Actual Problems in Machine Building 2018 Vol. 5 No. 3-4

Актуальные проблемы в машиностроении. Том 5. № 3-4. 2018 Материаловедение в машиностроении ____________________________________________________________________ 101 Materials and methods Cylindrical rods of nominal compositions Ti-xNb, x= 25, 27.5, 30, 32.5 35 mass % Nb (hereafter mass % will be referred to as %) were synthesized by casting methods in two steps. First, button shaped ingots were obtained by arc melting the initial materials, weighted according to the nominal composition. Prior to melting the furnace was evacuated and flushed with argon three times. A water-cooled copper crucible and a non-consumable tungsten electrode were used for melting. A titanium ingot was used as oxygen getter and was melted prior to every melting procedure. Considering the big difference in density (Ti:4.5 g/cm³; Nb: 8.57 g/cm³) and melting point (Ti: 1941 K; Nb: 2750 K) of the initial elements the alloys were remelted 16 times and flipped after every second melt. The master alloy obtained in this way was than remelted and casted into a water-cooled copper crucible with a diameter of 5 mm. The weight loss was evaluated by weighing the initial materials and the samples after casting. After the casting the rods were cut into pieces and embedded into phenolic resin. The samples were mechanically ground using SiC paper up to grade 1500, followed by polishing with Al 2 O 3 suspension. Final polishing was conducted using colloidal SiO 2 . The microstructure of the samples was revealed by etching with a Kroll`s reagent (5 vol. % HF, 10 vol. % HNO 3 and 85 vol. % H 2 O). The microstructures were examined using an optical microscope (Carl Zeiss AxioObersver Z1m). Scanning electron microscopy (SEM) was conducted using a Carl Zeiss EVO 50 in back scattered electron (BSE) mode. The elemental composition was checked by Oxford Instruments X-Act energy dispersive X-ray spectrometer (EDX) coupled with SEM. The hardness measurements, an average of 30 readings, were carried out using a WOLPERT Group 402 MVD Vickers hardness tester under a load of 50 g and a dwell time of 10 s. Synchrotron X-ray diffraction (SXRD) study was conducted at the P07 beamline of the “Deutsches Elektronen Synchrotron” in Hamburg, Germany. Diffraction patterns were recorded by a 2-dimensional (2D) image plate detector with 2048 x 2048 pixels centred on the beam. An X-ray wavelength of λ=0.124 Å and a beam cross section of 0.5 x 0.5 mm² was used. Results and discussion The experimental results are summarized in Table 1. The weight loss of the samples after the casting was found to be 0.16 and 0.23 %, which indicates that the compositions of the as-cast samples were close to the nominal compositions. Table 1 Chemical composition, weight loss and microhardness of researched alloys Alloys code Nb, wt % Ti, wt % Weight loss, % Microhardness, HV 0.05 Ti-30Nb 30 ± 0.4 Balance 0.16 301 ± 7 Ti-35Nb 35 ± 0.6 balance 0.23 222 ± 5 Fig.1 a-d show the microstructures of as-cast samples. A typical bimodal microstructure, a dendritic phase dispersed in a matrix, has been obtained in both alloys. Different dendritic morphologies and different size of the dendritic phase among the alloys are attributed to the different composition and the different cooling rate. The microstructure changes with increasing the distance from the regions which are in contact with the copper mold. The outer edges exhibit a very fine dendritic structure, like shown in Fig. 1b. Afonso et.al proposed that this refinement results in the formation of metastable phases [18]. If the radial distance from the border is increased the arm spacing of the dendrites becomes bigger and the dendrites are orientated into the direction of heat flow. In the center of the sample, where the smallest cooling rate is achieved, the dendrites show the biggest arm spacing. It is supposed that the dendritic phase is a bcc β-Ti solid solution which is