Formation Of Nano-sized Grains Research Articles

Enhancing the thermoelectric properties for hot-isostatic-pressed Bi2Te3 nano-powder using graphite nanoparticles

Bismuth telluride (Bi2Te3) is a promising thermoelectric material produced commercially. However, its poor electrical conductivity and low figure of merit, caused by grain boundaries and high thermal conductivity, limit its effectiveness in powder metallurgy production. Herein, effects of adding Graphite nanoparticles (GTNPs) to Bi2Te3 on thermoelectric properties were studied. Three ratios of GTNPs (0.2, 0.35, 0.5 wt%) were added to ball-milled Bi2Te3 nano-powder. The hot isostatic pressing (HIP) sintering technique was employed to prepare the pristine Bi2Te3 and the BT-xGTNPs samples for testing. The crystallographic measurements showed a reduction in the crystallinity of the BT-xGTNPs samples compared to the pure Bi2Te3, whereas the electron microscopy measurements showed smaller grain sizes. This was also confirmed with an increase in the samples’ relative density implying the formation of nano-sized grains. Full electrical, thermal, and thermoelectric measurements were performed and comprehensively discussed in this report for all samples in the temperature range from room temperature (RT) to 570 K. The measurements demonstrated an enhancement for x = 0.35 wt% GTNPs at 540 K up to 43% in the power factor and 51% in the ZT compared to pristine Bi2Te3, which was attributed to the optimum grain size, the lower grain boundaries, and better electrical and thermal conductivity aroused from the precise addition of GTNPs. The best electrical conductivity of ~ 8.2 × 104 S/m and lowest thermal conductivity of ~ 1 W/m·K for the Bi2Te3-containing 0.35 wt% GTNPs at RT even though the sample with 0.5 wt% attained the highest Seebeck coefficient of 154 µV/T at 540 K.

Formation of nano-sized grains in Ti-10Zr-5Nb-5Ta biomedical alloy processed by accumulative roll bonding (ARB)

Formation of nano-sized grains in Ti-10Zr-5Nb-5Ta biomedical alloy processed by accumulative roll bonding (ARB)

Formation of Refined Grain Size Less Than 5 nm and Nano-sized Undulations in the Bonding Interface Region of an Ultrasonic Spot Welded Cu/Ni Joint

The microstructure in the bonding interface region of an ultrasonic spot welded Cu/Ni joint was investigated. At the copper side near the bonding interface, all the refined grains with random orientations and high-angle grain boundaries are < 5 nm in size. Nano-sized undulations occur at the Cu/Ni interface. The formation of nano-sized grains and undulation characteristics is ascribed to the dynamic recrystallization as well as shear instability in ultrasonic spot welding.

Characterization of plastic deformation induced by machining in a Ni-based superalloy

The surface integrity characteristics of Udimet 720Li subjected to slight damage and damage machining conditions have been studied using a complementary range of techniques such as FIB-SEM, EBSD, TKD, TEM-EDS and nano-indentation. The results indicate the existence of nano-sized grains and no observable tertiary γ′ in regions of severe plastic deformation in the machined surface. The correlation between the machining condition and the resulting plastic deformation is established. Grain refinement of this alloy via machining was achieved by dislocation slip. The nanohardness of the surface of damage machined sample is 40% higher than that of bulk material, which is attributed to the formation of nano-sized grains and high density of dislocations in the superficial layer.

Fabrication, microstructure and mechanical property of a novel Nb-rich refractory high-entropy alloy strengthened by in-situ formation of dispersoids

We fabricated Nb42Mo20Ti13Cr12V12Ta1, a novel refractory high-entropy alloy (RHEA) enriched with niobium (Nb), by following a high-energy ball milling and spark plasma sintering (SPS) method. The microstructure of the bulk RHEA formed a single body-centered cubic (BCC) matrix with in-situ-formed carbide dispersoids. The combination of the effect of the severe deformations caused by the milling, the short sintering time, and the impedance of the grain growth caused by the dispersoids led to the formation of nano-sized grains, in spite of the high sintering temperature of 1200 °C. This resulted in outstanding mechanical properties. The bulk Nb42Mo20Ti13Cr12V12Ta1 RHEA exhibits a compressive yield strength of 2680 MPa, a maximum strength of 3896 MPa, and a Vickers hardness of 741 HV. The excellent mechanical properties of the alloy can be attributed to the strengthening of the dispersion and the solid solution, coupled with the grain refinement effects.

Experimental study on surface integrity of cryogenically machined Ti-6Al-4V alloy for biomedical devices

Titanium and its alloys are widely used in the biomedical sector. In this field, titanium and its alloys are the material of choice for biomedical devices such as hip and knee replacements. Usually, a Total Hip Replacement (THR) is based on four components, made out of different materials due to the material properties associated with the functional performance. One approach to lower the overall manufacturing costs and enhance the reliability of THR’s is to manufacture the prosthesis out of one material. The titanium alloy Ti-6Al-4V is, therefore, feasible as it exhibits better osseous integration compared to other metallic materials used as orthopedic devices. The sole use of Ti-6Al-4V alloy requires improvements of surface integrity (SI) and characteristics that are sensitive to SI. One possible way to improve the tribological properties of the THR and the biocompatibility of Ti-6Al-4V alloy is to deliberately decrease the material grain size in the surface layer from the micron scale (> 1 µm) to the region of nano-sized grains (< 100 nm).The objective of this paper is to study and prove the formation of nano-sized grains within the surface as well as the characterization of surface integrity when machining Ti-6Al-4V alloy. Therefore, different cryogenic cooling strategies are used where liquid nitrogen (LN2) is applied to the flank and rake face, and just to the flank face respectively. To compare the effect of cryogenic machining, conventional flood cooling was applied as third cooling strategy. As cutting tool, a roughing tool, having a large cutting edge radius, was used, since severe plastic deformation (SPD) has shown to be capable to produce nano-sized grains in the surface. The results showed, that cryogenic machining using a large cutting edge radius tool is able to decrease the materials grain size to the region of nano-sized grains.

Effects of cold rolling on microstructure, texture evolution and mechanical properties of Ti-32.5Nb-6.8Zr-2.7Sn-0.3O alloy for biomedical applications

The effects of cold rolling on the microstructure, texture evolution and mechanical properties of Ti-32.5Nb-6.8Zr-2.7Sn-0.3O (TNZSO, wt%) alloy were investigated. The results showed that the TNZSO alloy exhibits multiple plastic deformation mechanisms and excellent workability during cold rolling. The grains of the alloy were refined and no stress-induced α" phase transformation occurred after cold rolling. The dislocation slipping and {112}<111> type twins appeared in the alloy deformed by 25% and 50%. When the deformation reduction was up to 75% and 90%, dislocation slipping became the main mode of deformation accompanying with the formation of nano-sized grains. With the increase of cold deformation reductions, it was found that the strength and hardness increased owing to the increase of dislocation density and grain refinement, and the elastic modulus obviously decreased owing to the increased dislocation density as well as the enhanced orientation density of <110>α-fiber textures (including {001}<110>α-fiber and {111}<110>α-fiber) and the weakened orientation density of {111}<112>γ-fiber texture. The 90% cold deformed alloy exhibited a great potential to become a new candidate for biomedical applications since it possesses low elastic modulus (54.1GPa), high tensile strength (1093MPa) and high strength-to-modulus ratio (20.2×10−3), which are superior than those of Ti-6Al-4V alloy.

Hysteresis properties of nanostructured Nd-Ho-Fe-Co-B alloys

The structure and magnetic properties of a rapidly quenched (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2. alloy are investigated by the X-ray and electron diffraction and by the standard magnetometry techniques. It is found that using the quenching velocity of Vq = 30 m/s, it is possible to attain the high-coercive state (iHc > 20 kOe) at room temperature as a result of formation of nano-sized grains of the main 2-14-1 phase. The low-temperature study of hysteresis properties of the material obtained is performed for the first time.

Effects of the pulse duration in laser modification of nano-sized WTi film on Sisubstrate

A study of morphological and composition changes of the WTi/Si system induced bynanosecond and picosecond laser pulses is presented. A 190 nm thick WTi film was depositedon a silicon substrate of n-type (100). The pulsed laser systems used were: nanosecond TEACO2 laser(emission, 10.6 µm; pulse FWHM; pulse duration 120 ns) and picosecond Nd:YAG laser (emission,532 nm; pulse FWHM; pulse duration 40 ps). During experiments the used fluences (Φ) had similarvalues, Φ1 = 20 J cm − 2 in caseof the TEA CO2 laser and Φ2 = 16 J cm − 2 for the Nd:YAG laser. The laser-induced morphological and composition modificationsshowed a dependence on pulse duration. Generally, the following morphological changeswere observed: (i) ablation/exfoliation of the WTi thin film, (ii) appearance of hydrodynamicfeatures such as resolidified material, and (iii) formation of nano-sized grains and globules.Overall morphological modifications were more pronounced after the picosecond laseraction. The surface composition analysis showed a quite different distribution of samplecomponents depending on the pulse duration. Formation of the silicon dioxide (SiO2) was recorded only in the case of irradiation of the WTi/Si system by picosecond laserpulses.

Ultrahigh strength of nanocrystalline iron-based alloys produced by high-pressure torsion

Microstructural evolution of an Fe–18.1%Ni–34.9%Co–9.3%Ti (in at.%) in processing by high-pressure torsion (HPT) was investigated by electron backscatter diffraction. After 10 turns of HPT straining at room temperature with a rotation speed of 1 rpm under a pressure of 6 GPa, the alloy was composed of body-centered cubic-structured grains with sizes of 20–50 nm having high density crystal defects inside the grains. The mechanism of this significant grain refinement was discussed in relation to the stress-induced martensitic transformation and transgranular shear near ideal strength. The actual shear stress of the nanocrystalline iron-based alloy was estimated to be 1.37 GPa and as high as 38% of the ideal shear stress, which is similar to Gum Metal exhibiting deformation without dislocation activity. It is inferred from these results that the dislocation motion can be suppressed up to ultrahigh stress level near ideal strength by the formation of nano-sized grains.

Fabrication of nano-sized grains by pulsed laser surface melting

In this work, the formation and characterization of nano-sized grains on the modified surfaces of GCr15 and H13 steels have been investigated. The material was processed by pulsed laser surface melting (LSM) under different depths of de-ionized water. The microstructures and phases of the melted zones were examined by x-ray diffraction, environmental field emission scanning electron microscopy and high resolution transmission electron microscopy. The results indicate that LSM under water can successfully fabricate nano-scaled grains on the surfaces of steel, due to the rapid solidification and crystallization by heterogeneous nucleation. The elemental segregation of chromium and activated heterogeneous nucleation mechanism of austenite in liquid metal play a key role in the formation of nano-sized grains at high cooling rates. This one-step technique provides us a new way to prepare uniform nano-scaled grains, and is of great importance for further understanding the growth of nano-materials under extreme conditions.

The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures

Cast AZ80 alloy was subjected to conventional extrusion pressing at 250 °C, 300 °C, and 350 °C. In order to characterize the changes in their microstructure a thorough study was done through various microscopy analyses including Optical Microscope, SEM, and TEM. Corrosion performance of each condition was investigated in 3.5% NaCl solution saturated with Mg(OH) 2 (pH ≈ 10.5) using immersion and AC and DC polarization tests. The local potential difference on the surface resulting from different compositions of second phase particles to the matrix was investigated using scanning Kelvin probe force microscopy (SKPFM) technique. The results show grain refinement by a factor of about 15–20 and obvious evidence of dynamic recrystallization were identified leading to the formation of nano-sized grains after the extrusion process. The corrosion resistance of cast AZ80 alloy drastically decreases after the thermo-mechanical processes and the main factor is high dependence on different phase rearrangements before and after the extrusion process, especially β phase. For the extrusion conditions, different corrosion resistances are attributable especially to dislocation rearrangement results by grain growth after dynamic recrystallization.

Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy

By means of surface mechanical attrition treatment, nanometer-sized grains (with an average size of 30±5nm) were generated in the surface layer of a single-phase AZ91D alloy. Transmission electron microscopy investigations showed that the strain-induced grain refinement process in AZ91D alloy includes three steps. At the initial stage twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets. With increasing strain, double twins and stacking faults form and a number of dislocation slip systems are activated, including basal plane systems, prismatic plane systems and pyramidal plane systems. As a result of the dislocation slip along these systems and of the cross slips, high-density dislocation arrays are formed which further subdivide the twin platelets into subgrains. Obvious evidence of dynamic recrystallization were identified within the high-strain-energy subgrains with a further increase of strain, leading to the formation of nano-sized grains in the surface layer.

Influence of microalloying on glass formation and crystallization

The influence of extremely small amounts of early transition metals on glass formation has recently been demonstrated in Al–Y–Fe alloys. Although rapidly quenched samples of Al 88Y 7Fe 5 have X-ray diffraction patterns that are typical of amorphous metallic alloys, the isothermal differential scanning calorimetry (DSC) data do not show the expected nucleation and growth peak associated with the formation of nano-sized grains of α-Al. Samples prepared with Ti (Al 88− x Y 7Fe 5Ti x with x = 0.5, 1, 2) transform at higher temperatures and show the DSC isothermal peak expected for glass crystallization. It is often claimed that the absence of an isothermal DSC peak indicates that the samples are not glasses, but are actually fully transformed nanocrystal composites, with the monotonic DSC trace indicating a coarsening of the nanocrystals. However, calculations presented here show that it could also be explained by diffusion-controlled nucleation and growth, with a high nucleation rate due to a change in the local structure of the glass.

Formation of nano-sized grains in Cu and Cu−Fe−P alloys by accumulative roll bonding process

Mechanical properties and formation of nano-sized grains in Cu and Cu−Fe−P alloys by the accumulative roll bonding (ARB) process were investigated. Nano-sized grains were successfully obtained in OFC and PMC-90 alloys by the ARB process after the third cycle. Once the 200 nm grains formed, further reduction in the grain size was not observed up to 8 ARB process cycles. For both alloys, the tensile strength values increased drastically in the initial stage of the ARB process. The tensile strength values of both alloys tended to saturate after the third ARB process cycle. The tensile elongation value greatly decreased by 1 cycle of the ARB process due to the strain hardening. After the third cycle of the ARB process, each alloy showed a gradual increase in tensile elongation due to the dynamic recovery. For PMC-90 alloy, the strength value was higher than that of OFC due to addition of the alloying elements. With increased annealing temperature, the nanosized grains tended to grow in OFC at 150°C, and after annealing at 200°C, coarse grains formed. On the other hand, in PMC-90 alloy, there was no grain growth up to 250°C due to the alloying elements (Fe, P).