Low-energy Milling Research Articles

Microstructure evolution and bending fracture mechanism of as-rolled gradient-laminated network titanium matrix composites

Titanium matrix composites (TMCs) have drawn significant interest due to decent specific strength and heat resistance. Introducing gradient and laminated structure can effectively enhance the toughness of TMCs. To achieve sheet forming, the gradient-laminated network titanium boride reinforced Ti6Al4V (TiB/TC4) composite was fabricated by low-energy milling, powder stacking, vacuum hot pressing sintering and β-phase-region hot rolling. After hot rolling, the layer thickness decreased, the layer interface was kept straight, and no macroscopic defects were observed. In the meanwhile, the flattened network structure, rotation and fracture of TiB whiskers, and grain refining of the matrix were observed. TiB whiskers can also promote the dynamic recrystallization of nearby matrix and tend to maintain orientation relations. While retaining the gradient-laminated network structure, the bending strength of the composite was enhanced to 2205 MPa after hot rolling, which is 13.07% higher than that of the as-sintered composite. The strengthening was attributed to grain refining of the matrix and alignment-induced load transfer strengthening of TiB whiskers. Digital image correlation (DIC) revealed the upward movement of the neutral surface away from the center during the bending test, which led to increased tensile stress and crack initiation in the center layer, rather than the region with maximum tensile strain. Upon observing the fracture side face, the crack propagation path was determined to be influenced by layer interfaces, the network structure connectivity, and the outmost alloy layers.

Oxidation behavior of lightweight Al2NbTi3V2Zr high entropy alloy at 1000°C

AbstractLightweight high entropy alloy (LWHEA) has great potential in aeroengine hot component applications. Anticorrosion property of LWHEA at high temperatures is one of the key determining factors. In this study, an LWHEA containing Al, Nb, Ti, V, and Zr elements with a molar ratio of 2: 1: 3: 2: 1 was designed and prepared by low energy (LE) and high energy (HE) mechanical milling followed by vacuum hot pressing. The sintered LE alloy consisted of a simple cubic matrix and α intermetallic while the BCC matrix along with α and β intermetallic phases were observed in HE alloy. The oxidation of LE and HE at 1000°C for 48 h was mainly concentrated on α and β phases, and the resultant oxides were ZrO2 and TiO2. Weight gain curve of HE sample within oxidation time of 48 h followed the parabolic law while a two‐stage parabola was found in LE bulk. The steady‐state oxidation of α precipitate was dominant within 12 h and showed a preferential oxidation mode. High temperature induced simple cubic matrix to stable BCC phase transformation and thus LE alloy with longer oxidation time and HE exhibited a similar uniform oxidation. The oxidation rate of the alloy is related to the content of α and β phases.

Macrokinetics of thermal explosion in a 3Ni-Al system mechanically activated in a low-energy mill

Two-step mechanosynthesis in the 3Ni - Al system has been investigated by experimental and theoretical methods using the developed macrokinetic theory of mechanochemical synthesis (MS). The first step included the grinding and mechanical activation (MA) of the starting reaction mixture in the mill, and the second step included the explosive synthesis of the Ni3Al intermetallic compound outside the mill. The effect of mechanical pre-treatment on the temperature-time parameters of the synthesis of the Ni3Al intermetallic compound was found. The phase composition of the activated mixture of nickel, aluminum, and synthesized products was characterized by X-ray diffraction. Preliminary low-energy mechanical activation (LEMA) of the mixture in a laboratory mill eliminates the negative phenomena associated with the grinding and formation of intermediate phases. Kinetic and thermophysical constants characterizing the two-step MS were determined using the inverse problem. The contribution of mechanical activation factors to the acceleration of Ni3Al synthesis was quantitatively evaluated.

Achieving high strength and high ductility in submicron-MgO/AZ31 composites with an innovative honeycomb-like structure

Honeycomb-like AZ31 matrix composites with 1, 2, and 4 wt% submicron-MgO were prepared using low-energy milling and extrusion. The composites consist of fine-grained and coarse-grained zones. The fine-grained zones enriched with submicron MgO form a honeycomb-like structure, and the coarse grains without MgO are filled into unit cells. Both the strengths and elongations of the newly developed MgO/AZ31 composites are enhanced compared with AZ31. Specifically, the tensile strength of composites containing 1 wt% MgO is improved to 344.1 MPa, and at the same time, its elongation is increased up to 21.5%. The excellent strength-ductility combination is due to the introduction of submicron-MgO reinforcements with unique distribution. Such achievements could be beneficial to the structural design of composites.

Effect of low energy milling processes on the magnetic properties of Fe–Ni–Co soft magnetic materials

ABSTRACT This work reports the effect of micro strain of soft magnetic materials obtained by low-energy ball milling. It has been observed that the low energy of the ball milling changes the magnetic domain size, increasing magnetisation, remanence and magnetic permeability of the milled powders as ferromagnetic particles. It is due to the increase of linear defects and the increase of lattice strain. The magnetisation M S increases up to 70% by the accumulation of crystalline defects, although M S decreases with the reduction of particle size, due to the connections of voids between the lines of magnetisation across the crystalline defects obtained by milling. This response is produced by the cold working through the ball milling.

Ga+ Focused Ion Beam Damage in n-type Ga2O3 and Its Recovery after Annealing Treatment

In this study, the focused ion beam (FIB) Ga+ irradiation damage on β-Ga2O3 Schottky diode structure under different ion energies were tested, and the effective recovery of diode characteristics after rapid thermal annealing (RTA) post-annealing were also studied.β-Ga2O3 based Schottky diodes have attracted increasing attention because of the prospects for use in next-generation high-power electronics. Focused ion beam (FIB) machining is one of the newest processing techniques, which is mainly used in semiconductor and chip manufacture and transmission electron microscopy (TEM) sample preparation. During FIB sample preparation, the energetic Ga ions are incident onto the sample to induce physical sputtering of the material, which in turn, modifies its chemical composition, crystallinity and electrical properties. It has been found in many studies that 20-30 nm amorphous layer can form on silicon after irradiation with 30 keV Ga ions. From our previous study, the FIB ion beam induced damages could cause significant degradation on diode on-resistance and turn-on voltage. Milling at lower energy has been used to reduce the electrical damage. However low energy milling is more time consuming and less precise. Short-time, high-temperature post-annealing treatments, such as RTA has been proved could remove the crystal damage. Thus, there is a great need to determine the maximal ion beam energy and post-annealing temperature for FIB without degrading diode characteristics.In this work, energies ranging from 2 keV to 30 keV were employed to mill off a part of Ga2O3 epi-layer and Schottky diodes were fabricated on milled surface. Forward IV characteristic shows less degradations with lower ion beam energies. For the diodes exposed to 2, 5, and 10 keV Ga+ ion beams exhibited significant recovery of diode on resistance and turn-on voltage after 300°C RTA annealing. For the diodes exposed to 30 keV Ga+ ion beams recovered significantly after 400 °C annealing. In conclusion, decreasing the FIB energy shows effective alleviations on the FIB damage, and post annealing treatment effectively recovered the damage from Ga+ implantation.This is the first time the thermal annealing treatment was performed for Ga2O3 Schottky diode recovering from FIB cutting, it is an effective and simple way to mitigate the degradation, especially for higher ion beam energies. This work could broaden the FIB’s usability and release tremendous potential as a specimen preparation and imaging tool in materials science applications. Figure 1

Achieving superior strength-ductility synergy by constructing network microstructure in Ti-based high-entropy alloy matrix composites

In-situ (Ti,Nb)B whiskers ((Ti,Nb)Bw) reinforced Ti-based high-entropy alloy (HEA) matrix composites ((Ti,Nb)Bw/Ti1.5ZrNbAl0.3) with a quasi-continuous network microstructure were successfully fabricated via low energy milling (LEM) and hot-pressed sintering (HPS). The results show that the in-situ (Ti,Nb)Bw reinforcements were distributed around Ti1.5ZrNbAl0.3 matrix particle forming a special network architecture. This microstructure resulted in remarkable grain refinement of Ti-based HEA matrix, which contributed to the superior strength-ductility synergy. Consequently, the as-sintered (Ti,Nb)Bw/Ti1.5ZrNbAl0.3 composites exhibited a high yield strength (σYS = 1015 MPa) while maintaining superior ductility (ϵe = 17.5%), which were simultaneously increased by 11.5% and 96.6% compared with the monolithic Ti1.5ZrNbAl0.3 alloy. The network microstructure design of HEA-based composites could be therefore exploited to overcome the strength-ductility trade-off.

(TiB+La2O3)/Ti-V-Al lightweight high temperature shape memory composites with high strength fabricated by reaction hot pressing and hot rolling

In the present study, lightweight high-strength Ti-V-Al based high temperature shape memory composites were fabricated to broaden the application of shape memory alloys. In-situ TiB whiskers and La2O3 particles were introduced and the distribution of reinforcements was optimized by the combination of low-energy milling, hot-press sintering and subsequent hot rolling. The in-situ reinforcements formed a network structure, which evolved from an equiaxed shape to an ellipsoidal shape. Furthermore, the direction of the reinforcement was inclined to the rolling direction. In the reinforcement-lean area, the phase constitution evolved from the β parent phase to the α″ martensite phase, and the width of the α″ martensite gradually widened with increasing distance to the in-situ reinforcements. The composites maintained high martensitic transformation temperature characteristics. Excellent mechanical properties with a high tensile strength of 885 MPa and a large elongation of 13.1% are obtained in the composite with a 1.0 wt% LaB6 addition and 70% deformation ratio. Meanwhile, the composite realized 3.49% recoverable strain under 6% pre-strain conditions.

Highly oxidation-resistant Ti-Mo alloy with two-scale network Ti5Si3 reinforcement

There is keen interest in using Ti alloys as lightweight structural materials for aerospace and automotive industries. However, a long-standing problem for these materials is their poor oxidation resistance. Herein, we designed and fabricated a Ti5Si3 reinforced Ti-4(wt.%)Mo composite with two-scale network architecture by low energy milling and spark plasma sintering. It displays superior oxidation resistance at 800 °C owing to the in-situ formation of a multi-component surface layer. This oxide layer has a dense grain size gradient structure that consists of an outer TiO2 layer and an inner SiO2-padding-TiO2 layer, which has remarkable oxidation resistance and thermal stability. Furthermore, it was revealed that the hitherto unknown interaction between Ti5Si3 reinforcement and nitrogen during oxidation would contribute to the formation of a TiN nano-twin interface layer, which accommodates the thermal mismatch strain between the oxide layer and matrix. This, along with high adhesion, confers excellent thermal cycling life with no cracking or spallation during long-term oxidation. In this regard, the secure operating temperature of this new composite can be increased to 800 °C, which provides a design pathway for a new family of Ti matrix composites for high-temperature applications.

Preparation of α-cordierite through mechanochemical activation of MgO–Al2O3–SiO2 ternary system

Samples of the ternary system MgO–Al2O3–SiO2 with stoichiometric composition in relation to α-cordierite (Mg2Al4Si5O18), consisting of 22.2 mol% MgO, 22.2 mol% Al2O3, and 55.6 mol% SiO2, were activated in a low energy mill with a constant speed of 100 rpm, in an aqueous medium. The precursors used were corundum (Al2O3), silica gel HF254 type 60 (SiO2), and periclase (MgO). The objective of the present study was to evaluate the effect of mechanochemical activation on the solid-state synthesis of α-cordierite, using a low energy ball mill. Another objective was to shed light on the effect of mechanochemical activation on the steps of α-cordierite formation. For this end several grinding conditions were evaluated, varying the time and mass ratio of precursors/grinding elements, as well as calcination at different temperatures between 950 °C and 1350 °C for 2 h. The samples were analyzed for the determination of the formed phases by Infrared (IR) and X-ray Diffraction (XRD). The phases identified in uncalcined samples were brucite (Mg(OH)2), forsterite (Mg2SiO4), enstatite (MgSiO3), spinel (MgAl2O3), amorphous silica (SiO2), corundum (α-Al2O3), and zirconia (monoclinic and tetragonal ZrO2). The lowest temperature corresponding to the formation of α-cordierite (α-Mg2Al4Si5O18) was 1150 °C and a considerable amount of this phase (16.2%) was observed at this temperature, for the sample with the higher mechanochemical activation. In a solid-state reaction, α-cordierite is normally obtained at around 1400 °C, therefore, the formation of this phase at 1150 °C confirms that the mechanochemical activation method, using a low-cost ball mill, is efficient in reducing the solid-state reaction temperature.

High-energy ball milling and spark plasma sintering of molybdenum - lanthanum oxide (Mo-La2O3) and molybdenum – lanthanum zirconate (Mo-La2Zr2O7) composite powders

The current study is focused on the preparation of Mo-10 vol%La2O3 and Mo-10 vol% La2Zr2O7 composite powders via low- and high-energy ball milling approaches as potential candidates for near-future high-temperature structural applications. The mechanical milling parameters play a critical role on the final powder's microstructure. When using the high-energy milling mode (using 800 rpm, ball-to-powder ratio (BPR) 100: 6), the homogeneous powder agglomerates are formed with refined laminated microstructure and more uniform ceramic phase distribution in both Mo-La2O3 and Mo-La2Zr2O7 systems compared to the powders produced by means of the low-energy milling mode (using 350 rpm, BPR 100: 6), where inhomogeneous powder mixture with less embedding of ceramic phases into Mo agglomerates was obtained. This study also focuses on the evaluation of high-temperature phase and microstructural stability of the produced composite powders treated at the temperature of 1300 °C under the different gaseous environments, including ambient, inert and reducing atmospheres. The Mo-10 vol% La2Zr2O7 composite powder exhibited better thermal stability during the high-temperature exposure in all tested atmospheres in comparison with the Mo-La2O3 composite powder, since it revealed less intensive formation of the intermediate phases, such as lanthanum oxymolybdates. Therefore, the Mo-10 vol% La2Zr2O7 composite powder was used further for consolidation by means of spark plasma sintering at 1600 °C. The successful production of Mo-La2Zr2O7 composite with homogeneous distribution of ceramic phase, the grain size about of 5 μm, and hardness of 3.4 GPa was not reported so far.

The enhanced mechanical properties and strain-hardening capability of CNT/Al composites achieved by heterogeneous micro-laminated architecture

A smart approach was adopted to fabricate a heterogeneous 1.5 wt% CNT/Al matrix possessing enhanced mechanical properties. The heterostructured-composite showed an excellent ultimate compressive strength of 372.19 MPa, which was approximately 33% and 76% higher than that of its homogeneous counterpart (279.83 MPa) and the monolithic Al (211.56 MPa), while keeping almost the same ultimate strain (40.5%) to that of the Al matrix (41.1%). It was proved that the discontinuous addition of CNTs to the matrix via several stages of low-energy milling (in the presence of PCA) followed by a high-energy dry-milling stage (without PCA) brought about a nonhomogeneous dispersion of CNTs in the matrix providing the opportunity for the development of heterogeneous microlaminated architecture, and finally enhanced the combination of strength and ductility of the resultant composite. The process would be technologically applicable for developing strong and ductile metal-matrix composites with different nanofillers.

Highly Anti-Oxidized Ti-4Mo Alloy With Two-Scale Network Architectured Ti <sub>5</sub>Si <sub>3</sub> Reinforcement

Titanium alloys are desirable for applications in advanced hypersonic aircraft engines and gas turbines, but currently few materials can satisfy the associated high temperature oxidation requirements. Herein, we designed and fabricated a Ti5Si3 reinforced Ti-4Mo composite with a two-scale network architecture by low energy milling and spark plasma sintering. It displays superior oxidation resistance at 800°C owing to the in-situ formation of a multi-component surface layer. This oxide layer has been revealed to have a dense grain size gradient structure that consists of an outer TiO2 oxide layer and an inner TiO2 and SiO2 mixed oxide layer with a core-shell structure, which has remarkable oxidation resistance and thermal stability. Furthermore, it was revealed that the hitherto unknown interaction between Ti5Si3 reinforcements and nitrogen during oxidation would contribute to the formation of a TiN nano-twin interface layer, which accommodates the thermal mismatch strain between oxide layer and matrix. This along with high adhesion confers excellent thermal cycling life with no cracking or spallation during long term oxidation. In this regard, the secure operating temperature can be increased to 800°C, which can provide a design pathway for a new family of titanium matrix composites for high-temperature applications.

Development and characterization of coal fly ash through low-energy ball milling

A paradigm shift from monolithic materials to composite materials has been observed in recent times to achieve the ever changing demands of the manufacturing sector. The composites are being explored in terms of industrial and thermal power plant wastes used as reinforcements. Being a prominent thermal power plant waste, fly ash disposal and utilization is gravitating environmentalists and researchers to propose innovative ideas and feasible implementations. To trace and adjoin an effort to add value to aforementioned cause, an investigation was conducted on fly ash which was milled in tumbler ball mill with predefined input parameters viz. spindle speed, ball-to-powder ratio, and milling time. Low-energy mill has an extensive application in preparation of new materials, their activation and process of synthesis for potential merits like cost-effectiveness, reliability, low maintenance and ease of operation over its contemporary high-energy mill. The modifications in chemical and morphological properties of milled fly ash sample were characterized through X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results inferred that the ball-milling process aimed at reducing crystallite domain size of milled fly ash, and also affected the microstructure, functional groups and led to decline in degree of crystallinity. The milling time was observed as a significant factor to impact extent of these changes. The particles of the raw spherical-shaped fly ash (regularly 5 μm size) were broken down into smaller particles of average crystallite domain size ∼ 30 nm, this change indicated an increase in amorphousness which helped in achieving better compatibility and significantly higher reactivity. The pattern and peaks of XRD for original crystallinites significantly diminished and resulted in broadened peaks. This can be interpreted by the smaller particle size and low degree of crystallinity. The research defined the achievement of nanostructured fly ash with low energy mill technique.

The Sticking Out Mechanism and Growth of TiB Whiskers from TiBw/Ti-6Al-4V Composites during Sintering

The process of TiB whiskers (TiBw) squeezed out and the growth of sticking out TiBw were investigated in TiBw/Ti-6Al-4V composites via adding 5 vol% TiB2 powders. TiB2 distributed along the surface of Ti-6Al-4V powders by low energy milling, resulted in the enrichment region of B and Ti elements. During sintering, TiBw nucleated and grew along the [010] direction with the diffusion of B and Ti atoms in the enrichment region. Due to larger thermal expansion ratio of TiBw, the TiBw were squeezed out from the Ti matrix, and the sticking out TiBw grew gradually with the diffusion of B and Ti atoms from the enrichment region. Because of the high interfacial strength between TiBw and Ti matrix, the matrix adhered to TiBw, and hindered the squeezing out of TiBw. Eventually, the cone TiBw stuck out the Ti matrix, and the other end of TiBw pinned in Ti matrix. Moreover, the diameter of TiBw near surface between TiB2 and Ti matrix was larger than other areas, resulting from the high concentration of B and Ti atoms. The solubilized B atoms near TiBw were absorbed by TiBw, which was considered as the other B source of TiBw.

Fabrication and mechanical properties of network structured titanium alloy matrix composites reinforced with Ti2AlC particulates

Network structured Ti-6.5Al–2Zr–1Mo–1V (TA15) matrix composites reinforced with Ti2AlC particles were fabricated by low energy ball milling and subsequent spark plasma sintering (SPS). Microstructures, mechanical properties, and wear resistance properties of the composites were evaluated as a function of Ti2AlC weight fraction. Experimental results showed that the fine Ti2AlC powders were adhered on the surface of the spherical TA15 powders after low-energy milling process and Widmanstatten α+β microstructure with a network architecture was formed after SPS. The Ti2AlC reinforcements were reacted with Ti matrix and formed a Ti3AlC phase at the interface. The hardness, tensile, and compressive strength of the TA15/Ti3AlC composites show significant improvement with less ductility loss. The TA15/1.0 wt% Ti2AlC exhibited tensile yield strength of 929 MPa and ultimate tensile strength of 1013 MPa with ductility of 13.06%. The TA15/Ti3AlC composites had a lower friction coefficient and decreased wear rate with delamination and abrasive wear modes.

Influence of ball milling parameters on the dispersion characteristics and structural integrity of MWCNTs in nickel aluminide matrix powders

The novelty of incorporating carbon nanotubes (CNTs) in various matrices is becoming increasingly significant for the development of various composites to match the rising technological demands in industry. However, for the potentials of CNTs to be fully realized, the mandatory requirement to uniformly disperse them must be achieved. To effectively disperse these nanotubes in metallic matrices, ball milling has emerged as an effective method, but concerns still persist as to the ability of this method to achieve uniform dispersion without significant damage to the CNTs. In this work, multi-walled carbon nanotubes (MWCNTs) were dispersed in a nickel aluminide matrix via three different milling methods, using both low and high energy milling regimes. The admixed powders were characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry and transmission electron microscopy techniques. Results show that a two-stage milling, comprised of a 6-h low energy milling with a short-term follow-up of a low speed 2-h high energy milling achieved the best MWCNTs dispersion and retained their structural integrity.

Effect of ball milling on the powder characteristics and welding mechanism of Mg2B2O5w/6061Al matrix composite

Mg2B2O5w/6061Al matrix composites were fabricated through the powder metallurgy (PM) technique. The morphology of the particles, particle size and the effect of powder characteristics on densification behavior of the compacted samples were investigated. It was found that milling way, milling time, milling speed and whisker content all had different effects on powders characteristics and densification. During the low-energy milling process, the particles morphology changed from spherical to flattened shape and the reinforcement accelerated the process to some extent; during the high-energy milling process, the particles experienced the welding-fracturing process, the Al particles morphology changed from spherical to flattened shape and then back to more equiaxed morphology, the reinforcement whisker apparently accelerated the milling process. During the uniaxial cold pressing process, the particles after high-energy milling achieved better densification, the reverse for the low-energy milled powders because of whiskers clusters. A good linear correlation coefficient was obtained when the experiential data were fitted by the Panelli and Ambrosio Filho equation.

Influence of Additives of Carbon Nanotubes and Oxide Nanofibers on Mechanical Properties of Aluminum Composite

Composite material with aluminum matrix reinforced by carbon nanotubes (CNT) and oxide nanofibers (NF) was made by powder metallurgy method. Powder mixtures were made by low energy milling on drum ball mill. Composite samples were manufactured by spark plasma sintering system. Microstructure analysis showed agglomeration of CNT and NF in composite materials. Correlation between flexural strength and unit mass of composites was revealed. Nevertheless, the use of low energy milling allows minimizing the damage of carbon nanotubes and oxide nanofibers during the preparation of mixtures. From this point of view it is suitable for making composites with high elastic modulus. It was shown that addition of 5-12%wt of CNT can increase flexural modulus by 30-189%. Also was shown that addition of 5%wt of oxide nanofibers increase flexural modulus by 78%

Influence of high-energy milling and heat treatment on the thermal and mechanical properties of a ZrMgMo3O12p/2024Al composite

In this study, ZrMgMo 3 O 12particle (p) /2024Al composites were fabricated with 10 vol% of a near-zero expansion material, ZrMgMo 3 O 12 , as a reinforcement via high-energy milling followed by vacuum hot pressing (VHP). The influences of the milling conditions and the heat treatment on the hardness, thermal expansion properties and dimensional stabilities of the composites were studied. The results show that high-energy (HE) ball milling refined the ZrMgMo 3 O 12 particles and the grains of the α-Al matrix, increased the hardness and decreased the coefficients of thermal expansion (CTEs) of the ZrMgMo 3 O 12p /2024Al composites. Compared with those of the ZrMgMo 3 O 12p /2024Al composites prepared via low-energy (LE) milling, the hardness value increased to 183 HV from 91.8 HV and the CTE decreased to 16 × 10 −6 /°C (RT - 400 °C) from 22 × 10 −6 /°C (RT - 300 °C). Heat treatment of the solution for 24 h followed by ageing at 190 °C for 5 h (i.e., P2 process) further increased the hardness value to 277 HV and decreased the CTE to 13.4 × 10 −6 /°C (RT - 400 °C), which represent an increase of 83% and decrease of 39%, respectively, compared with those of the heat-treated LE ZrMgMo 3 O 12p /2024Al composites. High-energy ball milling and heat treatment could improve the dimensional stabilities of the composites as well. In particular, the HE ZrMgMo 3 O 12p /2024Al composites heat treated with the P2 process showed no variation in their hysteretic strain or CTEs during thermal cycling from RT to 400 °C. Therefore, the P2 HE ZrMgMo 3 O 12p /2024Al composites are in a thermostable state and have the best dimensional stabilities of the tested samples.