查看更多>>摘要：? 2022 The AuthorsThe development of energy resolving photon counting detectors (PCD) has paved the way to spectral X-ray Computed Tomography (CT), with which one can simultaneously extract the energy dependence of a material's linear attenuation coefficient (LAC). Spectral CT has proved to be an advanced technique to classify materials based on their physical properties such as electron density (ρe) and effective atomic number (Zeff). However, the application of spectral CT may be hindered by the poor image reconstruction quality when used for material identification in security screening where rapid scanning is required. Image reconstruction from few projections or low radiation exposure time would enable rapid scanning. The reconstruction quality for each energy bin may also be degraded since the division of photon counts into multiple energy bins naturally leads to higher noise levels. In this work, we explore how to perform accurate spectral CT reconstructions from such data, and propose L∞norm-based vectorial total variation (L∞-VTV) regularization that uses correlations between multiple energy bins. Using experimental data acquired with a custom laboratory instrument for spectral CT, the L∞-VTV is tested on “real life” phantoms consisting of materials in the range of 6 ≤ Zeff ≤ 15. For material classification from only 7 projections, the L∞-VTV gives the relative deviations of 3.5% for ρe and 2.4% for Zeff, whereas the total nuclear variation (TNV) of another state-of-the-art joint reconstruction and the total variation (TV) yield those of 3.4% and 3.1%, and 3.8% and 4.2%, respectively. The L∞-VTV is now ready for use in security screening.
查看更多>>摘要：? 2022 Elsevier Inc.The deformation microstructure of single crystal superalloys CMSX-4 was investigated during uniaxial tensile with a strain rate of 5 × 10?3 s?1 at room temperature. The objection of the current study is to evaluate the deformation twinning and investigate the deformation mechanism. Experimental results followed by transmission electron analysis show that deformation nanotwins and elemental segregation in single-crystal Ni-based superalloys were observed after tensile deformation at room temperature with a strain rate of 5 × 10?3 s?1. Furthermore, the relationship among deformation twinning, elemental segregation, and high local stress due to plastic deformation with increasing plastic strain was estimated and discussed. These findings provide valuable insights into understanding twinning and elemental segregation in the plastic deformation of single-crystal Ni-based superalloys.
查看更多>>摘要：? 2022 Elsevier Inc.To develop potential biomedical materials for laser direct energy deposition, five Ti-Fe-Sn-Y alloys with varying Sn contents were designed using a dual cluster model. Systemic investigation for microstructure and properties of the alloys fabricated by laser additive manufacturing were then performed. The results show that the microstructure of as-deposited alloys evolves from diphasic hypereutectic-structure to triphasic hypoeutectic-structure with the increase of Sn addition. As a result, the hardness, strength, ductility, and Young's modulus of the as-deposited alloys gradually decrease in the whole, whereas tribological properties increase with the increase of Sn addition to 4.42 at.%, and decrease afterwards. Moreover, all as-deposited alloys do not present any cytotoxic effect. These results suggest that the near-eutectic alloy with 4.42 at.% Sn has the optimum combination of mechanical, chemical, tribological, and forming properties, being a promising candidate as L-DED biological implant material.
查看更多>>摘要：? 2022Dissimilar-metals welding is essential to construction of a fusion reactor by using advanced blanket structural materials of oxide dispersion strengthened (ODS) steels. In the present study, Dissimilar-metals bonding between 9Cr-ODS and CLF-1 steels was carried out by friction stir welding (FSW). Effect of post-weld heat treatment (PWHT) on microstructure and bonding properties of the joint was investigated. The results show that, quenched martensite was found in the weld metal (WM) after FSW. PWHT at 740-800 °C can recover the microstructure of the WM to tempered martensite. Mid-size particles and nano-particles were observed and analyzed in the WM. Besides, in the WM on the CLF-1 side, recrystallization was found after PWHT. As the PWHT temperature increased, the recrystallization width and grain size grew up. However, the recrystallization in the WM did not degrade but may be beneficial to the bonding properties of the joint due to support ductile deformation during mechanical tests. The recommend PWHT temperature is 740-760 °C for keeping the mechanical properties of the CLF-1 base metal (BM) as that before FSW.
查看更多>>摘要：? 2022Based on high Li content, optimizing the mechanical properties of Mg-Li alloy can bring about the good combination of ultra-low density and high strength, causing high specific strength. Reducing the deformation temperature can effectively maintain the strength improvement of deformed β-based Mg-Li alloy. In this work, the microstructures and mechanical properties of room-rolled (RR) and cryogenic-rolled (CR) Mg-16Li-4Zn-1Er (LZE1641) alloy were investigated. Results show that cryogenic temperature delays the dynamic recovery(DRV) and dynamic recrystallization(DRX). A tensile strength of 267 MPa can be obtained at CR60%, which is attributed to the nanocrystalline grains and the significantly higher dislocation density than that of the RR alloy. Further CR with 80% reduction makes the form of some alloying elements transform from solid state to precipitation phases, causing relatively decreased strength
查看更多>>摘要：? 2022 Elsevier Inc.AZ31 magnesium alloy and DC01 steel were friction stir lap welded (FSLW) with Z-axis force controlled at 7.5 kN. The microstructural features and element distributions of the joint were studied in detail. Mg17(Al, Zn)12 particles with significantly high concentration of Zn were found to precipitate in the stir zone on the AZ31 side. A bowl-shaped region formed with a maximum depth of ~770 μm on the DC01 side, with the grain size decreasing rapidly from the base material to the area at the interface (~120 nm) due to the operation of dynamic recrystallization. Significant amount of Mg and Al atoms was found in DC01 grains near interface, which can be attributed to local mechanical intermixing between AZ31 and DC01 and the enhanced solid solubility via severe deformation. A thin Al-rich layer of ~50 nm thick formed within DC01 grains upside at the interface with the Al atoms crossed the interface from AZ31, while abundant nanoscale Mg particles precipitated out at the boundaries of nano-sized DC01 grains due to low solid solubility of Mg in Fe. The tensile shear strength of the joint reached 295.6 N/mm, with the crack passing through the root of hook-like structure and the stir zone of AZ31 instead of the interface, indicating a high bonding strength of such an interface microstructure.
查看更多>>摘要：? 2022 Elsevier Inc.TA15 and TC17 titanium alloys were joined by linear friction welding (LFW) method. The microstructure investigation indicated that the TC17 side welding zone (WZ) was composed of coarse β phases, α phase at grain boundaries and martensite α’ phase at the grain interior, while the TA15 side WZ consisted of abundant acicular martensite α’ phases and few retained β phases. Electrochemical test result indicated that TA15 base metal (BM) has better corrosion resistance than the TC17 BM. Numerous corrosion pits were formed on the TC17 BM and TC17 side WZ after immersion and salt spray tests. The worse corrosion resistance of the TC17 BM could be attributed to its high content α/β phase boundaries.
查看更多>>摘要：? 2022 Elsevier Inc.In this paper, the pulsed electromagnetic field treatment (PEFT) is designed between the pre-stretching and artificial aging of the thermomechanical treatment. Using electron microscopy and hardness test, the effect of the PEFT on dislocation evolution and subsequent artificial aging behavior of spray deposited 2195 Al-Li alloy is studied. The results show that although the PEFT reduces the Vickers microhardness of the specimen, it improves the hardening kinetics and precipitation strengthening of the specimen during the subsequent artificial aging process. Besides, it is found that the PEFT reduces the dislocation density of the pre-stretching specimen and improves the dislocation uniformity. The PEFT makes the T1 precipitate obtained by the subsequent artificial aging process more finely dispersed, a decrease of the average thickness by 57.6%. From simulation combined with theoretical analysis, it is found that the static recovery induced by temperature rise is an important reason for the decrease of dislocation density on the specimen during the PEFT. The magnetically induced stress is an important driving force for a more uniform dislocation distribution. The mechanism of the microstructure evolution of the material is systematically discussed and summarized.
查看更多>>摘要：? 2022 Elsevier Inc.0.5 mm thick AA 7075-T6 ultrathin sheets were butt jointed by conventional friction stir welding (C-FSW) and high speed friction stir welding (HS-FSW) under a constant rotational speed (ω)/welding speed (v) ratio of 6.67 rad/mm using a pinless tool. In this study, the conventional ω of 2000 rpm and conventional v of 300 mm/min were adopted by C-FSW, and the high ω of 8000 rpm and high v of 1200 mm/min were adopted by HS-FSW. Compared with C-FSW, HS-FSW presented a fast heating and cooling rate thermal characteristic with a slightly higher peak temperature and significantly shorter elevated-temperature exposure time. Kissing bond defect was appeared in the weld bottom for C-FSW. While, no obvious defect was found in the weld obtained by HS-FSW. Compared with C-FSW, the proportion of deformed grains in the center of nugget zone (NZ) fabricated by HS-FSW was increased. For the joint fabricated by HS-FSW, the upper NZ was dominated by recrystallized grains, and the lower NZ was dominated by deformed grains. The ultimate tensile strength of the joint fabricated by HS-FSW can reach 87% of base metal (BM), and its elongation, yield strength and ultimate tensile strength were increased by 58.8%, 23.3% and 22.8% respectively compared with C-FSW. Both the joints fractured in the center of the NZ. The joint fabricated by C-FSW broke due to the kissing bond defect. While the joint fabricated by HS-FSW may be broken due to the weld thickness reduction.
查看更多>>摘要：? 2022 Elsevier Inc.The effects of various Dy addition on the microstructure and mechanical properties of Mg-6Zn-1Mn-3Sn (wt%, ZMT613) alloys were investigated. The results showed significant grain refinement and new MgSnDy phases were detected with Dy addition of the as-cast ZMT613 alloy. A small amount of Dy (≤0.8 wt%) can hinder the proportion of dynamic recrystallized (DRXed) grains during hot-extrusion process and enhance the basal texture of the alloy. The MgSnDy high-temperature phase (HTP) particles inhibited the recovery process and retained substructures during solution treatment, which promoted the precipitation of the second phase. The as-aged ZMT613–0.2Dy alloy showed optimum mechanical properties, where yield strength, ultimate tensile strength, and elongation were 336 MPa, 399 MPa, and 4.4%, respectively.
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