查看更多>>摘要：The present study deals with the thermal stability of the microstructure of the rapidly solidified ribbon-consolidated Mg97.94Zn0.56Y1.5 alloy. In the consolidated state, the material has a very fine-grained microstructure with an average grain size of similar to 790 nm and contains Zn- and Y-rich stacking faults (SFs) in basal planes. SFs are dispersed in an individual manner or organized in the blocks forming a Mille-feuille structure (MFS). The alloy is characterized by a weak basal texture with a more pronounced intensity at the (10 (1) over bar0) pole. In order to study the thermal stability of the microstructure, isothermal annealing in a temperature range of 300-500 degrees C was applied. The microstructure is thermally stable up to 400 degrees C, which is exceptionally high compared to conventional ultra-fine grained magnesium alloys. At higher temperatures, the growth of the grain size and redistribution of the texture intensity is related to the recrystallization process. The order of dispersion of the solute-segregated SFs is independent of the annealing temperatures. However, there is a change in the arrangement of SFs in the grains: the thickness of the SFs blocks increases with increasing temperature, i.e., the stacking faults became more agglomerated. Nevertheless, even after annealing at 500 degrees C, there is still a mixture of several polytypes of the long-period stacking ordered (LPSO) phase rather than a single-ordered LPSO phase.
查看更多>>摘要：The effect of extrusion and heat treatment processes on the microstructure and mechanical properties of the spray formed 7055 alloy were systematically studied by three-dimensional X-ray microscope, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atom probe tomography and tensile testing. It was found that the main second-phases in the as-deposited 7055 alloy include theta (Al2Cu) and Mg(Zn,Al,Cu)(2) phases with similar structure to the eta (MgZn2) phase dissolved with Al and Cu. The Al2Cu particles embedded in Mg(Zn,Al,Cu)(2) phase are widely distributed on the grain boundaries. Compared with the as-cast 7055 alloy, the S (Al2CuMg) phase and T (Al2Mg3Zn3) phases were not detected in the as-deposited alloy due to the change of solidification paths caused by rapid solidification. The microporosity caused by spray forming is always observed in the as-deposited alloy, these micro-pores can be effectively eliminated by the extrusion processing. Meanwhile, compared with extrusion directly after preheating at 420 degrees C/24 h, additional high temperature two-stage homogenization treatment before extrusion can reduce the adverse effect of S (Al2CuMg) phase and promote the precipitation of high density of Al3Zr dispersoids, which significantly increases the recrystallization resistance and improve the mechanical properties of the alloy.
查看更多>>摘要：This work presents the mechanical properties and elaborately discusses microstructural aspects of LSPwC (laser shock peening without coating) peened 7075-T6 aluminum alloy using the results coming from various characterization techniques such as surface roughness, Vickers microhardness, residual stress analysis, tensile test, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). The possibilities and consequences of thermo-mechanical interaction, deformation behavior, grain refinement, fractional change of grain boundaries, compressive residual stress and nucleation of precipitates during LSPwC are discussed along with the underlying mechanisms. Microhardness increased by about 25% post LSPwC with a work hardened layer of more than 1000 mu m. Maximum compressive residual stress of -227 MPa (in sub-surface region of peened specimens) was induced with effective depth (i.e. thickness of compressive layer) of 900 mu m. The defect (dislocation) concentration, volume fraction of precipitates, area fraction of fine grains (of below 50 mu m) and number fraction of low angle grain boundaries (especially of 2 degrees-5 degrees misorientation) increased significantly after LSPwC. Besides, the effects of observed improvements (by LSPwC) in tensile behavior were also investigated. The near surface strengthening or modifications observed in this investigation are collectively attributed to LSPwC induced plastic deformation, compressive residual stress, additionally induced precipitates, grain refinement and increased fractions of LAGBs.
查看更多>>摘要：The influence of Cr addition on the microstructure and stress corrosion cracking behavior of Al-6.5Zn-2Cu-1.5 Mg-0.05Ti alloy was investigated by microscopy characterization and slow strain rate tensile tests. In Cr-containing alloys, E (Al18Mg3Cr2) phases precipitated from the matrix, which could serve as energetically preferred heterogeneous nucleation sites for eta' (Mg(Zn, Cu)(2)) phases. Due to the decrease of eta' (MgZn2) strengthening phases and the formation of If (MgZn2) precipitate free zones, the strength of Cr-containing alloys was reduced to some extent. However, the crack propagation could be suppressed by the discrete distribution of grain boundary precipitates and the inhibition of recrystallization, leading to the increase of the elongation and the stress corrosion cracking resistance. With the addition of 0.15 wt% Cr, the comprehensive performance of Al-6.5Zn-2Cu-1.5 Mg-0.05Ti alloy was improved. Compared with the base alloy, the susceptibility index of stress corrosion cracking (I-SCC) decreased by 82% and the elongation increased by 42% at the expense of partial strength (less than 5%).
查看更多>>摘要：The hetero-deformation induced (HDI) strengthening of alloys can achieve the superior combinations of strength and ductility. However, there are not enough examples to reveal quantitatively the contributing proportion of HDI strengthening to yield strength, especially the systematic comparisons between classical strengthening mechanisms and HDI mechanism. In this paper, we performed calculations on four microstructures of Al-Mg-Sc alloy that friction stir processing (FSP) and rolled deformation (R80%) were used, including initial material (IM), FSP, IM + R80%, and FSP + R80%. It was found that the FSP + R80% had higher yield strength and elongation than IM + R80%, and strain hardening rate presented two modes in the two materials. Based on the complementation between classical strengthening mechanisms and HDI strengthening, the quantitative calculation implied that the better performance was related to the higher HDI stress (sigma HDI) in FSP + R80%. Precisely, for the contribution ratio of sigma HDI to yield strength, FSP + R80% was 1.39 times that of IM + R80%. Finally, at the small strain, IM + R80% was dominated by dislocation strengthening, while HDI strengthening in FSP + R80% played an important role, thus the mode of strain hardening rates of the two were inconsistent.
查看更多>>摘要：To investigate the hot deformation behaviour of Al-3%Mg2Si, isothermal compression tests were carried out in the temperature range of 300-500 degrees C and strain rate range of 0.01-5 s-1. The deformed microstructures were characterized by optical microscope, scanning electron microscope, transmission electron microscope and electron backscatter diffraction. The results show that the flow stress curves reveal a typical dynamic recovery characteristic. A modified Arrhenius-type constitutive equation was built to describe the flow behaviour of Al-3% Mg2Si alloy. Furthermore, based on the processing map of Al-3%Mg2Si alloy established under the strain of 0.8, microstructure evolution was analyzed. The high efficiency of power dissipation regions is mainly due to the softening mechanism of dynamic recovery and partial dynamic recrystallization. Mg2Si particles also play a positive role in dynamic recrystallization. The damage mechanism of unstable domains is the fracture and debonding of Mg2Si particles.
查看更多>>摘要：We present a study on laser nitriding on NAK80 steel alloy. The alloy contains Al, which results in many faceted AlN precipitates formed in the subsurface region. The AlN facets have three different crystallographic orientations, and their relationships with respect to martensite matrix were [(2) over bar 110](AlN) parallel to [(1) over bar 11](Fe), [2 (1) over bar 11](AlN) parallel to [010]F-e, and [(2) over bar 110](AlN) parallel to [001](Fe). Among these, [(2) over bar 110](AlN) parallel to [(1) over bar 11](Fe) exhibited a basal plane of AlN at approximately -5 degrees with respect to the {110}(Fe) plane, suggesting that rotation reduced the interfacial energy between AlN and the matrix. The mismatch parameters of each facet direction were calculated as 6.19%, 4.03%, and 0.86% for [(2) over bar 110](AlN) parallel to [(1) over bar 11](Fe), [2 (1) over bar 11](AlN) parallel to [010]F-e, and [(2) over bar 110](AlN) parallel to [001](Fe), respectively. Consequently, a greater degree of mismatch requires more elemental segregation to reduce the interfacial energy. The analysis results of the interfacial energy between the matrix and precipitate makes it possible to consider the efficient formation of precipitates.
查看更多>>摘要：The effects of cerium (Ce) on the microstructure, mechanical properties and strain hardening behavior of Fe18Mn-0.6C twin-induced plasticity (TWIP) steel were investigated by electron backscatter diffraction, X-ray diffraction and transmission electron microscopy. Compared with the addition of Al, the addition of Ce significantly reduced the stacking fault energy of TWIP steel, which promotes the formation of deformation and annealing twins. The dynamic strain aging behavior of TWIP steel was inhibited by Al and Ce atoms during plastic deformation. The optimal mechanical properties were obtained when 0.015 wt% of Ce was added, resulting in a tensile strength of 1023 MPa; and an elongation was 92%. The interaction of twin variants in the Ce-containing alloys exhibited an X shape, which was significantly different from the T shape in the non-Ce containing alloy. This is mainly attributed to the fact that Ce promotes dislocations near grain boundaries, where for the nucleation of deformation twins at grain boundaries is favored. Compared with the T-shaped twin variants, the dynamic Hall-Petch effect caused by the X-shaped staggered twin variants was stronger, which improved the strain hardening ability of TWIP steel.
查看更多>>摘要：The initiation mechanism of the discontinuous precipitation (DP) reaction driven by migrating grain boundaries (GBs) in Alloy 33 (Cr-Fe-Ni-N) isothermally aged at 800 degrees C has been studied using analytical electron microscopy, which includes scanning/transmission electron microscopy (STEM/TEM), X-ray energy-dispersive spectroscopy (XEDS), and electron diffraction. Precipitation products including both the types of precipitated phases (FCC M23C6 and M6N with diamond-cubic structure) and the GB morphology generated in the early stages of the aging process have been investigated in detail to correlate the experimental findings of the present work with the DP initiation mechanisms reported in the literature. STEM-XEDS elemental maps and electron diffraction data confirmed the FCC M23C6 was the first precipitated phase at the GBs, generating a Cr-depleted zone along the boundary and around the intergranular precipitates. The most remarkable characteristic in the initiation of DP reaction observed in this alloy system refers to the consistent evidence that the GB migration occurred to a significant extent while the M23C6 precipitates remain at the original GB position. In all observations, GB migration occurred against the capillarity force of the concave-forward boundary curvature, thereby suggesting a strong chemical force acting on the GB. Micro and nano-scale analytical STEM-XEDS results corroborated the existence of a compositional gradient of Cr across all migrating GB, confirming that a chemical driving force is responsible for the displacement of the GB. It is concluded that in the overall precipitation phenomenon the necessary solute partitioning is operated by interface diffusion mechanism through the moving GB acting as reaction front. Extensive STEM-XEDS analyses of the precipitation products observed in this investigation have confirmed that diffusion-induced grain boundary migration (DIGM) plays a major role as precursor to the initiation of DP reaction in Alloy 33.
查看更多>>摘要：The hot deformation behavior of a novel Sc, Zr alloyed Al-Zn-Mg-Cu alloy was explored in the isothermal hot compression test with a temperature range of 300-500 degrees C and a strain rate of 0.001-10 S-1. The stress-strain curve, constitutive equation, and processing map were integrated to evaluate the flow behavior and hot work-ability of the studied alloy. Besides, the relationship between dynamic recrystallization and precipitation behavior and the Zener-Hollomon (Z) parameter during hot deformation was deeply explored. As lnZ decreased, the dislocation density decreased, while the misorientation angle increased. Meanwhile, with the decrease of lnZ, the amount of low-angle grain boundaries and medium-low-angle grain boundaries decreased. Moreover, dynamic recovery is the dominant dynamic softening mechanism, while three different dynamic recrystallization (DRX) mechanisms were the main DRX mechanisms at different lnZ values. Discontinuous DRX dominated at high lnZ value (lnZ >= 33.33); Continuous DRX (CDRX) dominated at middle lnZ value (23.4 < lnZ < 33.33); Geometric DRX dominated at low lnZ value (lnZ <= 23.44). Meanwhile, increasing lnZ value is conducive to precipitation behavior. The intragranular precipitates and grain boundary precipitates are identified as the phase containing Al, Fe, Mn, and Si, the equilibrium eta phase containing Cu and Al-3(Sc, Zr) particles.
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Elsevier