Effect of stoichiometry and Cu-substitution on the phase structure and hydrogen storage properties of Ml-Mg-Ni-based alloys

To improve the electrochemical properties of rare-earth–Mg–Ni-based hydrogen storage alloys, the effects of stoichiometry and Cu-substitution on the phase structure and thermodynamic properties of the ...     

Pre-martensitic phenomena in Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy

Pre-martensitic phenomena such as abnormal resistivity growth, diffusion scattering, “tweed” contrast and internal friction peak were observed in Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy prior to the forward martensitic transformation on cooling. It was shown that all the observed phenomena were due to the formation of quasi-static strain nanodomains in the B2 phase prior to the forward martensitic transformation. This led to accumulation of the elastic energy before the phase transition and resulted in the variation in thermodynamic balance for the forward martensitic transformation and, as a result, influenced the parameters of the phase transition. The appearance of elastic energy prior to the forward transformation caused a decrease in the forward and reverse martensitic transformations' start temperatures, a widening of the temperature range of the reverse transformation and an increase in the hysteresis of the transformation.     

Accelerated exploration of multi-principal element alloys for structural applications

A strategy for accelerated discovery and exploration of multi-principal element alloys was developed and used to identify new alloys within a design window of desired microstructures and properties. As an example, the strategy was applied to analyze thousands of 3- 4-, 5- and 6-component alloys at equiatomic compositions of the alloying elements. Currently available thermodynamic databases were used to assess equilibrium phase diagrams for these alloys. The validity and reliability of the calculated phase diagrams were estimated based on the extent of experimental binary and ternary data used to build the respective thermodynamic databases. Alloys with specific characteristics, such as single-phase solid solution alloys with the use temperature above 1000 °C, were identified using an automated analysis of the calculated phase diagrams. The density, elastic moduli and costs of these alloys were estimated using the rule of mixtures of pure elements and were used as additional criteria for alloy selection. This approach allowed rapid, albeit preliminary, screening of many thousands of alloys and identification of promising candidate compositions, some of which are reported in this paper, for more time intensive experimental validations and assessments.     

Microstructure and magnetostrictive performance of NbC-doped<100>oriented Fe-Ga alloys

The<100>oriented Fe83Ga17 alloy rods with various NbC contents less than 1at%were prepared by the directional solidification method at a growth rate of 720 mm?h?1. Low NbC-content was found to affect t...     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.     

Silica based hybrid organic-inorganic materials for PEMFC application

It is of key importance to develop membrane assembly electrodes (MEAs) offering high conductivity, thermal stability and suitable performance in the fuel cell. The mesoporous materials functionalized with acid groups are appropriate candidates to improve membrane's properties. The goal of this work was to assess the addition of functionalized porous silica, bearing different acid groups, on the MEA performance in a PEM type single fuel cell. Ni₅₉Nb₄₀Pt_0.6Fe_0.4 -based amorphous alloys were applied as anode electrocatalysts. The synthesis of functionalized mesoporous silica (UGM-fx) with different acid groups, namely [SO₃H], [COOH] and [PO(OH)₂], was carried out following a nonaqueous sol gel method. The results showed that the MEA containing silica with PO(OH)₂ groups leads to an outstanding fuel cell performance compared to that of the other organic groups-based MEAs and that it outperformed a commercial Pt-based sample. This might be due to the higher proton conductivity exhibited by the phosphonic groups.     

Study of residual stress variation with depth of friction stir welded aluminium plates with different thicknesses

ABSTRACT

The residual stress may greatly vary through thickness due to the large temperature gradients and severe plastic deformation in depth for thick section friction stir welded plates. AA 2024-T351 plates with 6.5, 12 and 20?mm thicknesses were joined by friction stir welding to investigate the differences of residual stress variations with depth between thin and thick plates. The surface residual stresses were compared between the X-ray diffraction and contour measurements. Significant variations of stress peaks, root central residual stresses and widths of ‘M’ profiles were observed along the thickness for thick plates. The origins of the aggravated variations with depth were investigated from the temperature gradients and material flow variations through thickness.     

Thermodynamic property of ternary compound MgCaSi: A study from ab initio Debye-Grüneisen model

The thermodynamic properties of MgCaSi and its mother phase Ca₂Si are comparatively investigated from ab initio calculations and quasi-harmonic Debye-Grüneisen model. At 0 K, MgCaSi is more thermodynamically stable. Under high temperature, the advantage of higher thermodynamically stability of MgCaSi is reduced, originating from the less negative entropy contribution because the thermodynamic entropy of MgCaSi increases more slowly with temperature and the entropy values are slightly smaller. With increasing temperature, the anti-softening ability for MgCaSi is slightly smaller due to the slightly faster decrease trend of bulk modulus than that of Ca₂Si, although the bulk modulus of MgCaSi is higher in the whole temperature range considered. The thermal expansion behaviors of both MgCaSi and Ca₂Si exhibit similar increase trend, although thermal expansion coefficient of MgCaSi is slightly lower and the increases is slightly slower at lower temperature. The isochoric heat capacity and isobaric heat capacity of MgCaSi and Ca₂Si rise nonlinearly with temperature, and both are close to the Dulong–Petit limit at high temperature due to the negligibly small electronic contribution. The Debye temperature of both phases decrease with increasing temperature, and the downtrend for MgCaSi is slightly faster. However, MgCaSi possess slightly higher Debye temperature, implying the stronger chemical bonds and higher thermal conductivity than the mother phase Ca₂Si. The Grüneisen parameter of MgCaSi and Ca₂Si increase slightly with temperature, the values of MgCaSi are slightly larger. The investigation of electronic structures shows that with substitution of partial Ca by Mg in Ca₂Si, the stronger MgSi, MgCa and SiSi covalent bonds are formed, and plays a very significant role for the structural stability and mechanical properties.     

Effect of Si/Ti additions on physico-mechanical and chemical properties of FeNiCrCo high entropy alloys manufactured by powder metallurgy technique

FeNiCrCoSi_x and FeNiCrCoTi_x (x=0, 0.3, 0.6, and 0.9 wt.%) high entropy alloys (HEAs) were prepared via the powder metallurgy technique. A homogenous distribution of the elements in all alloys due to the formation of a solid solution phase is observed. The density and hardness of the prepared HEAs are improved by Si and Ti additions, compared to FeNiCrCo HEA. The wear rate of the prepared alloys was studied at different loads and the results indicate that the alloys that contain 0.3 wt.% Si and 0.9 wt.% Ti have the lowest wear rates. X-ray diffraction, SEM, and EDX were used to understand the phases, grain sizes, and microstructures in different investigated HEAs. The effects of Si and Ti content on the corrosion behavior and surface morphologies of sintered FeNiCrCoSi_x and FeNiCrCoTi_x HEAs were studied by immersion in H₂SO₄, HNO₃, and HCl solutions. Uniform corrosion and localized pitting are observed in different sizes in the corrosive media used. Because of the smaller pit size and the reduced pit density, the FeNiCrCoSi_0.3 HEA has an excellent microstructure.