Papers by Author: Lembit A. Kommel

Abstract: The structurization of a high purity niobium from double electron-beam melted cast microstructure to fine-grained microstructure was completed by equal-channel angular pressing by the Bc route up to a Von Mieses strain of 13.8. In addition, for the viscoplastic behavior study as well as nanostructure and properties improving the hard cyclic viscoplastic deformation, die forging at room temperature and followed heat treatment with low heating rate were conducted. The nanostructure of processed samples was characterized by transmission electron microscopy and X-ray diffraction testing. This paper focuses on several new trends in the study of improved mechanical and physical properties of pure niobium, to what purpose these materials will be used in industry. The crystallite size, microstrains and dislocation density in severe plastic deformed pure niobium were calculated and electric conduction was measured. The nanocrystalline microstructure with minimal crystallite size down to 62 nm as mean in cross-section of sample was received. By this the dislocation density varies from 5.0 E+10 to 2.0 E+11 cm-2 and was maximal for pure niobium which has minimal electrical conductivity, maximal value of hkl-parameter and maximal relative microstresses. The microhardness was maximal for sample after 12 passes by Bc route and for samples with 8 and 10 passes followed heat treatment at 170 and 350°C. The mechanisms answerable for the electronic conduction were discussed according to the microstructure evolution in the different directions and for different strain levels.

Abstract: Deformation-enhanced diffusion in single-crystalline Ni-based superalloy specimens have been investigated under the conditions of hard cyclic viscoplastic tension-compression deformation. The chemical composition of phases before and after cyclic deformation was investigated by filed-emission scanning electron microscopy. At low strain amplitude values (0-0.05%; 0-0.2%; 0-0.5%) the material shows upscaled viscoelastic behavior and microstructural stability. At the increase of strain amplitude in the γ+γ’-phase (0-1%), the Ni, Re and Co content decreases, whereas Al and Mo content increases significantly. On the contrary, in the single γ’-phase area, the Ni and Co content was increased, which was accompanied by a decrease of Nb, Cr, Ta and Al content. The length of dendrite arms was significantly decreased as compared to primary dendrite arms and γ+γ’-rafts were formed parallel to the stress axis direction. As a result of the deformation-enhanced, diffusion the necking of dendrites accompanied with longitudinal cracking by the dendrite axis and cross-sectional radial cracking by interdendritic region of single crystalline specimen occurs.

Abstract: Annealed pure copper was subjected to equal-channel angular pressing (ECAP) by route Bc for different passes number. Tensile test specimens were manufactured and subjected to hard cyclic viscoplastic (HCV) deformation by means of the materials testing installation Instron 8516 in strain control regime at room temperature. The specimens were cyclically deformed with a frequency of 0.5Hz at different strain amplitudes, step-by-step increased from ±0.05 to ±2.5% for 30 cycles, up to seven test series in this study. The microstructure of ECAP and HCV deformed samples were characterized by optical- and transmission electron microscope, X-ray diffraction, tensile- and hardness testing methods. The ECAP processed metal has mainly elongated subgrains with low-angle grain boundaries and texture, oriented in direction of metal flow during latest pressing. We demonstrate that during HCV deformation the dislocations density of ECAP processed UFG copper was decreased. The ECAP texture was reoriented under cyclic load applied as elongated subgrains were jointed to small pieces under this same angle to axis as texture before. The grain- and crystallite sizes were decreased, which were accompanied with dislocation ribbons forming nearby new formed high-angle grain boundaries. This paper builds on knowledge that the combined treatment by ECAP and followed HCV deformation enable to improve UFG microstructure and ductility with lowering the strength and hardness of UFG metals due to the lower dislocation density while coarse grained copper exhibits increasing the strength and hardness.

Abstract: High purity (99.99, wt %) niobium ingots were received by double electron beam melting technology. The specimens of pure metals with cast microstructure were processed using cold equal-channel angular pressing (ECAP) followed by a heat treatment at around 1080 °C in vacuum furnace for 2 h. Shear bands and microstructure evolution was studied in shear region at first pass and at next passes of ECAP. Microstructure processing characterization was performed using light optical and field emission scanning electron microscopy. Accompanied to microstructure the changes of mechanical properties were determined by micro- and universal hardness testing. The shear bands forming, evolution of large crystals to elongated laminar and dislocation fine-grain structures during processing was characterized and discussed in view of metal hardening-softening and viscoplastic behavior under subsequent hard cyclic straining.

Abstract: Lightweight B4C/Al composites were produced from powders of boron carbide and aluminum by self-propagating high-temperature synthesis (SHS). The effects of postdensification heat treatment at different temperatures and environmental conditions on phase transformations and properties evolution were studied. Heat treatment processing that followed the synthesis was applied using low heating rate in temperature range from 400°C up to 1500°C. An interconnected multiphase (B4C, Al3BC, and c-BN) microstructure was produced in composite as a result of heat treatment at temperatures below 1080°C. The formation of hard and brittle reaction products (AlN, AlB2, Al4C3, and Al8B4C7) at temperatures above 1150°C causes decrease in bending strength and increase in resistance to unlubricated sliding wear.

Abstract: Diffusion in the interface regions of lightweight heatproof quality titanium and titanium/aluminum alloys was investigated. We studied the diffusion of aluminum from intermetallide to titanium alloy. The concentration of other chemical elements and microhardness has been measured in diffusion region formed in the solid titanium alloy. The interface region includes a transition zone from the initially solid Ti-alloy and the molten TiAl-Nb intermetallic substrate. The width of the interface region after diffusion bonding is 45-60 µm. The titanium content decreases and aluminum content increases starting from surface up to 120-150 µm in depth in solid titanium alloy. As a result of diffusion, the intermetallic Ti3Al thin layer was formed in the transition zone in the Ti-alloy substrate. The microporosity was also formed in the interface region.