Materials Characterization最新文献

In metal additive manufacturing (AM), understanding the process-structure-performance relationships requires a combination of multi-scale characterization techniques that allows for the measurement of the melt pool shape and boundary and classifying various defects and flaws in the AM parts. Such approaches can be destructive, only 2D in nature, or have a small field of view and can be complex to co-register and analyze. In this work, we present a non-destructive 3D inspection technique that employs dual-energy X-ray computed tomography (XCT) along with a model-based iterative reconstruction (MBIR) and a new segmentation algorithm. The proposed approach and algorithm are not only capable of classifying and quantifying flaws such as pores, cracks, and inclusions, but they also allow for the extraction of microstructural features such as melt pool boundaries (MPB) and melt pool regions (MPR), that can help understand process-structure-performance relationships for alloys under study. As an exemplar application, we employed the method for characterization of an additively manufactured aluminum alloy crept under tensile stress at 300 °C for 1064 h. Our results demonstrate high quality segmentation and classification of various flaws and MPB and MPR, for the first time, using 3D X-ray CT inspection. The delineated MPB and MPR in the crept samples reveal the preferential growth paths of cracks that formed during creep deformation. The technique was used for successfully quantifying the characteristics (number of defects, their density, volume fraction, etc.) of the manufacturing-induced pores and creep-induced cracks, which is necessary to better understand the creep failure mechanisms of the material.

The mechanism of performance enhancement of magnetic high-entropy alloys (MHEAs) under powder metallurgical processes is unknown, especially in terms of precipitated phases. This work reports the doping behavior of Fe_1.25CoNiAlMn_0.21V_x (x = 0, 0.2 and 0.5) MHEAs, caused by the change of V content. Fe_1.25CoNiAlMn_0.21V_x MHEAs were prepared by the mechanical alloying (MA) and the spark plasma sintering (SPS). Special attention was paid to the effect of the V content on the microstructure evolution, hardness, compressive strength, and magnetic properties of HEAs. Interestingly, the phase structure of these alloys remained relatively stable regardless of the variation in V content. The addition of a small amount of V helps to increase the saturation magnetization Ms of the alloy up to 117.2 emu/g and also promotes the solid-solution strengthening effect, with a maximum compression strength of 2724 MPa and a hardness of 531 HV. The excess V atoms could not completely enter the FCC phase to form a solid solution, but would instead and form a tough and brittle phase, reducing the mechanical properties of the alloy.

The compositional sensitivity of phase transformation and resulting microstructure and texture has been systematically studied for a new generation γ-TiAl-based intermetallic system (Ti, Al, Nb, Cr, Mo, C, B). Systematic increase of Al and decrease of Nb content in this complex multi-component system revealed that the solidification path varied in the sequence of β solidification, hypo-peritectic solidification, hyper-peritectic solidification, and α solidification, consequently resulting in very different cast microstructure, texture, and elemental segregation. The alloy system involves multitudes of solid-state phase transformations, which are very sensitive to compositional variation, resulting in varieties of microstructural and crystallographic characteristics.

In the aerospace industry and the medical field, there is a demand for materials that combine the functional properties of NiTi with the excellent ductility of Nb. However, creating high-strength interfaces between these two materials has been difficult due to their inherent differences in physical and chemical properties. In this study, a laminated heterogeneous structure (LHS) combining NiTi and Nb was prepared using wire arc additive manufacturing (WAAM) with different layer thickness ratios. The resulting microstructure of the NiTi/Nb LHS components consisted of a large amount of NiTi and β-Nb eutectic structures. This was due to the metallurgical bonding between the NiTi and Nb layers. The NiTi/Nb LHS components exhibited excellent compressive strength, measuring at 2607.6 ± 31 MPa. Additionally, the interface strength of the NiTi/Nb LHS component was remarkable, showing an ultimate tensile strength of 789.3 ± 8 MPa. The enhanced strength of the NiTi/Nb LHS component can be attributed to the gradient microstructure of the NiTi layer, which promoted heterogeneous plastic deformation generation. Furthermore, this study unveiled the relationship between the formation mechanism of the heterogeneous eutectic microstructure and the strength-ductility synergistic mechanism. As a result, this study provides an innovative approach for additive manufacturing in the strengthening of laminated multi-material interfaces.

The evolution of eutectic Si, the precipitation behavior of dispersoids and microhardness change of the α-Al matrix in a Mn + Cr modified Al-Si-Mg-Cu alloy during solution treatment were investigated. The results revealed that the evolution of eutectic Si was divided into four paths during solution treatment: Dissolution, splitting, spheroidization and coarsening. Meanwhile, numerous α-Al(FeMnCr)Si dispersoids were precipitated in the eutectic region and the α-Al dendrite arms after solution treatment for 30 min. Due to differences in precipitation behavior, three different dispersoid precipitation zones were formed in the dendrite arms, including the dispersoid-free zone (DFZ) near the grain boundaries, coarse dispersoid zones (CDZ) located in the dendrite cores, and the remaining fine dispersoid zones (FDZ). The bimodal size distribution of the dispersoids within intradendritic regions were closely related to the micro-segregation of Mn and Cr. The precipitation of coarse grain boundary phases exhausted the Fe solute near the grain boundary, which was mainly responsible for the formation of the DFZ. Moreover, there was a partial coherent interface between the dispersoids and the α-Al matrix. The evolution mechanism of matrix microhardness can be well explained by dispersion strengthening effect

In this work, Fe_x(CoCrMnNi)_100-x (x = 40, 60 and 80 at.%) medium-entropy ferrous alloys (MEFAs) were fabricated through laser powder bed fusion (LPBF) of mixed powders composed of FeCoCrNiMn and Fe powders, and the dynamic compressive properties and deformation mechanisms were studied with combination of experiment and molecular dynamics simulation. The results demonstrate that all MEFAs exhibit good processability. The Fe40 and Fe60 samples were predominantly composed of FCC phase with coarse grains, while the Fe80 mainly consisted of BCC phase with a smaller grain size. When subjected to quasi-static and dynamic compression at 3000/s, the yield strengths of Fe40, Fe60 and Fe80 increases by 131, 220 and 256 MPa, respectively, illustrating the positive strain rate sensitivity and influence of Fe on the mechanical properties of MEFAs. Further, both the single-crystal and the polycrystalline models suggested that the incompatibility between the BCC and FCC phases is responsible for the enhancement of strength. Therefore, it is more likely to produce deformation twins, dislocations and thus stress concentration around the BCC phase, which is consistent with the TEM observations.

Metallic materials or alloys consisting of single face-cubic centered (FCC) phases typically face difficulties in obtaining high strength without sacrificing ductility. The introduction of different crystalline defects via pre-straining and the activation of multiple deformation mechanisms have been shown to be effective in achieving superior strength–ductility synergy. In this work, we investigated the role of varying rolling conditions and subsequent annealing on the mechanical response in an established twinning- and transformation-induced plasticity dual phase high-entropy alloy (TWIP-TRIP-DP HEA). We found that all of the annealed samples exhibited similar nano-twinned structures. Furthermore, phase transformation from FCC γ to hexagonal close-packed (HCP) ε prevailed along the nano-twins under mechanical loading. With increasing rolling temperature, the phase stability of the matrix revealed a downward trend, resulting in a significant increase in TWIP/TRIP effects along with a prominent change in the deformation microstructure. Our study presented a simple and feasible strategy for manipulating mechanical performance by modulating the microstructural characteristics and associated deformation modes.

Many industries including aerospace make extensive use of lightweight materials like aluminium alloys because of their extraordinary properties. Wire Arc Additive Manufacturing-Cold Metal Transfer (WAAM-CMT) as a part of modern direct energy deposition technique can be recommended for developing aerospace components of aluminium alloys. However, it is still necessary to investigate the impact of the combined change of developing parameters on some of the crucial qualities including metallurgical and mechanical properties. Therefore, in this part of research work, practically, various process parameters were tried for development of aluminium (Al-5356) alloy walls. Finaly, a specific set of parameters namely wire feed speed, scanning speed and inter layer temperature parameters with two levels of each were selected and with the assistance of these, four walls have been fabricated. Th preliminary analysis showed that low heat input accompanied by higher scanning speed of 60 cm/min and lower wire feed rate of 6 m/min was found to best suitable set of parameters for developing dimensionally stable Al wall with flaw less microstructure. The said set of parameters also showed higher hardness that also accompanied with finer grains, lower percentage towards high angle grain boundary and lower KAM. Even slight variation in these parameters that deal to high heat input can give rise of porosity and misalignment in the wall. Details of microhardness reported high hardness in the bottom part of the wall. However, in a single bead the hardness is higher in the top region, which may be owing to finer grains in the specified region. The above said set of parameters also able to deliver better tensile strength and toughness for Al-5356 alloy. The developed alloy showed improved strength in longitudinal orientation owing to intra layer failure with ductile mode of fracture in contrast to transverse orientation, where the failure mode was reported to mixed (ductile as well as brittle) owing to inter layer fracture.

C/C composite was successfully brazed with TiZrHfTa/Ni composite interlayers at lower temperature far below the melting point of TiZrHfTa refractory high entropy alloy. The influence of brazing parameters on the joint morphology, indentation fraction toughness of the reaction layer, shear strength at room temperature and at 1000 °C, and fracture behavior was investigated. The results show that all of the C/C composite joints contained two main phases: an equimolar (Ti-Zr-Hf-Ta)C hard phase and a near-pure Ni binder phase. The maximum indentation fracture toughness of the obtained joint reaction layer material was 15.52 ± 0.64 MPa·m^1/2. This was proved to be beneficial to improve the strength and toughness of the joint. Meanwhile, the maximum shear strengths of C/C-TiZrHfTa/Ni-C/C joint at room temperature and at 1000 °C reached 33.24 ± 1.85 MPa and 21.70 ± 2.12 MPa, respectively. Abundant slip lines, in-situ dimples, and tearing ridges resulting from Ni plastic deformation suggest a predominantly ductile fracture mode in the joint. This work introduces an innovative method, which can not only ensure the service of C/C composite in high temperature environment, but also effectively reduce the joining temperature.

The soft zone is deemed unwanted for grade 91 steel weldments as it constitutes a cracking-sensitive region deleterious for long-term creep performance. In the present work, formation mechanisms of the soft zone in grade 91 steel weldments have been elucidated and an approach for its elimination has been proposed. It has been demonstrated that a soft zone forms within the intercritical heat-affected zone (ICHAZ) after post-weld tempering. Post-weld normalizing and tempering enables the elimination of the soft zone. Further analysis reveals that the ICHAZ of the as-welded specimen possesses a mixed structure, consisting of over-tempered martensite and fresh martensite with a fine prior austenite grain size. During post-weld tempering, the increase in grain size and the reduction in dislocation density are the main factors contributing to the decrease in hardness. The recrystallization rate of the ICHAZ is faster than that of other regions, resulting in a decrease in hardness from 350.8 HV0.3 to 204.6 HV0.3 and the formation of the soft zone. Increased prior austenite grain size in the ICHAZ, achieved through post-weld normalizing and subsequent tempering, facilitates the formation of tempered martensite and prevents the formation of the soft zone.