Al–Ti powder produced through mechanical alloying for different times

A powder mixture of aluminum, 10 wt% titanium, and 1.5 wt% of a wax acting as process control agent (PCA), has been attrition-milled for 2–20 h. Titanium powder had been previously ground to a lower particle size to make it similar to the as-received aluminum particle size. The overall aim of this work was to achieve a metastable titanium solution in the aluminum matrix. Changes with milling time of particle size and shape, microstructure, hardness and other powder characteristics have been studied. Given the used experimental-conditions, a process time of 10 h has been selected for the mechanical alloying (MA) of Al–10Ti powder, attaining a compromise between uniform microstructure development and a not so long processing time. At this milling time aluminum dissolves about 9 wt% Ti, increasing its Vickers microhardness (202 VH₂₀) more than 10 times with reference to the starting Al powder (20 VH₂₀). Milled particle size is smaller than the starting one (17 vs. 44 μm). Increasing milling for longer times, up to 20 h, does not produce important changes in powders structure.     

Evolution of manufacturing parameters in Al/Ni3Al composite powder formation using blending and mechanical milling processes

Aluminu–matrix composites produced by Ni₃Al intermetallic particles are increasingly used in aerospace and structural applications because of their outstanding properties. In manufacturing of metal–matrix composites using powder metallurgy blending and milling are important factors. They control the final distribution of reinforcement particles and porosity in green compacts which in turn, strongly affect the mechanical properties of the produced PM materials. This paper studies different conditions for producing composite powders with uniform dispersion of Ni₃Al particles in aluminum powders and improved physical and mechanical properties. The results indicated that an intermediate milling time for fabrication of composite powder, better than prolonged and shortened ones, causes better microstructure and properties. It was shown that addition of 5 vol.% Ni₃Al particles, produced by 15 h mechanical alloying to aluminum powders, and then 12 h blending operation provides an appropriate condition for producing Al–Ni₃Al composite powder.     

Preparation of NiAl–TiC nanocomposite by mechanical alloying

NiAl–TiC nanocomposite was successfully synthesized via a ball-milled mixture of Ni, Al, Ti, and graphite powders. The structural and morphological evolutions of the powders were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Results show that NiAl–TiC composite was obtained after 6 h of milling. The mean grain sizes of 6 and 10 nm were attained for NiAl and TiC at the end of milling, respectively. An annealing of 3 h milled sample at 600 °C led to the formation of Ni (Al, Ti, C) solid solution. NiAl–TiC nanocomposite that was formed in the 12 h milled sample is stable during an annealing at 600 °C. The mean grain size of NiAl at the 12 h milled powder increased during annealing at 600 °C. Maximum micro hardness value of 870 kg/mm² (8.7 GPa) was acquired from the 12 h milled powder. SEM images and particle size measurement showed that very fine spheroid particles (1 μm) were procured at the end of milling.     

多重结构Ti-B4C/Al2024复合材料的组织和力学性能

为了研究多重结构对铝基复合材料力学性能的影响,将气雾化态Al2024合金粉末与球磨不同时间的Ti-10%(质量分数,下同)B_4C复合粉末混合,采用热压烧结和热挤压的方法制备多重结构Ti-B_4C/Al2024复合材料。通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)和拉伸试验机对不同材料的显微组织与力学性能进行观察和测试,并对多重结构复合材料的强韧化行为进行讨论。结果表明:Ti-B_4C/Al2024复合材料多重结构包括基体Al2024、核壳结构Ti/Al₁₈Ti_2Mg_3组织和B_4C颗粒。向Al2024中加入5%预先球磨6h后的Ti-B_4C粉末时,其屈服强度从107MPa提高到122MPa,并且表现出与热挤压Al2024合金几乎相同的伸长率。当球磨时间延长至12h时,试样5TB-12h的伸长率可达到16.4%。然而,复合材料的伸长率随着Ti-B_4C添加量的增加而降低。     

Effect of high energy ball milling on the displacement reaction in particulate reinforced Al–Si–Cu alloy matrix composite powders

The displacement reaction between Al and SiO₂ in an Al–3wt%Cu–3wt%Si–9wt%SiO₂ powder mixture has been studied when the mixture had been ball-milled, and compared with the reaction in the as-mixed powder. Diffusion couples consisting of Al/SiO₂ were formed during ball milling. The size of the composite powder particles and the diffusion couples was reduced by increasing the ball milling time. Differential thermal analysis and X-ray diffraction results showed that the displacement reaction between Al and SiO₂ did not occur in the as-mixed powder, but occurred in the as-milled powders in the temperature range of 640–680 °C. Furthermore, the onset temperature of the displacement reaction shifted to lower temperatures after increasing the ball milling time. On the basis of these results the milled powder was sintered at 640 °C in order to produce an Al–Cu–Si matrix composite reinforced with homogeneously distributed submicron-sized Al₂O₃ particles. This is much lower than the temperature required for the same reaction in other processes which are used to produce such composites, such as the melting infiltration process.     

Dry sliding tribological behavior and mechanical properties of Al2024–5 wt.%B4C nanocomposite produced by mechanical milling and hot extrusion

In this paper, tribological behavior and mechanical properties of nanostructured Al2024 alloy produced by mechanical milling and hot extrusion were investigated before and after adding B₄C particles. Mechanical milling was used to synthesize the nanostructured Al2024 in attrition mill under argon atmosphere up to 50 h. A similar process was used to produce Al2024–5 wt.%B₄C composite powder. The milled powders were formed by hot pressing and then were exposed to hot extrusion in 750 °C with extrusion ratio of 10:1. To study the microstructure of milled powders and hot extruded samples, optical microscopy, transmission electron microscopy and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) were used. The mechanical properties of samples were also compared together using tension, compression and hardness tests. The wear properties of samples were studied using pin-on-disk apparatus under a 20 N load. The results show that mechanical milling decreases the size of aluminum matrix grains to less than 100 nm. The results of mechanical and wear tests also indicate that mechanical milling and adding B₄C particles increase strength, hardness and wear resistance of Al2024 and decrease its ductility remarkably.     

Effects of ball milling and high-pressure torsion for improving mechanical properties of Al–Al2O3 nanocomposites

Pure Al powders were mixed with a 30 % volume fraction of Al₂O₃ powders having particle sizes of ~30 nm. The mixed powders were first subjected to ball milling (BM) and thereafter consolidated by high-pressure torsion (HPT) at room temperature under a pressure of 3 GPa for 10 turns. The Al–Al₂O₃ composite produced by BM and HPT (BM + HPT) had a more uniform dispersion of the nano-sized Al₂O₃ particles in the Al matrix. Hardness values of the BM + HPT composites were higher than those of the composites without BM. It is shown that the use of BM powders for HPT is more effective in achieving a uniform dispersion of the nano-sized Al₂O₃ particles and in improving mechanical properties of the Al–Al₂O₃ nanocomposites.     

Tensile properties and fracture behaviour of an ultrafine grained Ti–47Al–2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti–47Al–2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 °C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 °C, respectively, but very high ductility (elongation to fracture 70–100%) when tensile tested 800 °C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700–800 °C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 °C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 °C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 °C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 °C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing.     

Sintering characteristics of nano-sized Ag–Pd–glass composite powders with high Pd content

Nano-sized Ag–Pd (50–50) alloy powders coated with Pb-based glass material with low and high glass transition temperature are directly prepared by high-temperature flame spray pyrolysis. Nano-sized Ag–Pd–glass composite powder is formed from the evaporated vapors by nucleation and growth process, and then glass material moves out to the outside of the powder by crystallization process of alloy. The thickness of the glass coating layer measured from the TEM image is 2.8 nm. The mass changes of the Ag–Pd alloy and Ag–Pd–glass composite powders in the TG analysis under 900 °C are 10.9 and 6.8%, respectively. Glass materials improve the uniformity and density of the Ag–Pd electrode layers by act as sintering agent and adhesion improvement. The Ag–Pd electrode formed from the composite powders with high glass transition temperature glass material has thin and uniform thickness. The specific resistances of the electrodes formed from the nano-sized Ag–Pd–glass composite powders are 0.27, 0.09, and 0.03 mΩ cm at firing temperatures of 700, 800, and 900 °C, respectively.     

Preparation and mechanical properties of SiC-reinforced Al6061 composite by mechanical alloying

In this paper, an Al6061–10 wt% SiC composite was prepared using the mechanical alloying route. The morphology and the structure of the prepared powder, which change with milling time, were evaluated using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques, respectively. Moreover, the relationships among the stages of mechanical alloying (MA), relative density and hardness of both pressed and hot extruded materials were investigated. The morphological evolutions showed that relatively equiaxed powders could be synthesized after 9 h of milling. The evolution of relative density and hardness with milling time is due to the morphological and microstructural changes imposed on the composite powder. High-relative densities are typical of the hot extruded samples. The effect of mechanical alloying process on hardness is more significant compared to reinforcement particles. The aging behaviors of the mechanically alloyed, commercially mixed and unreinforced Al6061 were compared. The results showed that MA composites exhibit no aging-hardenability.     

Effects of stearic acid on synthesis of nanocomposite WC–MgO powders by mechanical alloying

The effects of stearic acid as a process control agent (PCA) on the synthesis of WC–MgO by mechanical alloying have been investigated. 0–2.0 wt% of stearic acid is added into the mixture of WO₃, Mg, and graphite powders in high-energy planetary ball milling experiments, and the as-milled powders are characterized by XRD and TEM. Results show that the mechanochemical reaction among WO₃, Mg, and graphite to form WC–MgO can be changed from a mechanically induced self-propagating reaction (MSR) to a gradual reaction by the addition of stearic acid in the range from 1.2 to 1.8 wt%, when other milling parameters are maintained at the same level. It has also been found that with the addition of stearic acid, the crystallite and particle size of WC–MgO powders can be refined, the homogeneity of particle size can be improved and the powder yield can be increased.     

Fabrication and characterization of nanostructured Ti6Al4V powder from machining scraps

In recent years researches on properties of nanocrystalline materials in comparison with coarse-grained materials has attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder by means of high energy mechanical milling. In this regard, Ti6Al4V powder was produced by ball milling of machining scraps of Ti6Al4V. The structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy, and microhardness measurements. The results revealed that ball milling process reduced the size of the coherent-scattering region of Ti6Al4V to approximately 20 nm. Also a remarkable change in morphology and particle size was occurred during ball milling. Moreover, phase evolution during milling was taken into consideration. The as-milled Ti6Al4V powder exhibited higher microhardness comparing to the original samples.     

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

This article describes the microstructure and Charpy impact properties of an Fe–14Cr–2W–0.3Ti–0.3Y₂O₃ oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe–14Cr–2W–0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y₂O₃ nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 °C under a pressure of 200 MPa for 4 h and heat treatment at 850 °C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 °C) than the elemental ODS steel (about 25 °C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM–EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15°) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Microstructural studies on nanocrystalline oxide dispersion strengthened austenitic (Fe–18Cr–8Ni–2W–0.25Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) alloy synthesized by high energy ball milling and vacuum hot pressing

In the present work, nanostructured (Fe–18Cr–8Ni–2W) austenitic base and oxide dispersion strengthened (ODS) alloy powders were produced through mechanical alloying and these nano powders were consolidated by vacuum hot pressing. The results showed that initially bcc solid solution formed in both the alloys and this transformed to fcc with continued milling. The bcc solid solution formation and the subsequent transformation to fcc were significantly faster in the ODS alloys when compared to the base alloy. In the ODS alloy, a grain size of ~25 nm is achieved within 5 h of milling. Study of variation of microhardness of mechanically alloyed powder particles with grain size showed linear Hall–Petch kind of behavior. Following vacuum hot pressing of mechanically alloyed powders, nearly fully dense (>99% of theoretical density) compacts were obtained with a grain size of ~80 nm. The bulk hardness of base and ODS alloys are ~530 and ~900 HV, respectively. These are significantly higher than the values reported in the literature so far. The enhanced strength the ODS alloy is due to increased dislocation density and presence of fine dispersoids along with the nanocrystalline grains.     

Mechanical alloying of biocompatible Co–28Cr–6Mo alloy

We report on an alternative route for the synthesis of crystalline Co–28Cr–6Mo alloy, which could be used for surgical implants. Co, Cr and Mo elemental powders, mixed in an adequate weight relation according to ISO Standard 58342-4 (ISO, 1996), were used for the mechanical alloying (MA) of nano-structured Co-alloy. The process was carried out at room temperature in a shaker mixer mill using hardened steel balls and vials as milling media, with a 1:8 ball:powder weight ratio. Crystalline structure characterization of milled powders was carried out by X-ray diffraction in order to analyze the phase transformations as a function of milling time. The aim of this work was to evaluate the alloying mechanism involved in the mechanical alloying of Co–28Cr–6Mo alloy. The evolution of the phase transformations with milling time is reported for each mixture. Results showed that the resultant alloy is a Co-alpha solid solution, successfully obtained by mechanical alloying after a total of 10 h of milling time: first Cr and Mo are mechanically prealloyed for 7 h, and then Co is mixed in for 3 h. In addition, different methods of premixing were studied. The particle size of the powders is reduced with increasing milling time, reaching about 5 μm at 10 h; a longer time promotes the formation of aggregates. The morphology and crystal structure of milled powders as a function of milling time were analyzed by scanning electron microscopy and XR diffraction.     

Fabrication of Al2O3–20 vol.% Al nanocomposite powders using high energy milling and their sinterability

In this study, alumina-based matrix nanocomposite powders reinforced with Al particles were fabricated and investigated. The sinterability of the prepared nanocomposite powder at different firing temperature was also conducted. Their mechanical properties in terms of hardness and toughness were tested. Alumina and aluminum powder mixtures were milled in a planetary ball mill for various times up to 30 h in order to produce Al₂O₃–20% Al nanocomposite. The phase composition, morphological and microstructural changes during mechanical milling of the nanocomposite particles were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) techniques, respectively. The crystallite size and internal strain were evaluated by XRD patterns using Scherrer methods.A uniform distribution of the Al reinforcement in the Al₂O₃ matrix was successfully obtained after milling the powders. The results revealed that there was no any sign of phase changes during the milling. The crystal size decreased with the prolongation of milling times, while the internal strain increased. A simple model is presented to illustrate the mechanical alloying of a ductile–brittle component system. A competition between the cold welding mechanism and the fracturing mechanism were found during powder milling and finally the above two mechanisms reached an equilibrium. The maximum relative density was obtained at 1500 °C. The harness of the sintered composite was decreased while the fracture toughness was improved after addition Al into alumina.     

In situ TiC particles reinforced Ti6Al4V matrix composite with a network reinforcement architecture

TiC particles reinforced Ti6Al4V (TiCp/Ti6Al4V) composite with a network TiCp distribution has been successfully fabricated by reaction hot pressing of coarse Ti6Al4V particles and fine carbon powders. TiC particles are in situ synthesized around the boundaries of the Ti6Al4V particles, and subsequently formed into a TiCp network structure. Contrary to the typical Widmanstätten microstructure for the monolithic Ti6Al4V alloy, an equiaxed (α + β) microstructure for the Ti6Al4V matrix of the composite is formed. This is due to the isotropic tensile stress generated by the network TiCp structure and the mismatch of coefficients of thermal expansion (CTE) during the phase transformation. The prepared composite exhibits superior compressive strength before and after heat-treatment due to the reinforcement network architecture and the relatively large matrix region with an equiaxed microstructure.     

Consolidation of ultrafine-grained Cu powder and nanostructured Cu–(2.5–10) vol%Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powders by powder compact forging

An as-received ultrafine-grained Cu powder and four nanostructured Cu–(2.5–10) vol%Al₂O₃ composite powders produced by high-energy mechanical milling of mixtures of the Cu powder and an Al₂O₃ nanopowder were consolidated using warm powder compaction followed by open die powder compact forging. The circular discs produced in the experiments achieved full densification. Tensile testing of the specimens cut from the forged discs showed that the Cu-forged disc had a fairly high yield strength of 330 MPa, UTS of 340 MPa and a plastic strain to fracture of 15%, but the Cu–Al₂O₃ composite-forged discs did not show any macroscopic plastic yielding. The fracture strength of the composite-forged discs decreased almost linearly with the increase of the volume fraction of Al₂O₃ nanoparticles. This study shows that a high level of consolidation of the ultrafine-grained Cu powder and the nanostructured Cu–2.5 vol%Al₂O₃ composite powder has been achieved by warm powder compacting at 350 °C and powder compact forging at 500 and 700 °C. However, this is not true for the nanostructured Cu–(5, 7.5 and 10) vol%Al₂O₃ composite powders, possibly due to their higher powder particle hardness at elevated temperatures in the range of 350–800 °C.     

In situ boron carbide–titanium diboride composites prepared by mechanical milling and subsequent Spark Plasma Sintering

Boron carbide–titanium diboride composites were synthesized and consolidated by Spark Plasma Sintering (SPS) of mechanically milled elemental powder mixtures. The phase and microstructure evolution of the composites during sintering in the 1,200–1,700 °C temperature range was studied. With increasing sintering temperature, the phase formation of the samples was completed well before full density was achieved. The distribution of titanium diboride in the sintered samples was significantly improved with increasing milling time of the Ti–B–C powder mixtures. A bulk composite material of nearly full density, fine uniform microstructure, and increased fracture toughness was obtained by SPS at 1,700 °C. The grain size of boron carbide and titanium diboride in this material was 5–7 and 1–2 μm, respectively.