INFLUENCE OF ALUMINUM POWDER CONCENTRATION ON MECHANISM AND KINETIC REGULARITIES OF CRYSTALLIZATION OF COMPOSITES BASED ON LOW DENSITY POLYETHYLENE

The results of the study of the influence of the aluminum powder concentration on the mechanism and the regularity of changes in the dependence of specific volume on temperature in the range of 25-180 °C in composites based on low density polyethylene are presented. By extrapolating the upper and lower branches of the dilatometric curve, approximate values of the glass transition temperature of the composites are found. Extrapolation of the lower branch of the dilatometric curve of the considered polymer composites to the absolute temperature allowed us to determine the value of the "occupied" specific volume and the "free" specific volume. Using the step dilatometry method, the temperature of the first-order phase transition of composites was found, which varies depending on the aluminum powder concentration in the composition of low density polyethylene. The concentration of aluminum powder with a particle size 1-2 µm varied between 0.5 – 30 wt. %. It has been established that the loading of 0.5 wt. % of aluminum powder practically does not affect the regularity of change in the dilatometric curve of the initial polymer matrix. At a filler concentration of 1.0 wt. % and higher, a significant change occurs in the regularities of change in these curves, expressed in a decrease in the value of the specific volume of the composites. A sharp decrease in the specific volume or an increase in the density of the composites in viscous-flow and solid states clearly indicated the reinforcing role of aluminum powder. In composites based on low density polyethylene, the dependence of the free specific volume on temperature and on the concentration of aluminum powder is determined. It is shown that an increase in the concentration of aluminum powder in the composition of low density polyethylene is accompanied by a regular decrease in the free volume of the polymer matrix. The obtained data confirms our idea that particles of aluminum powder are involved not only in the formation of heterogeneous crystallization centers, but are also forced into the amorphous region as the spherolite crystalline formations grow. In the coordinates of Avraami, the kinetic regularities of the process of isothermal crystallization in the region of the first-order phase transition are investigated. It has been established that with an increase in the degree of filling of the polymer matrix, the mechanism of crystallization from spherical (three-dimensional) type of growth of crystalline formations passes into a plate-like (two-dimensional) with the continuous formation of crystallization centers. Recommendations are given on the practical use of the results of a study of the crystallization process of composites as applied to the technology of their processing by injection molding.

ДОСЛІДЖЕННЯ ВПЛИВУ ЛЕГУВАННЯ НА ТЕМПЕРАТУРНИЙ КОЕФІЦІЄНТ ЛІНІЙНОГО РОЗШИРЕННЯ ПОКРИТТІВ

The effect of doping with yttrium-containing ligatures on the thermal coefficient of the linear expansion of sealing coatings is studied. KHA-82 coating was taken as the base material for the research. It is used in the aircraft engines produced by Motor Sich JSC. This coating contains nickel (base), silicon, aluminium, and solid lubricants (graphite and boron nitride). To enhance the physical-mechanical properties of the coating, an yttrium-containing ligature was added to the size prepared by the serial process. Three different compositions of the ligature with varying content of yttrium were studied, namely Ni-Y is composition No.1, Y is composition No. 2, Co-Ni-Cr-Al-Y is composition No. 3, and KHA-82 is composition No. 4. The coatings were applied onto specially prepared samples by a gas-flame technique. Analysis of the volumetric and possible structural phase changes in the material of the coatings under consideration was carried out on the Shevenar differential dilatometer. It was found out that during the first heating within 650-700° C, there is a change in the shape of the dilatometric curve. Recurring heating also causes the change in the trajectory of the dilatometric curve. The obtained result is probably related to the development of the oxidization processes, appearance of intermetallide phases, and changes in the porosity of the coatings during the first and recurring heating. Reduction of the pore space is beneficial in terms of energy as it is accompanied by the reduction of free surface, which in its turn can trigger the development of phase changes that are connected with the formation of oxide, nitride, and intermetallide inclusions. The smallest difference in the shape of dilatometric curves is observed in the coating of composition No. 3, which is presumably due to the fact that double spinels with a more compact structure are formed in the structure of the material obtained. The estimation of the difference in the temperature coefficient of the linear expansion of the base and coating materials indicates an insignificant difference in these values that can be reduced if the suggested yttrium-containing ligatures are added.

Interpretations of Cooling Dilatograms in the Range of the Bainitic and Martensitic Transformations/ Intwerpretacja Dylatogramów Chłodzenia W Zakresie Przemiany Bainitycznej I Martenzytycznej

The method of interpretation of cooling dilatograms of undercooled austenite during its transformation into upper and lower bainite (UB and LB) and into martensite (M), applied in the Laboratory of Phase Transformations of AGH University of Science and Technology in Krakow, is presented in the hereby paper. The relation of the mechanism of the mentioned above transformations to places on the dilatometric curve where transformations start and where they are continued was indicated. An appeal for the unified, correct interpretation of dilatograms of phase transformations, in all laboratories of the world, was formulated. This will allow the formation of equally accurate diagrams of the kinetics of phase transformation of undercooled austenite at its continuous cooling (CCT). In consequence, this could provide a development of more advanced heat treatment technologies. Such approach will also allow for the correct assessment of the influence of the alloying elements on the stability of austenite and phase transformation temperatures at its cooling.

Improper Interpretation of Dilatometric Data for Cooling Transformation in Steels

Dilatometry is the most commonly method of producing CCT diagrams and analyzing phase transformations during cooling (as well as transformations during heating) and it permits the real-time monitoring of the extent of reaction in terms of dimensional changes due to phase transformation. All modern dilatometers are fitted with computerised systems which collect dimensional change signals versus temperature to plot a dilatometric curve and also to calculate and plot the derivative of the relative dimensional change with respect to temperature. Unfortunately, elaborated by dilatometers manufacturers software, in many cases could lead to wrong interpretation of phase transformations during cooling.

Intragranular Porous Aluminum Titanate Ceramics with Low Thermal Expansion and High Strength Simultaneously

Intragranular porous aluminum titanate ceramics were prepared by using graphite powder as pore-forming agent and magnesium-doped aluminum titanate powder as starting material. FESEM was employed to observe the microstructure. In order to investigate the expansion behavior and mechanical property, the dilatometric curve and three-point flexural strength of the prepared aluminum titanate ceramics were measured respectively. The results demonstrate that because of the formation of the intragranular pores, the aluminum titanate ceramics are of low thermal expansion and high strength simultaneously.