Mg-Nd-Y-Zr合金的无熔剂保护制备工艺研究

以金属镁、金属钕、镁钇中间合金和镁锆中间合金为原料,进行Mg-Nd-Y-Zr合金的无熔剂制备工艺的研究。通过考察熔炼温度、熔炼时间、合金液静置时间以及浇铸温度等工艺参数对合金收率、合金元素收率以及合金元素偏析程度等技术指标的影响,结合对合金铸态组织结构的研究,确定Mg-Nd-Y-Zr合金的均质纯净化制备工艺条件为:熔炼温度770℃、熔炼时间150 min、物料加入速率50 g/min、保护气中SF6体积分数0.1%、合金液静置时间60 min以及浇铸温度740℃。该工艺条件下,合金收率大于96%,合金元素偏析程度为±(0.05~0.15)%(质量分数),合金元素含量偏差±(0.03~0.10)%(质量分数),合金内部无氧化缺陷,无杂质元素的引入富集。合金中的典型强化相为Mg41Nd5、Mg12Nd及Mg2Y相,呈网状分布,钕和钇元素对合金具有弥散强化作用;锆元素在合金中具有晶粒细化和铁杂质去除的双重作用。

A novel immiscible high entropy alloy strengthened via L1_(2)-nanoprecipitate

The low-cost Fe-Cu,Fe-Ni,and Cu-based high-entropy alloys exhibit a widespread utilization prospect.However,these potential applications have been limited by their low strength.In this study,a novel Fe_(31)Cu_(31)Ni_(28)Al_(4)Ti_(3)Co_(3) immiscible high-entropy alloy(HEA)was developed.After vacuum arc melting and copper mold suction casting,this HEA exhibits a unique phase separation microstructure,which consists of striped Cu-rich regions and Fe-rich region.Further magnification of the striped Cu-rich region reveals that it is composed of a Cu-rich dot-like phase and a Fe-rich region.The aging alloy is further strengthened by a L1_(2)-Ni_(3)(AlTi)nanoprecipitates,achieving excellent yield strength(1185 MPa)and uniform ductility(~8.8%).The differential distribution of the L1_(2) nanoprecipitate in the striped Cu-rich region and the external Fe-rich region increased the strength difference between these two regions,which increased the strain gradient and thus improved hetero-deformation induced(HDI)hardening.This work provides a new route to improve the HDI hardening of Fe-Cu alloys.

Microstructure and mechanical properties of high strength Al-Zn-Mg-Cu alloys used for oil drill pipes

Three Al?Zn?Mg?Cu alloys used for oil drill pipes (Alloy A: Al?6.9Zn?2.3Mg?1.7Cu?0.3Mn?0.17Cr; Alloy B: Al?8.0Zn?2.3Mg?2.6Cu?0.2Zr, Alloy C: Al?8.0Zn?2.3Mg?1.8Cu?0.18Zr) were studied by hardness tests, tensile tests and transmission electron microscopy (TEM). The results show that the ultimate tensile strength, yield strength and elongation for Alloys A, B and C are 736 MPa, 695.5 MPa and 7%; 711 MPa, 674 MPa and 12.5%; 740.5 MPa, 707.5 MPa and 13%, respectively after solid solution treatment ((450 °C, 2 h)+(470 °C, 1 h)) followed by aging at 120 °C for 12 h. The dominant strengthening phases in Alloy A are GPII zone andη′ phase, the main precipitate in Alloy B isη′ phase, and the main precipitates in Alloy C are GPI zone, GPII zone andη′ phase, which are the reason for better comprehensive properties of Alloy C. The increase of zinc content leads to the improvement of the strength. The increase of copper content improves the elongation but slightly decreases the strength. Large second-phase particles formed by the increase in the manganese content induce a decrease in the elongation of alloys.

Anisotropic and temperature-dependent growth mechanism of S-phase precipitates in Al-Cu-Mg alloy in relation with GPB zones

By employing atomic-resolution imaging and first principles energy calculations, the growth behavior of S-phase precipitates in a high strength A1-Cu-Mg alloy was investigated. It is demonstrated that the nucleation and growth of the S-phase precipitate are rather anisotropic and temperature-dependent companying with low dimensional phase transformation. There are actually two types of Guinier-Preston （GP） zones that determine the formation mechanism of S-phase at high aging temperatures higher than 180 ℃. One is the precursors of the S-phase itself, the other is the structural units or the precursors of the well-known Guinier-Preston-Bagaryatsky （GPB） zones. At high temperatures the later GPB zone units may form around S-phase precipitate and cease its growth in the width direction, leading to the formation of rod-like S-phase crystals; whereas at low temperatures the S-phase precipitates develop without the interference with GPB zones, resulting in S-phase orecioitates with lath-like momhology.

Investigation on sintering and deformation strengthening of Mo-Cu alloy

Mo-Swt%Cu nanocomposite powders were fabricated by mechanical alloying, and full density alloy was obtained via liquid-phase sintering and post-treatment process. The microstructure of Mo-8wt%Cu alloy was investigated by scanning elec-tron microscope （SEM） , and the effects of process parameters on relative density, tensile strength and elongation were stud-ied. The results indicate that the relative density of Mo-Cu alloy is 98. 6% after sintering at 1 250℃ for 30 min, and its micro-structure is composite network The full density of Mo-Cu alloy can be obtained when specimens are treated through deforma-tion strengthening process of rotating forging and hydrostatic extrusion The tensile strength and elongation rate are 576 MPa and 5. 8% ,respectively, when hydrostatic extrusion deformation degree is 40%.

Effects of trace Ag on precipitation behavior and mechanical properties of extruded Mg−Gd−Y−Zr alloys

The effects of trace Ag element on the precipitation behaviors and mechanical properties of the Mg−7.5Gd−1.5Y−0.4Zr(wt.%)alloy by means of tensile test,X-ray diffractometry,scanning electron microscopy,electron backscattered diffractometry,and scanning transmission electron microscopy.There is an unusual texture(á0001ñ//extrusion direction)in the extruded Mg−Gd−Y−Zr alloys containing 0.5 wt.%Ag.During the aging periods at 225℃,the addition of the trace Ag does not form new precipitates,just accelerates aging kinetics,and refinesβ′precipitates,thereby increasing the number density of theβ′precipitates by Ag-clusters.Moreover,the Mg−Gd−Y−Zr alloy containing 0.5 wt.%Ag shows the most excellent synergy of strength and plasticity(408 MPa of ultimate tensile strength,265 MPa of yield strength,and 12.9%of elongation to failure)after peak-aging.

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg−Zn−Cu−Zr−Ca samples were solidified under high pressures of 2-6 GPa.Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure.The mechanical properties of the samples were investigated through compression tests.The results show that Ca is mostly dissolved in the matrix and the Mg_(2)Ca phase is formed under high pressure,but it is mainly segregated among dendrites under atmospheric pressure.The Mg_(2)Ca particles are effective heterogeneous nuclei ofα-Mg crystals,which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy,with the grain size reduced to 22μm at 6 GPa.As no Ca segregating among the dendrites exists,more Zn is dissolved in the matrix.Consequently,the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg7Zn3 with a lower Zn/Mg ratio.The volume fraction of the intergranular second phase also increases to 22%.Owing to the combined strengthening of grain refinement,solid solution,and dispersion,the compression strength of the Mg-Zn-Cu-Zr-Ca alloy solidified under 6 GPa is up to 520 MPa.

Tailoring phase fractions of T'and η' phases in dual-phase strengthened Al−Zn−Mg−Cu alloy via ageing treatment

Hardness tests and transmission electron microscopy were used to investigate the strategy of tailoring the phase fraction of precipitates in an Al-Zn-Mg-Cu alloy strengthened by T’ and η’ phases. Different phase fractions of T’ and η’ phases are presented in samples subjected to either single or two stages of ageing treatments at 120 and 150 ℃.For both types of ageing, the precipitation of η’ phase is found to be promoted by ageing at lower temperature and its phase fraction increases with prolonging ageing time at 120 ℃;whereas the phase fractions of T’ and η’ phases almost remain constant during ageing at 150 ℃. Besides, the strain fields produced by T’ and η’ phases were analyzed by using the geometric phase analysis technique, and on a macroscale the contributions of T’ and η’ phases to precipitation strengthening have been quantitatively predicted by combining the size, phase fraction and number density of precipitates.

Effect of magnesium on dispersoid strengthening of Al-Mn-Mg-Si(3xxx) alloys

The effects of magnesium addition on the dispersoid precipitation as well as mechanical properties of 3xxx alloys wereinvestigated. The microstructures in as-cast and heat-treated conditions were evaluated by optical microscopy and transmissionelectron microscopy. The results reveal that Mg has a strong influence on the distribution and volume fraction of dispersoids duringprecipitation heat treatment. The microhardness and yield strength at ambient temperature increase with increasing Mg content. Thesolid solution and dispersoid strengthening mechanisms of materials after heat treatment are quantitatively analyzed. Dispersoidstrengthening for the alloys is the predominant strengthening mechanism after precipitation heat treatment. An analytical model isintroduced to predict the evolution of ambient-temperature yield strength.

Enhanced mechanical properties in oxide-dispersion-strengthened alloys achieved via interface segregation of cation dopants

With significantly enhanced irradiation resistance,high-temperature strength,and creep resistance,oxide-dispersion-strengthened tungsten(ODS-W)alloys present tremendous potential for high-temperature applications.However,the oxide particles tend to segregate at W grain boundary and grow up(even to micron),greatly suppressing their strengthening effect.It is always a great challenge to effectively refine and disperse the oxide particles at W grain boundary.Here,we successfully developed a new type of cation-doped W-Y2O3 alloy via a wet chemical method and subsequent low-temperature sintering.It was found that proper cation doping could not only significantly refine the intergranular Y2O3 second phase particles but also dramatically improve the sinterability of W matrix.These doping effects,as a result,simultaneously enhance the strength and ductility of the W-Y2O3 alloy.It was confirmed that the segregation of cation dopants at the W/Y2O3 interface is the origin of these doping effects.Furthermore,X-ray photoemission spectra(XPS)analyses confirmed that cation dopant segregation also obviously affects the chemical bonding(i.e.,W–O bond)along the W/Y2O3 interface.As a result,the ratelimiting mechanism for W grain growth is influenced remarkably,explaining well the difference of W grain size in various cation-doped W-Y2O3 alloys.For the refinement of intergranular Y2O3 particles,it can be understood well from both thermodynamic and kinetic views.Detailedly,W/Y2O3 interfacial energy and atom mobility for Y2O3 coarsening are all limited by cation dopant segregation.More importantly,this cation-doping approach can also be applicable to other ODS alloys for enhancing their comprehensive mechanical properties.

Low-temperature thermoplastic welding of metallic glass ribbons for in-space manufacturing

In-space manufacturing is an emerging and promising research field in space industry,which benefits the development of space explorations.Owing to the microgravity,high vacuum and complexity of the space environment,a special manufacturing strategy for alloys is highly demanded for the in-space manufacturing.Herein,a lowtemperature thermoplastic metallic welding method was proposed and employed for La-based metallic glass ribbons.With the sandwiched structures of La-and Zr-based ribbons,the welded samples exhibit a competitive fracture strength and an increased tensile strain than those of the welded la-based samples due to the second phase strengthening.This suggests that metallic glass is an ideal material candidate for in-space manufacturing,and can be well manufactured at a nonmelting state with good mechanical performances.In addition,the thermoplastic manufacturing method can also feature a moderate processing time window.In general,this work presents the promising potential of low-temperature thermoplastic welding strategy for in-space manufacturing of metallic materials.