Impact of severe plastic deformation on kinetics and thermodynamics of hydrogen storage in magnesium and its alloys

Magnesium and its alloys are the most investigated materials for solid-state hydrogen storage in the form of metal hydrides,but there are still unresolved problems with the kinetics and thermodynamics of hydrogenation and dehydrogenation of this group of materials.Severe plastic deformation(SPD)methods,such as equal-channel angular pressing(ECAP),high-pressure torsion(HPT),intensive rolling,and fast forging,have been widely used to enhance the activation,air resistance,and hydrogenation/dehydrogenation kinetics of Mg-based hydrogen storage materials by introducing ultrafine/nanoscale grains and crystal lattice defects.These severely deformed materials,particularly in the presence of alloying additives or second-phase nanoparticles,can show not only fast hydrogen absorption/desorption kinetics but also good cycling stability.It was shown that some materials that are apparently inert to hydrogen can absorb hydrogen after SPD processing.Moreover,the SPD methods were effectively used for hydrogen binding-energy engineering and synthesizing new magnesium alloys with low thermodynamic stability for reversible low/room-temperature hydrogen storage,such as nanoglasses,high-entropy alloys,and metastable phases including the high-pressureγ-MgH2 polymorph.This work reviews recent advances in the development of Mg-based hydrogen storage materials by SPD processing and discusses their potential in future applications.

Superfunctional high-entropy alloys and ceramics by severe plastic deformation

High-entropy alloys and ceramics containing at least five principal elements have recently received high attention for various mechanical and functional applications.The application of severe plastic deformation(SPD),particularly the high-pressure torsion method,combined with the CALPHAD(calculation of phase diagram) and first-principles calculations resulted in the development of numerous superfunctional high-entropy materials with superior properties compared to the normal functions of engineering materials.This article reviews the recent advances in the application of SPD to developing superfunctional high-entropy materials.These superfunctional properties include(ⅰ) ultrahigh hardness levels comparable to the hardness of ceramics in high-entropy alloys,(ⅱ) high yield strength and good hydrogen embrittlement resistance in high-entropy alloys;(ⅲ) high strength,low elastic modulus,and high biocompatibility in high-entropy alloys,(ⅳ) fast and reversible hydrogen storage in high-entropy hydrides,(ⅴ) photovoltaic performance and photocurrent generation on high-entropy semiconductors,(ⅵ) photocatalytic oxygen and hydrogen production from water splitting on high-entropy oxides and oxynitrides,and(ⅶ)CO_(2) photoreduction on high-entropy ceramics.These findings introduce SPD as not only a processing tool to improve the properties of existing high-entropy materials but also as a synthesis tool to produce novel high-entropy materials with superior properties compared with conventional engineering materials.

Recent advances in metastable alloys for hydrogen storage:a review

Development of new materials with high hydrogen storage capacity and reversible hydrogen sorp-tion performances under mild conditions has very high value in both fundamental and application aspects.In the past years,some new systems with metastable structures,such as ultra-fine nanocrystalline alloys,amorphous alloys,nanoglass alloys,immiscible alloys,high-entropy alloys,have been abundantly studied as hydrogen storage mate-rials.Many new hydrogen storage properties either from the kinetics or thermodynamics aspects have been reported.In this review,recent advances of studies on metastable alloys for hydrogen storage applications have been comprehensively reviewed.The materials preparation methods to synthesize metastable hydrogen storage alloys are firstly reviewed.Afterwards,hydrogen storage prop-erties of the metastable alloys are summarized and dis-cussed,focusing on the unique kinetics and thermodynamics properties by forming of such unique metastable structures.For examples,superior hydrogena-tion kinetics and higher hydrogen storage capacity have been achieved in Mg-based amorphous and nanoglass alloys.Destabilized thermodynamics properties can be obtained in the immiscible Mg-Mn and Mg-Zr alloys.In addition to highlighting the recent achievements of metastable alloys in the field of hydrogen storage,the remaining challenges and trends of the emerging research are also discussed.

Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation

To have an insight into the occurrence of inverse Hall-Petch relationship in ultrafine-grained(UFG) aluminum alloys produced by severe plastic deformation(SPD),ultra-SPD(i.e.inducing several ten thousand shear strains via high-pressure torsion,HPT) followed by aging is applied to an Al-La-Ce alloy.Average nanograin sizes of 40 and 80 nm are successfully achieved together with strain-induced Lomer-Cottrell dislocation lock formation and aging-induced semi-coherent Al_(11)(La,Ce)_3 precipitation.Analysis of hardening mechanisms in this alloy compared to SPD-processed pure aluminum with micrometer grain sizes,SPD-processed Al-based alloys with submicrometer grain sizes and ultra-SPD-processed Al-Ca alloy with nanograin sizes reveals the presence of two breaks in the Hall-Petch relationship.First,a positive upbreak appears when the grain sizes decrease from micrometer to submicrometer which is due to extra hardening by solute-dislocation interactions.Second,a negative down-break and softening occur by decreasing the grain sizes from submicrometer to nanometer which is caused by weakening the dislocation hardening mechanism with minor contribution of the inverse Hall-Petch mechanism.Detailed analyses confirm that nanograin formation is not necessarily a solution for extra hardening of Al-based alloys and other accompanying strategies such as grain-boundary segregation and precipitation are required to overcome such a down-break and softening.