查看更多>>摘要：This study was undertaken to investigate the influence of heat treatment on the stress corrosion cracking (SCC) behavior of a low-pressure sand-cast Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy in a 3.5 wt% NaCl spray environment. Results indicated that T4 heat treatment (540 degrees C x 10 h) could effectively improve the corrosion resistance, compared with as-cast and T6 alloys. The relatively lowest volume fraction of high-potential (HPOT) phases observed in the T4 alloy primarily accounts for its lowest corrosion rate. Slow strain rate tensile (SSRT) tests were conducted to evaluate the influence of heat treatment states on the SCC resistance. The lowest SCC susceptibility index (I-SCC I = 0.403 and I-SCC II = 0.165) was obtained in T4 state, while the as-cast alloy possessed the highest SCC susceptibility. The present work revealed that hydrogen-assisted cracking (HAC) played a key role in the SCC process. Cracking was effectively suppressed in the T4 alloy owing to its retarded hydrogen diffusion. The structure and compositions of the multi-layer corrosion product film were characterized in detail. Solution treatment applied in this work was confirmed to promote the formation of protective and dense RE oxides layer, primarily contributed to the relatively highest SCC resistance achieved in the T4 alloy.
查看更多>>摘要：A compositionally complex (Ca, Sr, Ba)ZrO3 (CC-CSBZ) fibrous membrane with perovskite structure was successfully prepared via a simple electrospinning method. The formation, phase transformation and microstructure evolution of CC-CSBZ fibers were fully discussed. X-ray diffraction, Raman spectra and transmission electron microscopy analyses confirmed the formation and phase transformation of CC-CSBZ solid solution at high temperatures. Scanning electron microscopy results indicated that a slower grain growth rate and more stable high-temperature microstructure were achieved for CC-CSBZ fibers compared with previous BaZrO3 and CaZrO3 fibers. CC-CSBZ fibrous membrane also exhibited excellent flexibility at 1000 degrees C and high thermal stability at 1200 degrees C. Furthermore, an average near infrared reflectivity of 98% at least at room temperature was obtained for CC-CSBZ fibrous membrane heat-treated from 800 to 1400 degrees C. For the slow grain growth rate, excellent high-temperature stability and near infrared reflectance, CC-CSBZ fibrous membrane would be a promising and novel ceramic fiber product for high temperature applications.
Biserova-Tahchieva, AlisiyaChatterjee, Dipanwitavan Helvoort, Antonius T. J.Llorca-Isern, Nuria...
15页
查看更多>>摘要：In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS 532750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (epsilon up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 degrees C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated.
查看更多>>摘要：In this paper, AlCoCrFeNi high-entropy alloy (HEA) particles was used as the reinforcement for aluminum matrix composites fabricated by friction stir processing. HEA particles were uniformly dispersed in the aluminum matrix. The formation of multiple intermetallic compounds and interface behavior were closely evaluated. The HEA-Al interface consisted of Al-13(CoCrFeNiMg)(4), Al-9(CoFeNi)(2) and Al18Cr2Mg3, and the interface formation and growth behavior were investigated by coupling the thermodynamics and kinetics factors. The low effective Gibbs free energy of Al-13(CoCrFeNiMg)(4) and Al-9(CoFeNi)(2) phases is the thermodynamic nature of their generation at the interface. Compared with as-received Al, the fabricated composites exhibited 16.8% higher yield strength and 28.1% higher tensile strength, and the elongation was retained to 15.3%. Both of brittle and ductile fracture characteristics were observed on the surface of the HEA particles.
查看更多>>摘要：Magnesium matrix composites usually possess high strength and stiffness as well as low coefficient of thermal expansion, which makes them promising in lightweight and energy-saving strategies. Although the interfacial bonding state has a crucial influence on the properties of magnesium matrix composites, the interfacial reaction mechanism of Mg-RE alloy matrix composites is still unclear. Herein, the interfacial reaction and microstructure of Mg-10Gd-3Y-1Zn-0.4Zr (wt%) alloy matrix composite reinforced with aluminum borate whisker (Al18B4O33w) were reported by heating the matrix to a semisolid region and inducing the interfacial reaction to continue. After heat treatment at 600 degrees C for 6 h, the thickness of the interface layer increased from 50-120 nm to about 335 nm, and the main interfacial reaction product was (Mg0.4Al0.6)Al1.8O4. With the increase of heating time to 12 h, a new double interface layer with (Mg0.4Al0.6)Al1.8O4 in the inner layer and MgO in the outer layer was formed. The difference of interfacial reaction products during isothermal heat treatment was related to the concentration gradient of Mg atoms along the radial direction of the whiskers and the hindrance of the migration of Mg atoms by reaction products formed in the early stage. These findings provide a further understanding of the interfacial reaction of Al18B4O33w reinforced Mg-RE alloy matrix composites.
查看更多>>摘要：The objective of this research is to study the effect of the Zr on the L-PBF processability, microstructure, and microhardness of an AlZnMgCu-Zr alloy. Two AlZnMgCu-0.5 and 1.5 wt% Zr pre-alloyed powders were produced by casting followed by gas atomization. In addition, an excess of Mg and Zn was added to the target compositions to compensate for vaporization during L-PBF. The as-atomized powders and the as-built and heat treated specimens were characterized via scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Crack-free samples with a relative density of 99.0 +/- 0.1% were obtained in the alloy containing 1.5 wt%Zr. The as-built microstructure of this alloy consisted of small equiaxed grains without preferred grain orientation near the melt pool boundary and slightly columnar grains between adjacent melt pools. After a solution and aging heat treatment (T6), the newly developed AlZnMgCu-1.5Zr alloy has an outstanding Vickers micro-hardness of 223 +/- 3 HV, which is significantly higher than that of the wrought Al7075-T6 aluminum alloy. This excellent mechanical behavior is attributed to the presence of large Al3Zr nucleant particles of up to 500 nm in size that inhibit grain growth during solidification and the subsequent heat treatment, and of a high density of nanoscale second phases (MgZn2 and Al3Zr) located within the aged alpha-Al grains.
查看更多>>摘要：Because of the special crystallization structure (hexagonal close-packed structure), it is difficult to achieve the synergy between strength and ductility in Mg alloys. High strength can be obtained through alloying addition or low-temperature deformation, but these methods usually cause the extremely poor ductility (less than 5%). Meanwhile, solid solution treatment or considerable alloying addition, such as RE, Li, can probably possess the superior ductility, whereas these techniques commonly result in the insufficient strength (poor than 100 MPa). However, Mg-1Mn binary alloy developed in our previous work extruded at a low temperature exhibiting the superior ductility (38.8%) with medium yield strength (204.3 MPa) has been recommended as one of the candidates to improve the strength-ductility synergy in Mg alloys. In this paper, a low-alloyed Mg-1Mn-1Al alloy has been developed to overcome the disequilibrium between strength and ductility. The alloy has a superior elongation capacity (- 32%) and a reasonably high tensile yield strength (- 280 MPa) at room temperature. The reasonably high yield strength of the alloy can be dominantly attributed to the refined DRXed grains based on the Hall-Petch relationship, intensified basal texture leading to the basal slip activity difficultly, a great number of nano-scaled Al8Mn5 secondary particles pinning the dislocation motion through the dispersion hardening and the multi-heterogeneous microstructure inducing bask-stress strengthening as dislocation pile-up. The superior elongation capacity of the alloy can be mainly explained as the ultra-fined microstructure through reducing the critical resolved shear stress (CRSS) ratio between prismatic slip and basal slip that favors prismatic slip activity to accommodate basal slip.
查看更多>>摘要：The widespread application of selective laser melting (SLM) fabricated high performance lightweight Al components is hindered by the available commercial alloys due to their limited mechanical properties and processability. The introduction of Sc/Zr elements to the Al matrix for SLM provides conspicuous benefits, including grain refinement, precipitation strengthening and high thermal stability. In the present study, the multiple effects of Sc/Zr elements in SLM fabricated Al alloy are systematically investigated, using the recently developed Al-MnSc alloy as an example. The application of Sc/Zr elements generated an unconventional heterogeneous grain structure characterised by the alternative growth of columnar-equiaxed grains in the molten pool, which effectively avoided the solidification cracks and alleviated the mechanical property anisotropy issues. The formation of the heterogeneous grain structure is mainly attributed to the precipitation of the primary Al3(Sc,Zr) particles which dependent on the local solidification conditions. The post heat treatment was also conducted to clarify the role of Sc/Zr additions on stress relieve effect and precipitation hardening effect. Moreover, the room temperature and elevated temperature mechanical properties of the SLMed Al-Mn-Sc alloy were also evaluated. The fundamental understanding of the Sc/Zr benefits in the SLM fabricated Al alloy is expected to shed light on future high performance Al alloy development specifically for the additive manufacturing process.
Fuh, Jerry Ying HsiChen, JieYang, YongqiangBai, Yuchao...
14页
查看更多>>摘要：This paper focuses on the influence mechanism of process parameters on the geometry morphology and microstructure characteristic of single-track and multi-track 316L/CuSn10 multiple materials manufactured by laser powder bed fusion (LPBF). The width of single-track increases with the increase of laser power and layer thickness, and decreases with the increase of scanning speed. Both variations in the copper content and wetting angle of the molten pool were investigated. In addition, the comparison of defect characteristics shows that the scanning speed and layer thickness have a greater influence on the formation of single cracks. The traces of Marangoni convection are observed directly in the centre of the molten pool due to the difference in microstructure between 316L and CuSn10 alloys. It is found that the copper penetration cracks appear on the steel side at the molten pool bottom. Moreover, fine grains appear in the copper-rich region, and the large-angle grain boundary distribution seems to be the reason for the concentration of dislocations. Four types of crack formation mechanisms are found in the cross section of the multi-track: crack formation inside the molten pool, passing through the track boundary, extending to the steel substrate, and copper penetration cracks.
NSTL
Elsevier