The Segregations Of Boron And Niobium At Grain Boundaries In Microalloyed Steel And High-purity Nickel

The segregations of boron and niobium at grain boundaries have great effects on the property of alloy materials.The clarification of the segregation mechanisms is significant to their manipulations,which are vital to the improvements of materials.In this work,the techniques of time-of-flight secondary ion mass spectrometry,Auger electron spectroscopy,transmission electron microscope and the electron backscattering diffraction are used to study the segregations of boron and niobium at grain boundaries.The positron annihilation lifetime spectroscopy technique is used to study the behaviour of vacancy-type defects during the cooling process.Finally,the solution types and diffusion mechanisms of boron are discussed by the first-principles calculations.By the technique of time-of-flight secondary ion mass spectrometry,the segregations of boron at both the random and annealing twin grain boundaries in hot compressed high-purity nickel are directly observed.The segregations at annealing twin grain boundaries are much weaker than that at random grain boundaries in the same sample.Two types of segregations are observed at random grain boundaries:in the sample that deformed at 1100 ?,the segregations of boron have high intensities with widthes about 1 ?m;in the sample that deformed at 700 ?,the segregations of boron have low intensities with widthes about 6 ?m.In some samples,two types of segregations are observed at the same grain boundaries.In the microalloy steel,the segregations of Nb at prior-austenite grain boundaries in microalloyed steel increase with the decreasing temperature during cooling.The positron annihilation lifetime spectroscopy results show that the concentration of defects decrease with the decreasing temperature.Our analyses attribute the increase of Nb segregation to the diffusion of vacancies and the migration of dislocations to the grain boundaries.By the technique of time-of-flight secondary ion mass spectrometry,the enrichment of Nb and Cu are directly observed along the prior austenite grain boundaries.The analyses of all the enrichment zones of Cu and Nb show that the enrichment of Nb can only be found in the Cu enrichment zones,which show that Cu has a promotion effect on the precipitation of Nb.The Cu-Nb precipitates are observed at prior austenite grain boundaries.First-principles calculations show that,in bcc-Fe,B atoms may occupy both the substitutional and the octahedral sites but diffuse via an interstitial mechanism.The calculated diffusion factor is D₀ = 1.05×10^-7 ×exp?-0.75eV/kBT?m²·s^-1,which coincides well with the experiment results.In fcc-Fe,B atoms may occupy the substitutional sites and diffuse with the B-vacancy complex mechanism,and the calculated diffusion factor is D₁= 6.3× 10^-6 ×exp?-2.01eV/kBT?m²·s^-1.