Solid State Phenomena Vol. 186

Abstract: The purpose of the work is the analysis and the identification of phases in alloy steels after various heat treatments by means of transmission electron microscopy. We have investigated the phases formed during austenitising followed by quenching with various rates or by isothermal quenching, as martensite, upper and lower bainite. In order to identify the phases, we used their morphological characteristics and the kind of orientation relationship between the given phase and the austenite. We concluded that the identification of phases occurring in steels after various heat treatments may be performed in a clear manner according to their morphological features combined with the diffraction patterns analysis as observed by transmission electron microscopy.

Abstract: The purpose of this study is to analyze the effect of glow discharge nitriding on hydrogen degradation of two types of steels: two-phase austenitic-ferritic and single-phase austenitic. The nitriding process resulted in formation of surface layers composed of expanded austenite (S phase), and in the case of two-phase steel of expanded austenite and expanded ferrite. Microstructural changes occurring under the influence of hydrogen on steels without and with nitrided layers were investigated with the use of scanning (SEM) and transmission (TEM) electron microscopy techniques. It was shown that the density of cracks formed during cathodic hydrogen charging is higher on the surface of the non-nitrided steels compared to the nitrided steels after identical hydrogen charging process. Moreover in non nitrided steel hydrogenation leads to considerable increase of dislocation density, which results from the high concentration of hydrogen absorbed to the steel during its cathodic charging. This leads in turn to high stress concentration and local embrittlement giving rise to cracks formation. Conversely nitriding reduces the absorption of hydrogen and prevents structural changes resulting in hydrogen embrittlement. The conducted studies show that glow discharge nitriding can be used to increase resistance to hydrogen embrittlement of austenitic and austenitic ferritic stainless steels.

Abstract: Two kinds of deformed carbon steel were used as a feedstock for thixoforming: X210CrW12 tool steel cold worked, of composition 2,1 %C, 10,5 %Cr, 0,7 %W, 0,4 %Si (all in weight %), and bearing steel 100Cr6: (0,9% C, 1,4% Cr, 0,4% Mn, 0,3% Si, 0,2% Cu). The microstructures of X210CrW12 steel after thixoforming process at temperature of 1230°C, performed at 30 % of liquid show presence of globular grains (average size from 30 µm 50 µm) surrounded by the eutectic mixture. Measured chemical composition of the austenite in globules was: 2,5% C, 0,4% Si, 0,7% Mn, 12% Cr, 1,3% W, 0,1%V, 87% Fe and that of the eutectic mixture: 5%C, 0,4% Si, 0,6%, Mn, 1,1%, W, 17,5% Cr, 75,4% Fe in thixocast of X210CrW12 steel. Measured carbon content is too high, most probably due to contamination effect. TEM studies of this steel after thixoforming shows that the eutectic mixture consists of ferrite α-Fe and Cr7C3 carbides. Thixoforming of 100Cr6 bearing steel was conducted at temperature of 1420°C at about 25 % of liquid fraction. Thixocast microstructure consists of globular grains consisting of martensite needles and residual austenite (20 μm to 40 μm in size and average composition 0,9%C, 1,4 %Cr, 0,5%Mn, 0,3%Si). The chemical composition of the eutectic is following: 2,4% C, 2,6% Cr, 0,2%Si, 0,6%Mn.

Abstract: The influence of long term annealing on microstructure of Al-Cu4-Ni2-Mg aluminum alloy was investigated. The castings were subjected to T6 heat treatment followed by annealing at 523 K and 623 K for 100, 150, 300, 500 and 750 hours. The soaking time and temperature was adjusted by corresponding to real service conditions of the elements of an aircraft and motor engines from investigated alloys. Microstructural examination of the alloy was carried out with optical microscope, as well as scaning and transmissiom electron microscopes. The result of microscopic analysis showed that applied heat treatment caused an increasing in the particles of hardening (θ’-Al2Cu) phase size. The significant growth of the length and changing the value of shape factor of hardening phase precipitations was observed. The phenomenon of the increase in size and change in shape of precipitations of hardening phases continually change with the prolonged holding time at high temperature. The microstructure degradation is connected to a decrease of mechanical properties of alloy, confirmed by the result of tensile tests.

Abstract: The effect of temperature and time of homogenization on intermetallic phase dissolution and transformation in 6066 alloy was studied. Microstructure examination has been carried out on the samples in the as-cast state and after different time of homogenization at 560°C using optical microscope - Nikon 300, scanning (SEM) electron microscopy combined with an energy dispersive X-ray microanalysis (EDS) and X-ray diffraction (XRD). The results show that the as-cast microstructure of 6066 contained β-Mg₂Si, Si, Q-Al₅Cu₂Mg₈Si₆, b-Al₅FeSi, α-Al(MnFe)Si and Al₂Cu intermetallic phases. Significant changes in microstructure followed after homogenization process were revealed. The majority of Al₂Cu, Q and Mg₂Si particles have dissolved in a-Al, and plate-like b-Al₅FeSi precipitates transformed into multiple, spherical α-Al(MnFe)Si.

Abstract: The effect of the Laser Shock Processing (LSP) on the microstructure of the surface layer of a commercially pure alluminum was studied. LSP process was performed with a high-power Q-switched Nd:YAG ReNOVALaser, operating in a pulse mode (18 ns), with a power density of 0,43 GW/cm2. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and a Scanning Transmision Electron Microscopy (STEM) have been used to study the microstructure of the surface layer of the investigated material after laser treatment. SEM investigation showed that after LSP process surface melting occurs but is restricted to a thin layer. However, both TEM and STEM images indicate that under the thin melting layer a high density of dislocations were visible. It has been found that the laser beam with employing parameters caused plastic deformation of the surface layer of the investigated aluminum.

Abstract: The metal matrix composites (MMC) were produced by force infiltration of preforms containing up to 10 vol % of Saffil fibers with AA6061 and EN AC 44200 aluminium alloys. The fibers were stabilized inside preform by dipping in liquid glass and heat treated at 800oC/2 hours. It allowed both to coat fibers with a thin layer of silica and bond fibers with amorphous silica bridges producing self supporting porous structure. The microstructure observations were performed using Tecnai FEG (200kV) transmission electron microscope. The performed investigations showed that infiltrating preform with AA6061 alloy with ~1 wt. % Mg caused reaction of silica with liquid metal substituting the former with porous amorphous Al2O3. A presence of MgO at the interfaces with aluminium matrix was noted. The infiltration with low magnesium EN AC 44200 (Mg <0.6 wt.%) alloy helped to produce composite with the silica bridges between fibers practically intact.

Abstract: This research characterizes the changes in microstructure that occur in friction stir welded extrusions of a novel 7042 aluminum alloy. Due to the presence of scandium the base material preserved the deformation microstructure with elongated grains and fairly high dislocation density. The temperature increase with simultaneous severe plastic deformation occurring during friction stir welding induced significant changes in the microstructure within the weld and its vicinity. The weld center (stir zone) was composed of fine equiaxed grains with residual dislocations and a modest density of small precipitates compared to the neighbouring thermomechanically and heat affected zones where the density of small precipitates was much higher.