Second phase effect on corrosion of nanostructured Mg-Zn-Ca dual-phase metallic glasses

Dual-phase metallic glasses(DP-MGs),a special member of the MGs family,often reveal unusual strength and ductility,yet,their corrosion behaviors are not understood.Here,we developed a nanostructured Mg_(57)Zn_(36)Ca_(7)(at.%)DP-MG and uncovered its corrosion mechanism in simulated body fluid(SBF)at the near-atomic scale utilizing transmission electron microscope(TEM)and atom probe tomography(APT).The 10-nm-wide Ca-rich amorphous phases allow oxygen propagation into the DP-MG,resulting in a micrometer thick hydroxides/oxides layer.This dense corrosion layer protects the DP-MG from further corrosion,enabling a corrosion rate that is 77%lower than that of Mg(99.99%purity).

高密度金属间化合物纳米颗粒强化型单晶高熵超合金的高温氧化行为

本文系统研究了单晶L_(12)强化富Co高熵超合金Co_(41)Ni_(35)Al_(11.5)-Ta_(2.5)Cr_(4)Ti_(6)(at%)在空气中800,900和1000℃下的氧化行为.结果表明,该合金在800至1000℃范围内均表现出优异的抗氧化性.其中,在800℃氧化后,合金表面形成了双层氧化膜,由TiO_(2)/Al_(2)O_(3)混合层和Cr_(2)O_(3)层组成.连续但不致密的Cr_(2)O_(3)层不能有效抑制内部氧化和内部氮化.在900和1000℃时形成复杂的分级结构氧化膜,主要成分为尖晶石、Cr_(2)O_(3)、TiTaO_(4)和Al_(2)O_(3).值得注意的是,在900至1000℃的温度范围内发现抗氧化性出现了反常提高.具体来说,在1000℃下形成的更复杂和连续的五层氧化层,特别是底部的连续致密的Al_(2)O_(3)层,赋予合金在1000℃下优异的抗氧化性.

Simultaneously improving mechanical properties and oxidation resistance of Ti-bearing high-entropy superalloys at intermediate temperature via silicon addition

Ti-bearing high-entropy superalloys(HESAs)often suffer from severe intergranular embrittlement and terrible oxidation degradation at intermediate temperatures.Here we showcase that minor Si addition can effectively mitigate the intergranular embrittlement and improve the oxidation resistance of the a(Ni_(2)Co_(2)FeCr)_(92) Ti_(4)Al_(4) HESA at 700℃ simultaneously.Experimental analysis revealed that the intergranu-lar G phase induced by 2 at%Si addition can effectively suppress the inward diffusion of oxygen along grain boundaries at 700℃,thus enhancing the tensile ductility of the alloy from∼8.3%to∼13.4%.Be-sides,the 2 at%Si addition facilitated the formation of a continuous Al_(2)O_(3) layer during oxidation,con-tributing to a remarkable reduction in the growth rate of the oxide scale to a quarter of the Si-free HESA.Our results demonstrate that Si can be a favorable alloying element to design advanced HESAs with syn-ergistically improved thermal-mechanical performance.

Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging

The multi-principal-component concept of high-entropy alloys(HEAs) generates numerous new alloys.Among them,nanoscale precipitated HEAs have achieved superior mechanical properties and shown the potentials for structural applications.However,it is still a great challe nge to find the optimal alloy within the numerous candidates.Up to now,the reported nanoprecipitated HEAs are mainly designed by a trialand-error approach with the aid of phase diagram calculations,limiting the development of structural HEAs.In the current work,a novel method is proposed to accelerate the development of ultra-strong nanoprecipitated HEAs.With the guidance of physical metallurgy,the volume fraction of the required nanoprecipitates is designed from a machine learning of big data with thermodynamic foundation while the morphology of precipitates is kinetically tailored by prestrain aging.As a proof-of-principle study,an HEA with superior strength and ductility has been designed and systematically investigated.The newly developed γ’-strengthened HEA exhibits 1.31 GPa yield strength,1.65 GPa ultimate tensile strength,and 15% tensile elongation.Atom probe tomography and transmission electron microscope characterizations reveal the well-controlled high γ’ volume fraction(52%) and refined precipitate size(19 nm).The refinement of nanoprecipitates originates from the accelerated nucleation of the γ’ phase by prestrain aging.A deeper understanding of the excellent mechanical properties is illustrated from the aspect of strengthening mecha nisms.Finally,the versatility of the current design strategy to other precipitation-hardened alloys is discussed.

Enhanced tensile ductility of tungsten microwires via high-density dislocations and reduced grain boundaries

Despite being strong with many outstanding physical properties,tungsten is inherently brittle at room temperature,restricting its structural and functional applications at small scales.Here,a facile strategy has been adopted,to introduce high-density dislocations while reducing grain boundaries,through electron backscatter diffraction(EBSD)-guided microfabrication of cold-drawn bulk tungsten wires.The designed tungsten microwire attains an ultralarge uniform tensile elongation of~10.6%,while retains a high yield strength of~2.4 GPa.in situ TEM tensile testing reveals that the large uniform elongation of tungsten microwires originates from the motion of pre-existing high-density dislocations,while the subsequent ductile fracture is attributed to crack-tip plasticity and the inhibition of grain boundary cracking.This work demonstrates the application potential of tungsten microcomponents with superior ductility and workability for micro/nanoscale mechanical,electronic,and energy systems.

Elemental partitioning as a route to design precipitation-hardened high entropy alloys

Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography(APT)technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design ofγ-γ’HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makesγ-γ’HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design ofγ-γ’HEA.