Fabrication of AMNC powders
To produce the AMNC powders, premixing of NaCl and KCl salt powders (from Fisher Chemical, ≥99.0%), TiC nanoparticles (with an average size of 40–60 nm, from US Research Nanomaterials, ≥99.0%) and Al microparticles (with average size of 20 µm, from Sigma-Aldrich, ≥99.0%) was carried out in a mechanical shaker for 30 mins. Then, the mixture was dehydrated in a vacuum furnace at 120 °C for 10 mins. The mixture was melted at 820 °C under argon protection in a graphite crucible with an outside diameter of 58 mm and a height of 88 mm. An ultrasonic niobium probe with a diameter of 12.7 mm and a length of 92 mm was then inserted 6 mm deep into the molten liquid, processing the melt for 15 mins before the specimens were taken out of the furnace and cooled down in air environment. The cooled specimens were then repeatedly dissolved four times in 400 mL distilled water in an ultrasonic bath for 30 mins. The solution was filtered through filter papers with a mesh size of 2.7 µm (Whatman plc) using vacuum filtration under room temperature for 20 mins. Eventually the AMNC powders were obtained and collected from the top of the filter papers. These powders were dried and dehydrated in a vacuum furnace for 10 mins at 150 °C before their use in laser experiments. To control the nanoparticle loading in the AMNC powders, we tuned the volume ratio, x, of the TiC nanoparticles to the Al microparticles. The volume ratio between the powder mixture and the salt was maintained constant at 3%. For the AMNC powder with a volume ratio factor of x = 0.25, we mixed 1.57 g TiC nanoparticles, 3.6 g Al micro particles, 27.1 g NaCl, and 34.6 g KCl. For AMNC powder with a volume ratio factor of x = 1, 4.39 g TiC nanoparticles were mixed 2.4 g Al, 27.1 g NaCl, and 34.6 g KCl. Ultrasonic amplitude of 30 µm and 45 µm were used for processing of the materials with x = 0.25 and x = 1, respectively.
Laser additive manufacturing of pure aluminum and AMNC
The schematic of a customized laser additive manufacturing system is illustrated in Supplementary Fig. 9. The experiment was conducted by a 1070 nm fiber laser (SP-200C-W-S6-A-B, SPI Lasers) tuned to a power output of 200 W, a scan speed of 0.2 m/s at continuous wave mode, a spot size of 50 µm, a hatching space of 30 µm, and a 90° scanning direction difference for each layer, as well as a customized stainless vacuum chamber (with a vacuum level at about 1 × 10−2 torrs). A temperature control system was installed for preheating of the powders. For each cycle of laser deposition, a AMNC powder layer (x = 0.25 and x = 1) with a thickness of about 50 µm was manually deposited on a pre-machined Al 1100 alloy substrate (≥99.0%, McMaster-Carr) with a dimension of 25.4 mm × 25.4 mm × 6.27 mm. The thickness of the powder layer was guided by a customized layer-thickness control device. A z-axis manual stage was placed in the chamber to manually adjust the laser focal point since the height of a new layer of powders will be different after laser melting of each layer. After the specimens were mounted onto the temperature control system inside the vacuum chamber, it was firstly evacuated to a vacuum level of about 1 × 10−2 torr, followed by a constant argon flow about 30 mins to reduce the oxygen content in the working environment. The argon purging process, i.e., vacuum pumping followed by argon purging, was repeated twice before of the preheating and laser selective melting. The laser scanning patterns of 3 mm × 3 mm, 3 mm × 18.5 mm, and 8 mm × 18.5 mm were processed the powder layers at temperatures of 25 °C (without preheating) or 300 °C (with preheating). The layer deposition process was repeated to obtain a designed layer thickness for fundamental study. In this study, the AMNC specimens with a thickness of 100 ± 16 µm (AMNC with17 vol.% TiC) and of 309 ± 16 µm (AMNC with 35 vol.% TiC) were deposited for characterization. Laser-processed pure Al specimens with a thickness of about 192 ± 30 µm were also obtained for comparison.
Light scattering (LS) particle analyzer (LS13 320, Beckman Coulter) was used to determine the size distribution of the AMNC powders (Supplementary Fig. 2c and Fig. 2d). Scanning Electron Microscopy (SEM, Supra 40VP, ZEISS) and Focused Ion Beam (FIB, Nova 600, FEI) were utilized to study the surface and the inner microstructures of the spherical AMNC powders. To reveal the distribution and dispersion of TiC nanoparticles in the AMNC powders, the specimens were mounted on a silicon wafer, tilted to 52° and then etched by FIB with gallium ions. The images of the 52° tilted cross-sectional SEM powders were acquired to reveal the distribution and dispersion of the nanoparticles. EDS (Energy-dispersive X-ray spectroscopy) with a mapping scan mode was used to characterize the chemical compositions of the AMNC powders. At least 40,000 elemental signals, i.e., net counts, were captured to determine the elemental compositions.
A UV3101PC spectrophotometer (SHIMADZU Cop.) was used to measure the reflectivity of the aluminum powder specimens without (i.e., Al ≥ 99.0%) and with the reinforcement nanoparticles (i.e., 17 vol.% and 35 vol.% of TiC nanoparticles). A wavelength range from 250 nm to 2500 nm was scanned using UV/visible/NIR detectors (photomultiplier and PbS cell) with a spectral resolution of 2.0 nm. A standard barium sulfate BaSO4 plate was used to perform a baseline correction over the required wavelength range to ensure a 100% reflectance. Powder specimens were then mounted and sealed on a powder specimen holder packed with the accessory barium sulfate BaSO4 for the reflectance measurement. The kinetic measurement mode was applied to record and analyze the reflectivity using a multifunctional UVProbe software (SHIMADZU Cop.).
Microstructure of laser layer-deposited AMNC specimens
SEM and TEM were used to reveal the dispersion and distribution of the TiC nanoparticles in the laser-processed AMNC specimens. The specimens were first vertically mounted in epoxy holders with an outside diameter of 30 mm, and then filled with a mixture of a transparent curable epoxy and a hardener (Allied High Tech Products, Inc.) with a ratio of 10–3, followed by grinding and polishing. To clearly expose the TiC nanoparticles on the surface of the Al matrix, polished specimens were 52° tilted and slightly etched by FIB with gallium ions, followed by obtaining SEM images with EDS (Energy-dispersive X-ray spectroscopy) analysis. High resolution TEM images were obtained by Scanning/Transmission Electron Microscopy (S/TEM, Titan, FEI). The TEM specimens prepared by FIB were obtained from the micropillars after compression tests (Supplementary Fig. 4a). Fourier-filtered high resolution TEM images were obtained to reveal the interface between the TiC nanoparticles and the Al matrix.
To reveal the grain size of laser-deposited specimens with and without nanoparticles, Electron backscatter diffraction (EBSD) was utilized to characterize the pure aluminum specimen while Transmission-EBSD was used to observe the FIB-prepared AMNC specimen (35 vol.% TiC) with a thickness less than 100 nm. Both specimens were placed in the SEM chamber, 70° tilted from horizontal towards the EBSD-diffraction camera. The EBSD/Transmission-EBSD scans were performed at a voltage of 20 and 30 kV and a current of 5.3 nA and 12 nA, respectively. The mapping scans were captured and evaluated by HKL Channel5 software. ImageJ software was used to further validate the grain size distribution as shown in Supplementary Fig. 5.
A MTS XP Nanoindenter was used to conduct microcompression tests to study the mechanical properties of the laser-processed specimens with and without TiC nanoparticles. FIB-machined micropillars with a size of 4.0 ± 0.1 µm in diameter and a height of 10 ± 0.5 µm were compressed by a flat punch probe with a size of 10 µm at room temperature using the displacement control mode, and a strain rate of 2 × 10−3 s−1.
In-situ microcompression tests test at elevated temperatures were conducted using a PI 95 PicoIndenter (Hysitron Inc.) with a flat punch diamond probe of 20 µm inside a FEI Quanta 3D SEM/FIB. FIB-machined micropillars (4 µm in diameter and 9 µm in height) from a AMNC (35 vol.% TiC, preheated at 300 °C) specimen were compressed using the load-control mode, and a strain rate of 2 × 10−3 s−1. The real time load-displacement data and in-situ deformation movies of micropillars were monitored, captured and recorded by TriboScan (Hysitron Inc.). In-situ microcompression was conducted at temperatures of 200, 300, and 400 °C by a resistive microelectromechanical systems (MEMS) heater. A AMNC (35 vol.% TiC, preheated at 300 °C) specimen was attached to the MEMS temperature-control specimen heater using high-temperature silver conductive epoxy (Ted Pella product #16014), followed by installing the specimen heater on the PicoIndenter system. The resistance of the heating element was utilized to elevate the specimen temperature to a desired value. Each temperature level was maintained 300 s before the compression tests. A resistive temperature detector (RTD) sensor was used to measure the real time temperature and provide feedback for the MEMS temperature controller.
Microcompression test of specimens after exposure at elevated temperatures were performed to further determine the thermal stability. AMNC (35 vol.% TiC) specimens, after a heating period of 1.0 h at 400 °C were cooled down to room temperature and further microcompression tests were conducted again under room temperature. Micropillars with a diameter of 4 µm and a height of 9 µm were prepared by FIB. All testing parameters and experimental setups remained the same as in the section of Microcompression test.
Measurements of elastic modulus were performed using nanoindentation tests by MTS Nanoindenter XP to evaluate the elastic modulus of the laser-processed materials. The specimens with nanoparticles (17 vol.% TiC and 35 vol.% TiC) and without nanoparticles (pure aluminum) were compressed by a Berkovich tip with an indentation depth of 2 µm. For each specimen, 20 randomly selected points were measured. Elastic moduli were calculated from the unloading curves.
Comparison of specific mechanical properties with other materials
To compare the AMNC (35 vol.% TiC) specimens with other representative engineering alloys, all testing data were collected from micropillar compression tests without size effect. The diameters of testing specimens were in the range of 3.5–7 µm. It should be noted that the properties reviewed were obtained by using different strain rates and the data points shown in the graph were the extreme values data presented in that reference, i.e., the highest values for the yield strength. Also, since the authors10,31 did not provide exact Young’s modulus data, the data were estimated using a superposition method, which would theoretically be higher than the experimental data. The references for each material are listed as follows: Al7075 T7332, Ultrafine-grained Al33, Mg2Zn (14 vol.% SiC)6, Mg10Al (1 diamondoids)34, 316 L stainless steel, and 316 L stainless steel (15vol.% TiB2)10, Duplex stainless steel35, Ti6AI4V36, and W7Cr9Fe37 and Inconel MA600038.
Comparison of yield stress at elevated temperature
The yield stress at elevated temperatures is sensitive to different strain rates. It should be noted that a high strain rate typically leads to a strengthening effect. Therefore, to scientifically compare our AMNC (35 vol.% TiC) specimens with other representative engineering alloys, we also listed the strain rate data here: a strain rate of 1 × 10−2 for aluminum alloys; of 2 × 10−3 for current strongest magnesium nanocomposite, and of 1 × 10−4 for SS304. The test of SS304 was conducted according to the ASTM Standard E21–92. For each material, the references are listed as follows: 7017 Aluminum39, SS30440, Mg2Zn (14 vol.% SiC)6, 7075 Aluminum41, and 6061 Aluminum42.
Mechanical strengthening mechanisms
To understand the strength obtained in as-deposited AMNC, the strengthening mechanisms for the AMNC (35 vol.% TiC) can be possibly attributed to Orowan strengthening28, Hall-Petch effect29, and load-bearing transfer. Here we discuss these potential strengthening contributions in the laser-deposited AMNC (35 vol.% TiC) specimens.
The contribution from Orowan strengthening can be estimated by28:
where r is the particle radius, and λ is the inter-particle spacing, dp is the particle diameter, b is the Burger’s vector, Vp is the volume fraction of nanoparticles, and Gm is the matrix shear modulus. The values for the AMNC specimens with 35 vol.% TiC are: b = 0.286 nm, Gm = 25.5 GPa, Vp = 0.35, r = 0.5 and dp = 79.5 nm. The particle size is determined by the TEM results in Supplementary Fig. 3. Theis thus determined to be ~294 MPa.
It is well known that the grain size has a significant influence on metal yield strength since grain boundaries act as obstacles for dislocation movement. The TiC nanoparticles can serve as nucleation sites and also the pinning points to inhibit the Al grain growth during solidification. From our EBSD mapping results, the average grain size of the laser-deposited AMNC (35 vol.% TiC) specimen is approximately 331 ± 95 nm, as shown in Fig. 2e (also see Supplementary Fig. 5). The yield strength gained from the Hall-Petch strengthening can be calculated by:
where k = 0.06 MPa m1/2 as the strengthening coefficient for aluminum42, as d the average grain size in the AMNC specimen. The calculated ∆σH_P is 104 MPa.
It is believed that the load-bearing effect significantly contributes to the strengthening of the AMNC specimen since the interfacial bonding between the nanoparticles and aluminum is excellent, as shown in as shown in Fig. 2d. A strong interfacial bonding and a dense homogeneous dispersion of TiC nanoparticles (see Supplementary Fig. 4b) can result in strengthening for the AMNC specimens, which can be estimated by43:
where Vp is the volume fraction of particles and σi is the interfacial bonding between Al matrix and TiC nanoparticles. The strong interfacial bonding as shown in Fig. 2d suggests that the theoretical value of σi would be ~1000 MPa (with a hypothetical ∆σload at 525 MPa). It should be noted that this is purely hypothesis as there is no interfacial bonding strength data available.