High-Temperature Corrosion Behavior Of Al_xTi_0.5Cr_y FeCoNi High Entropy Alloys

The so-called high-entropy alloy(HEA)refers to the alloy with a variety of elements as the principle elements,the number of the elements larger than 5,and each elemental atomic percentage within 5%-35%.And due to some basic characteristics of HEAs(such as high entropy effect,sluggish diffusion effect,lattice distortion effect and cocktail effect),HEAs have attract more and more attention,of which the high entropy effect and sluggish diffusion effect endows High-entropy alloys are superior in high-temperature performance,and the research of their high-temperature corrosion behavior can provide some guidance for the application of alloys in the field of high temperature.In this thesis,AlxTi0.5CrFeCoNi(x=0?0.5?1.0)and Al0.5Ti0.5CryFeCoNi(y=0?0.5?1.0)HEAs were prepared using the arc melting method.The salt immersion method was used to study the corrosion behavior of the alloys in molten mixed salt(75%Na2SO4+25%NaCl).The corrosion kinetics curves of the high entropy alloys at 800? and 900? were measured by discontinuous weighing method,the phase constituent of corrosion products was analyzed by X-ray diffraction,the surface and cross-sectional morphology and composition of corrosion products were analyzed by scanning electron microscope(SEM)and energy disperse spectroscopy(EDS).The main results are as follows.The addition of Al significantly improve the corrosion resistance of the AlxTi0.5CrFeCoNi alloys.With the increase of Al content,the corrosion resistance of the alloy increases firstly and then decreases,Al-0.5 alloy had the strongest corrosion resistance,Al-1.0 take the second place,And the corrosion resistance of Al-0 alloy is the last.With the increases of temperature,the properties of Al-0.5 alloy are the most stable.The corrosion behavior of the alloy does not change with the temperature.From outside to inside,the alloy can be divided into oxide layer,corrosion-affected zone and non-corrosive matrix after corrosion.The cross-sectional distribution of oxide layer from outside to inside is:relatively dense Cr2O3?TiO2(Ti3O5)?Al2O3 layer(Al-0 alloy without Al2O3 layer),as well as the trace amount of spinel structure oxide,of which the oxide layer of Cr2O3 and TiO2(Ti3O5)is the thickest.The corrosion affected zone is composed of a small amount of Fe/Cr/Ti and other corresponding sulfide,pores and microcracks.With the increase of the temperature and the prolongation of the corrosion time,it is obvious that the thickness of the oxide layer increases,the binding ability with the matrix becomes weaker,and even the separation from the matrix(corrosion affected zone)can be obviously seen.The density of holes increases,the number of cracks adds,and the cracks expands to the inside of the matrix,Which indicates that the corrosion degree of the alloy is more serious.With the increase of Cr content,the corrosion resistance of the Al0.5Ti0.5CryFeCoNi alloys improves,and with the increase of the corrosion temperature,this phenomenon becomes more obvious.The corrosion resistance of Cr-1.0 alloy is obviously superior to that of Cr-0.5 alloy at 900?,Cr-0 is the worst(only 10h corrosion).At 800?,the corrosion resistance of Cr-1.0 and Cr-0.5 alloys are almost the same,and both are better than Cr-0 alloy.The corroded alloy is also composed of an oxide layer and a corrosion-affected zone.The cross-sectional distribution of oxide layer from outside to inside is Cr2O3(not present in Cr-0)/TiO2(Ti3O5)/Fe3O4?Al2O3 layer and a trace amount of spinel structure oxide.The corrosion affected zone is also composed of a small amount of Fe/Cr/Ti and other corresponding sulfide,pores and microcracks.With the increase of temperature and the prolongation of time,the thickness of the oxide layer of the alloy also increases,being broken,uneven,and peeled off from the substrate and the width of the affected area increases.What's more,the density of the holes distributed thereon increases,and the cracks expand to the inside of the matrix meaning that the corrosion resistance of the alloy reduces.The corrosion process of AlxTi0.5CrF eCoN i and Al0.5Ti0.5CryFeCoNi HE As is divided into two stages.The early stage is corrosion incubation period,during which the oxidation of the alloy is the main reaction and the main oxides produced in this stage are Cr2O3,TiO2(Ti3O5),Al2O3 and Fe3O4 so on.The oxides have a certain impediment to corrosion.With the corrosion proceeding,the corrosion into the accelerated corrosion period.At this time,Na2SO4 in the mixed salt will undergo a series of chemical reactions that produce the S and SO42-,which react with the elements in the matrix.Meanwhile,the NaCl will react with the generated oxide and the elements in the alloys through a series of chemical reactions to form loose and porous granular Cr2O3,Al2O3 and TiO2.All these decrease the protective effect of oxidation layer,make it is possible for further oxidation-sulfidation of the alloy,and thereby accelerate the corrosion of the alloy.In general,the high temperature corrosion of the HEAs happens through oxidation-sulfidation process and chlorination of NaCl.It is also found that the corrosion resistance of the Al0.5Ti0.5CrFeCoNi high-entropy alloy is superior to that of the conventional alloy under the same experimental conditions.