Effects of B segregation on Mo-rich phase precipitation in S31254 super-austenitic stainless steels:Experimental and first-principles study

Precipitation in super-austenitic stainless steels will significantly affect their corrosion resistance and hot workability.The effects of Cr and Mo on precipitation behaviors were mainly achieved by affecting the driving force for precipitation,especially Mo has a more substantial promotion effect on the formation of theσphase than Cr.In the present study,B addition to the S31254 super-austenitic stainless steels shows an excellent ability to inhibit precipitation.The effect of B on the precipitation behaviors was investigated by microstructure characterization and theoretical calculations.The experimental observation shows that the small addition of B inhibits the formation of theσphase along grain boundaries and changes from continuous to intermittent distribution.Moreover,the inhibitory effect increased obviously with the increase of B content.The influence of B addition was theoretically analyzed from the atomic level,and the calculation results demonstrate that B can inhibit the formation ofσphase precipitates by suppressing Mo migration to grain boundaries.It is found that B and Mo are inclined to segregate atΣ5 andΣ9 grain boundaries,with B showing the most severe grain boundary segregation tendency.While B distribution at the grain boundary before precipitation begins,the segregation of Mo and Cr will be restrained.Additionally,B’s occupation will induce a high potential barrier,making it difficult for Mo to diffuse towards grain boundaries.

Influence of sigma phase on corrosion and mechanical properties of 2707 hyper-duplex stainless steel aged for short periods

2707 hyper-duplex stainless steels (HDSSs) contain high contents of alloying elements (Cr, Ni, Mo and N), which may shorten the incubation period and promote both nucleation and growth of sigma (σ) phase. 2707 HDSS was first aged at different temperatures and periods of time for drawing the time-temperature-transformation profiles. The results identified the main precipitates as σ phase, with nose temperature of about 950℃. Also, 2707 specimen was aged at 950℃ for a short time, and the morphology, distribution and amount of σ phase were examined through the scanning electron microscope and X-ray diffraction,σ phase was initially formed at the boundaries of ferrite and austenite and then transformed through the eutectoid reaction (α→σ+γ2). Finally, the precipitation and growth of σ phase in 2707 steels aged at nose temperature for a short period reduced the corrosion resistance and deteriorated the mechanical properties, and the corresponding reason was further analyzed.

Chi Phase after Short-term Aging and Corrosion Behavior in 2205 Duplex Stainless Steel

Correlation between pitting corrosion behavior and chi( χ )phase formed after a short-term aging(5,10 and 15 min)at 850 ℃ of 2205 duplex stainless steel(DSS)was investigated using potentiodynamic polarization tests,optical microscopy,and scanning electron microscopy equipped with energy-dispersive spectrum system.Results showed that after aging for 5min,the χ phase initially precipitated at ferrite grain boundaries,developed and then became linked with prolonging aging time.The χ phase was rich in Cr and Mo,resulting in formation of depleted zones nearby.The χ phase could reduce corrosion resistance of DSS and slightly influence its stability,but the specimens still displayed the capacity for repassivation.Some lines of evidence showed that stable pitting corrosion initiated at the boundaries of precipitates.The χ phase was selectively corroded during the first stage of corrosion and then the depleted zones nearby were attacked.In addition,the grain size and volume of precipitates also affected pit nucleation and progress,and suitable size and distribution of χ phase could aggravate pit initiation at precipitate boundaries.The χ phase with considerably low volume fraction and small size was not sensitive position for pit initiation.

Effect of compression deformation on precipitation phase behavior of B-containing S31254 super austenitic stainless steel

Effects of compression deformation on the sigma-phase precipitation behavior of B-containing S31254 stainless steel after solution treatment were studied using the Gleeble compression test. The cold and hot processing characteristics of B-containing S31254 stainless steel were evaluated, and the results show that the speed of compression deformation increased the precipitation rate of the sigma phase, and the location of precipitation extended from the austenite grain boundary to the original hot rolling deformation area. During cold deformation at room temperature, the precipitation rate increased when the deformation reached at 40%. Deformation at 950℃ affected precipitation more obviously. At 1074 ℃, when the deformation reached 20%, the precipitated phases started increasing, and above this deformation range, precipitation began decreasing. Also, at 1074℃ the deformation accelerated the precipitation of sigma phase;but with deformation, there was a change in critical temperature for the sigma-phase excursion. The precipitation position of the sigma phase is strongly related to the area of the original hot rolling deformation. With an increase in the deformation amount, precipitates in this region appeared as coarse-grained, skeletal, and network-like features.

Effect of boron on dissolution and repairing behavior of passive film on S31254 super-austenitic stainless steel immersed in H_(2)SO_(4) solution

The impact of boron on the dissolution and repairing behavior of passive films formed on S31254 super-austenitic stainless steel(SASS)was investigated.SASS was immersed in 0.5 mol/L of H_(2)SO_(4) for 0,2,6,10,and 14 days to explore the evolution of the passive film.The electrochemical impedance spectroscope(EIS),the Mott–Schottky analysis,and X-ray photoelectron spectroscope were utilized to analyze the semiconductor properties and compositions of the passive films.EIS showed a decrease and consequent increase over 14 days;the same pattern was observed for Cr_(2)O_(3) and Cr/Fe.However,the defect density of the passive film exhibited a reverse trend.The variation in film thicknesses indicated that the passive films possessed dissolution and repairing behavior.SASS passive film had a double-layer structure;the outer layer was found to be rich in Fe3+and Cr(OH)3,but low on Mo6+,while the inner layer was rich in Cr_(2)O_(3) and low in Mo4+.The addition of boron increased the corrosion resistance and could promote the efficiency of the passive film repair,likely by promoting the migration of Mox+,which promoted the repairing of the passive film.

Surface adsorption and diffusion of N on γ-Fe-Al (111) using first principles calculations

The adsorption and diffusion of N on γ-Fe-Al (111) surface have been investigated using the first principle calculations combined with density functional theory to explore the formation mechanism of AlN in the oxidation process of austenitic stainless steel. The results indicate that the most preferential adsorption site of N on the surface of γ-Fe (111) is fcc-hollow site. In addition, the stable positions are located at fcc adsorption site on clean and Al-doped γ-Fe (111) surface adsorbed 4.76 at.% N. Compared with the pure Fe system,γ-Fe-Al (111) system reduces the energy difference of N from the surface to the bulk. The system is most stable for 9.09 at.% N adsorbed on the octahedral interstice of the 2nd and 3rd atom interlamination of γ-Fe-Al (111) surface. Thus, the doping of Al makes it easier to spread N on the surface of γ-Fe (111). The increase in N in the atmosphere also accelerates the diffusion. Moreover, according to the density of states analysis, the interaction between Al and N was enhanced when 9.09 at.% N was adsorbed on the surface of γ-Fe-Al (111).

Effects of different alloying elements M(M=Fe,Ni,Mn,Si,Mo,Cu,Y)on Cr_(2)0_(3)with Cl:a first-principles study

Using the first-principles methods,the effects of different alloying elements M(M=Fe,Ni,Mn,Si,Mo,Cu,Y)on Cr_(2)0_(3)with Cl adsorption are studied.The results show that the layer distance of all doped models has been widened to different degrees with Cl adsorption.When Mo or Y is doped into the passive film,the difference of layer distance is reduced to a certain extent.The interaction between alloying elements and Cl is studied by calculating the adsorption height,bond population and electron density difference.The results show that Mo and Y can inhibit Cl erosion and improve the corrosion resistance of passive film.Furthermore,we investigate the CrMoFe and CrMoY co-doped system with Cl adsorption.The calculations point out that when Mo and Y are doped together in the passive film,the corrosion resistance of the system is more prominent than that of CrMo,CrY and CrMoFe co-doping systems.

Direct chemical vapor deposition growth of graphene on Ni particles using solid carbon sources

Graphene has attained a considerable amount of popularity as an attractive ultra-thin reinforcement for nickel(Ni)matrix composites in recent years.However,its excellent reinforcement efficiency is suffered from the agglomeration of graphene nanosheets in manufacturing process and the poor bonding strength of graphene with Ni matrix.To overcome these two problems,one of the efficient strategies is to in-situ grow graphene reinforcements on Ni particles for powder metallurgy.This work aims to synthesize uniform graphene@Ni composite particles by using polymethyl methacrylate(PMMA)as the solid sources for chemical vapor deposition(CVD)process.The results demonstrate that few-layer or multilayer graphene with different morphologies can be grown on the particles by controlling the PMMA content and annealed temperature,respectively.The optimum condition for the formation of high-quality few-layer graphene is 1.0 mg·ml^(-1) PMMA and 900℃.A competition mechanism rises from the growth kinetic,and the spatial confinement effect has led to the formation of graphene with different microstructures and morphologies.