Thermodynamic aspects of electron-beam surface modification of low-carbon steel

Multicomponent surface modification of carbon steels is of high interest in mechanical engineering due to its beneficial impact on machine components’ and structures’ surface properties. The present research was devoted to simulating the process of aluminides and borides formation on the surface of low-carbon steel during electron beam alloying and predict the phase composition of the obtained coatings. Computational thermodynamics and approximate calculation method were used to solve the problem mentioned above. Calculations were done in the temperature range between 200 and 2000 K at 10−3 Pa. It was discovered that the calculated and experimental data of the coating’s phase composition differs significantly. The only confirmed phase that was predicted by the calculations was sodium fluoride (NaF). It was established that NaF presence in the treatment paste was redundant for the electron beam alloying because of its low reactivity in a vacuum. XRD analysis revealed the following phases in the coating: Fe2B, Fe3C, and AlFe3.

Interplay between eutectic and dendritic growths dominated by Si content for Nb-Si-Ti alloys via rapid solidification

Rapid solidification techniques such as electron beam additive manufacturing are considered as promising pathways for manufacturing Nb-Si based alloys for ultra-high-temperature applications. Here we investigate the microstructure diversity of a series of Nb-Si-Ti alloys via electron beam surface melting (EBSM) to reveal their rapid solidification behaviors. Results show that the microstructural transition from coupled to divorced Nbss/Nb3Si eutectics can be triggered by increasing Si content. The formation of fully lamellar eutectics, evidenced by scanning transmission electron microscopy and atom probe tomography (APT), is achieved in the EBSM-processed Nb18Si20Ti alloy (at%), in contrast to the hypereutectic microstructures in arc-melted counterparts. The dendritic microstructures containing divorced eutectics are generated with a higher content of Si during rapid solidification. The transition from faceted to non-faceted growth of intermetallic Nb3Si occurs with the formation of primary Nb3Si dendrites. The interplay between eutectic and dendritic growths of silicides is discussed to provide insights for future alloy design and manufacture.

Electron-Beam Surface Modification of Boron-Based Diffusion Layers Obtained of the Surface of Steel H21

In recent years the interest in the development of new protective coatings with improved functional properties for machine parts’ surface have been of great fundamental and applied importance. The current study is devoted to the creation of coatings based on boron and aluminium on the surface of alloy steel using a cutting-edge method, combining thermal-chemical treatment (TCT) and subsequent electron beam processing (EBP). TCT was carried out in treatment pastes based on boron carbide and aluminum at 950°C and 1050°C for 2 hours. As a result of processing, diffusion layers with a thickness of up to 120 μm and 580 μm were formed on the steel surface after TCT at 950°C and 1050°C respectively. The subsequent EBP led to a complete transformation of the primary diffusion layer and an increase in its thickness to 1.6 mm. XRD analysis showed significant differences in composition before and after EBP: new compound, such as tungsten borides (WB, W2B9) and iron boride (Fe2B) were detected. In addition, it was determined that the distribution of microhardness and elemental composition (B, Al, W) over the layer thickness after EBP had a more favorable profile without significant fluctuations compared to the sample after TCT. The concentration of Al decreased significantly after EBP. It dropped from 18% after TCT to a low of 1%.

Influence of Beam Power on Young’s Modulus and Friction Coefficient of Ti–Ta Alloys Formed by Electron-Beam Surface Alloying

In this study, we present the results of Young’s modulus and coefficient of friction (COF) of Ti–Ta surface alloys formed by electron-beam surface alloying by a scanning electron beam. Ta films were deposited on the top of Ti substrates, and the specimens were then electron-beam surface alloyed, where the beam power was varied from 750 to 1750 W. The structure of the samples was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Young’s modulus was studied by a nanoindentation test. The coefficient of friction was studied by a micromechanical wear experiment. It was found that at 750 W, the Ta film remained undissolved on the top of the Ti, and no alloyed zone was observed. By an increase in the beam power to 1250 and 1750 W, a distinguished alloyed zone is formed, where it is much thicker in the case of 1750 W. The structure of the obtained surface alloys is in the form of double-phase α’and β. In both surface alloys formed by a beam power of 1250 and 1750 W, respectively, Young’s modulus decreases about two times due to different reasons: in the case of alloying by 1250 W, the observed drop is attributed to the larger amount of the β phase, while at 1750 W is it due to the weaker binding forces between the atoms. The results obtained for the COF show that the formation of the Ti–Ta surface alloy on the top of Ti substrate leads to a decrease in the coefficient of friction, where the effect is more pronounced in the case of the formation of Ti–Ta surface alloys by a beam power of 1250 W.