2019 Volume 60 Issue 5 Pages 714-717
This work concerns microstructure investigation of aluminum based composites strengthened with the TiC nanoparticles. The composites were fabricated by the casting method combined with in-situ formation of TiC particles. Transmission electron microscopy observation shown that the average size of TiC particles was close to 140 nm. HRTEM investigation of the interface between Al matrix and TiC particles shown the existence of misfit dislocation located in the Al-matrix, 2 nm far from interface boundary. Two other, bigger kinds of particles with size of several micrometers and blocky shape of α-Al2O3 and TiAl3 phases were also identified in the investigated composites which can influence both strengthening mechanism and plasticity.
Metal matrix composites (MMCs) have attracted lot of interest as a result of possible applications in aerospace and automotive industry. They show high specific modulus (E/ρ) and yield stress (σε/ρ), higher wear resistance and good elevated temperature resistance.1) TiC has been recognized as one of the most important reinforced ceramic for metal matrix composite materials due to its excellent properties such as high hardness and high temperature stability.2,3) There are different routes for achieving an Al–TiC metal matrix composite but recently the in-situ casting is one of the most popular because of the simplicity of the process and promising results.4–9) Moreover, the use of molten salt assisted solidification can lead to produce TiC nanoparticles in size of several nanometers10) and also by in-situ process the local reinforcement even in the Fe based alloy can be achieved.11) The microstructure of in-situ composites have a great influence their final mechanical properties. Especially, factors such as: particles size, particles distribution, quality of interface between particles and matrix, additional phases should be taken into account. There is a small amount of papers concerning detailed microcopy investigations especially high resolution electron microscopy of in-situ cast Al–TiC composites. Microstructural phenomena are strongly associated with the mechanism of strengthening of composites. Thus, it is very important to carry out advanced microstructural investigations providing comprehensive information on the microstructure of the composite. Therefore, in this paper we report the advanced microstructure investigations of Al–TiC composites obtained by in-situ formation of nanometer-sized TiC particles during casting.
Aluminum 1000 alloy was used as a matrix. Table 1 presents detailed chemical composition of Al 1000 according to the producer specification. Aluminum as moderator of in-situ synthesis, titanium and graphite powders were used as the reaction system. Powders were mixed in ball mill and then compacted into pellet form. The amount of Al–Ti–C added corresponding to the composition of Al–5 mass% TiC. An aluminum 1000 alloy ingot was melted in a graphite crucible in an induction furnace. Al–Ti–C pellets were introduced into the molten aluminum for the in-situ TiC synthesis. Next suspension was cast into a metal mold to form an ingot and cooled in air. Microstructure and chemical composition of the samples were examined with Leica DM IRM light microscope (LM), scanning electron microscope (SEM) FEI ESEM XL30 equipped with X-ray energy dispersive spectrometer EDAX GEMINI 4000 and a transmission electron microscope (TEM) Tecnai G2 operating at 200 kV equipped with an Energy Dispersive X-ray (EDX) microanalyser and High Angle Annular Dark Field Detector (HAADF). Thin foil samples for TEM observations were prepared by Focus Ion Beam (FIB) technique using FEI Quanta 2D Dual Beam operating at 30 kV equipped with Omni-probe system.
Figure 1 shows polarized light micrographs taken with two different magnifications. One can see granular microstructure with irregular shapes of grains with the size ranged from 100 to 200 µm. The fine second phase particles were distributed along the grain boundaries as well as grain inferiors. It is worth to notice that only small discrepancies of size and shape of grains were observed along the cross section of the casted composite. They were mainly related to the temperature gradient during crystallization process. Generally, it can be assumed that macrostructure of cast is homogenous and representative for each areas what is important for further microstructure investigations.
Polarized light micrographs taken with two different magnifications.
Figure 2 presents SEM BSE micrographs of cross-section of the cast. One can see, homogenous microstructure composed of three different areas. According to the EDS measurements we can distinguish areas with following chemical composition (all in at%): (1) Al-93.3, C-2.7 characterized by single nanoparticles of TiC embedded in Al matrix, (2) Al-36.6.3, Ti-43.6, C-19.8 with a conglomeration of TiC nanoparticles and (3) Al-68.6, 26.7, C-4.7 showing elongated blocky TiAl3 particles of length of about 25 µm. Almost the same microstructure was presented by Liu et al.5) in the case of TiC/Al composites fabricated by the in-situ casting but with the preheating treatment and ultrasonic vibration. Furthermore, the size of investigated TiC particles were close to 1 µm with spherical shape, quite different than in our case.
SEM BSE micrographs taken at two magnification of cross-section of cast.
In order to confirm the crystal structure of all existed phases in composites the TEM investigations were performed. Due to the difficulties connected with the preparation of thin foils from multiphase material by electropolishing method we decided to use the focus ion beam (FIB) technique. However, the FIB method has some limitations, especially, small size of samples, which decreases area of observation. Thus, it was necessary to prepare two lamellas assuring examination of all phases. Figure 3 presents STEM-HAADF images and corresponding elemental maps taken from two different areas of investigated composite.
STEM-HAADF images and corresponding elemental maps performed for two different areas of investigated composite.
In the first area we can distinguish at least three phases from the following systems: (i) pure Al as the matrix, (ii) Al–O and (iii) Ti–C, whereas in the second area additionally Ti–Al system must be considered. Bright Field (BF) image and corresponding Selected Area Diffraction Pattern (SAEDP) of TiC particle allowed to identification of TiC with the cubic crystal structure, Fm-3m space group and lattice parameter a = b = c = 0.4327 nm (Fig. 4). Their mean size estimated by secant method based on the STEM-HAADF images was 143 nm ± 55 nm. What is interesting, the particles possess the special faceted shape which means that particular walls of particles correspond to specific crystallographic planes of TiC crystal structure. Also this type of particles were observed in the Al-based composites obtained by reacting Al3Ti and graphite at 1000°C for about 1 min. prior ball-milling and compaction. They concluded that faceted shape of TiC crystals observed on TEM micrograph can be explained by the formation of these particles from a nucleation/growth mechanism in the Al liquid phase produced by the interaction between Al3Ti and graphite.12) In Fig. 4 one can see TiC particle with two pairs of parallel walls corresponds respectively to the (111) and (002) TiC crystallographic planes. It can be also well observed that TiC particle in the Bragg conditions (dark) neighboring with the others and also with the Al matrix. Since Al matrix is in bright contrast there is no crystallographic relationship between TiC particle and Al matrix, at least in this particulate case. We also performed High Resolution Electron Microscopy (HREM) investigations in order to check what happens at the interface between TiC particle and Al matrix in this diffraction conditions. Figure 5 presents HREM image and Fast Fourier Transform (FFT) from the area marked by the white square and inverse fast Fourier transform (IFFT) obtained after masking procedure of FFT image from area number 2.
TEM BF and corresponding SAEDP images showing faceted TiC nanoparticle.
HREM image and FFT images from the area marked by the white square IFFT obtained after masking procedure of FFT image from area number 2.
Corresponding FFT can be indexed according to the [101] Al zone axis while some reflections are missed or are very weak due to no proper Bragg conditions. FFT of area 3 can be well indexed as [110] TiC and FFT image taken from interface between Al and TiC (area 2) is characterized by the scattered reflections from TiC as well as very weak and only individual from Al matrix. Applying the mask procedure of FFT from area 2 and making the IFFT image we obtained very well defined interface between TiC nano particle and Al matrix in the atomic scale in which frequent every 5 nm misfit dislocations (indexed by ovals in IFFT image) in the Al matrix close to interface can be distinguished.
The misfit dislocation may forms due to lattices mismatch between fcc Al and fcc TiC structures which is close to 6.8% and mismatch in coefficient of thermal expansion (CTE) and in elastic modulus (EM) between the reinforcements and the metal matrix. Since these dislocations are located in the Al matrix they can give contribution to the their strengthening.13) Other strengthening mechanisms of metal matrix composite are the Load Transfer Effect (LTE), Hall-Petch and Orowan strengthening mainly associated with the size and distribution of hard particles. However, the most important contributions are achieved mainly due to CTE and EM mismatch and Orowan effect, especially when the particle diameter is lower than 50 nm as was determined for 2 mass% Al2O3 reinforced Al matrix processed at 400°C.14) Nevertheless the reinforced particles play an important role in the strengthening mechanism, therefore it is very important to analyze all particles in the fabricated composite. As was shown in Fig. 3 apart of TiC nanoparticles other bigger particles such as Al–O and Al–Ti system can be identified. TEM microstructure observation of these particles is presented in Fig. 6. One can see that Al–O particles are α-Al2O3 type with the hexagonal structure and R-3c space group. They neighbor with the Al-matrix as well as with the TiC particles having with them the crystallographic relationship as follow: ⟨4 11 5⟩Al2O3 // ⟨010⟩TiC and {322}Al2O3 // {220}TiC. Other big blocky particles were identified as TiAl3 very often noticed in the in-situ cast Al-based composites reinforced with TiC particles. It is worth notice that this two additional particles also can contribute in the strengthening of composite but also can cause decrease of their plasticity due to their big size, irregular shape and heterogeneous distribution.
TEM BF and corresponding SAEDP images showing Al2O3 and TiAl3 particles.
In this work, the type of crystal structure, size and distribution of different particles formed during in-situ cast Al-based composites were studied. Based on the experimental results following conclusion can be drawn:
This work was financially supported by Polish National Science Centre in frame of Project No. 2016/21/B/ST8/01181.
