainless steel (D-SS) that shows transformation induced plasticity was developed. The present D-SS exhibited an excellent strength–ductility combination over 1000 MPa tensile strength and 50% elongation. An analysis of the element partitioning during annealing revealed that the stacking fault energy of austenite was low enough for a strain induced martensite (SIM) transformation to occur. The strain hardening rate began to increase at ∼10% strain with the same manner of SIM fraction. The TEM and EBSD analyses showed that not only the ɛ martensite band intersections but the austenite grain boundaries acted as the SIM nucleation sites. The SIM transformation was saturated because of the austenite grain refinement and the corresponding austenite stabilization. The austenite grain refinement was caused by the mutual impingement of growing SIM and as a result by the engulfment of remaining austenite by SIM. The deformation behavior of the present D-SS was characterized by analyzing the kernel average misorientation (KAM) of the constituent phases with strain. The KAM distribution of austenite, ferrite and SIM exhibited different characteristics. The average KAM of austenite and ferrite increased as the strain increased, but its increasing rate of austenite was higher than that of ferrite. These KAM characteristics were discussed along with the dislocation glide modes of austenite and ferrite. By contrast, the average KAM of SIM was insensitive to strain and higher than that of the other two phases.