Structural and magnetic properties of nanocrystalline NiFeCuMo powders produced by wet mechanical alloying

The NiFeCuMo nanocrystalline soft magnetic powders were successfully obtained by wet mechanical alloying route in a planetary ball mill using benzene (C₆H₆) as process control agent (PCA). The milling time used was ranging from 2 up to 20 h. The synthesis conditions and alloy formation have been investigated by X-ray and neutron diffraction as well as their influence on the intrinsic physical properties. Nanometer scale (≈10 nm) crystallites were obtained. A decrease of the samples magnetization has been observed and attributed to the stresses induced during the milling and to the benzene adsorbed on the powders surface. Differential scanning calorimetry investigation shows the presence of an exothermic peak related to the presence of benzene. The adsorbed benzene, internal stresses and crystalline defects removal took place during the heat treatment at 350 °C for 4 h, leading to an improvement of the powders magnetization.     

Nanophase intermetallic FeAl obtained by sintering after mechanical alloying

The preparation of bulk nanophase materials from nanocrystalline powders has been carried out by the application of sintering at high pressure. Fe–50 at.%Al system has been prepared by mechanical alloying for different milling periods from 1 to 50 h, using vials and balls of stainless steel and a ball-to-powder weight ratio (BPR) of 8:1 in a SPEX 8000 mill. Sintering of the 5 and 50 h milled powders was performed under high uniaxial pressure at 700 °C. The characterization of powders from each interval of milling was performed by X-ray diffraction, Mössbauer spectroscopy, scanning and transmission electron microscopy. After 5 h of milling formation of a nanocrystalline α-Fe(Al) solid solution that remains stable up to 50 h occurs. The grain size decreases to 7 nm after 50 h of milling. The sintering of the milled powders resulted in a nanophase-ordered FeAl alloys with a grain size of 16 nm. Grain growth during sintering was very small due to the effect of the high pressure applied.     

机械合金化及真空热压制备纳米晶Fe3Al的组织及性能

采用机械合金化(MA)及热压烧结工艺制备纳米晶Fe3Al块体材料。采用X射线衍射、透射电镜、扫描电镜等对MA粉体及热压块体的相及显微组织进行分析,并对热压块体的力学性能及断口形貌进行了测试分析。结果表明:Fe72Al28混合粉在球磨过程中,Al逐渐溶入Fe中,形成Fe(Al)过饱和固溶体,纳米晶粉体的结构有序度较低。在1200℃,保温1h下真空热压烧结,Fe(Al)转变为有序的DO3-Fe3Al,同时发生晶粒长大。Fe3Al块体晶粒尺寸为40.1nm,相对密度大于96%,维氏硬度626.8 HV,三点弯曲强度985MPa;弯曲断口为脆性断口,但也呈现出一定韧性断裂特征。     

In situ nanocrystalline Fe–Si coating by mechanical alloying

A new approach for deposition of in situ nanocrystalline Fe–Si alloy coating on mild steel substrate by mechanical milling has been proposed. The thickness of nanocrystalline coating was a function of milling time and speed. Milling speed of 200 rpm was the optimum condition for development of uniform, hard, adherent and dense 200–300 μm thick nanocrystalline coating. A possible mechanism, consisting of three steps like repeated impact, cold welding and delamination, has been proposed for the formation of coating. These coatings have resulted in the increase of the hardness to almost double the value before coating.     

Synthesis of nanocrystalline NiAl by mechanical alloying

The nanocrystalline NiAl intermetallic compound was synthesized by mechanical alloying of the elemental powders. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometery, scanning electron microscopy and microhardness measurements. The mechanical alloying resulted in the gradual formation of nanocrystalline NiAl with a grain size of 20 nm. It was found that NiAl phase develops by continuous diffusive reaction at Ni/Al layers interfaces. The NiAl compound exhibited high microhardness value of about 1035 Hv.     

Structure and phase transformations in Fe–Ni–Mn alloys nanostructured by mechanical alloying

Ternary Fe₈₆Ni_xMn_14−x alloys, where x = 0, 2, 4, 6, 8, 10, 12, 14, 16 at.%, were prepared by the mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. X-ray diffraction analysis and Mössbauer spectroscopy were used to investigate the structure and phase composition of samples. Thermo-magnetic measurements were used to study the phase transformation temperatures. The MA results in the formation of bcc α-Fe and fcc γ-Fe based solid solutions, the hcp phase was not observed after MA. As-milled alloys were annealed with further cooling to ambient or liquid nitrogen temperatures. A significant decrease in martensitic points for the MA alloys was observed that was attributed to the nanocrystalline structure formation.     

Partial martensitic transformation of nanocrystalline NiAl intermetallic during mechanical alloying

Nanocrystalline NiAl intermetallic powders were synthesized by mechanical alloying (MA) in a planetary ball mill. Microstructural characterization was accomplished using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The nanocrystalline NiAl powders were formed by a gradual exothermic reaction mechanism during MA. Prolonged milling resulted in partial martensitic transformation of B2-NiAl to tetragonal L1₀-NiAl structure. It is believed that the martensitic transformation is induced by mechanical stress during MA.     

Fabrication of Ti-Cu-Ni-Al amorphous alloys by mechanical alloying and mechanical milling

Ti-based amorphous alloy powders were synthesized by the mechanical alloying (MA) of pure elements and the mechanical milling (MM) of intermetallic compounds. The amorphous alloy powders were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Scanning electron micrographs revealed that the vein morphology of these alloy powders shows deformation during the milling. The energy-dispersive X-ray spectral maps confirm that each constituent is uniformly dispersed, including Fe and Cr. The XRD and DSC results showed that the milling time required for amorphization for the MA of pure elements was longer than that of the MM for intermetallic compounds. The activation energy and crystallization temperature of the MA powder are different from those of the MM powder.     

Application of controlled and electrical discharge assisted mechanical alloying for the synthesis of nanocrystalline MgB2 superconducting compound

In this paper we present the results of our efforts to synthesize the nanocrystalline MgB₂ superconducting compound from elemental Mg and B powders by combination of controlled mechanical pre-alloying in a magneto-mill Uni-Ball-Mill 5 under shearing mode followed by electrical discharge (ED) assisted mechanical alloying (MA). There is no conclusive evidence of MgB₂ formation in the Mg-2B mixture using crystalline boron after controlled mechanical alloying (CMA) under protective argon or helium atmosphere as well as subsequent ED assisted alloying. There seems to be some XRD evidence of the strongest (1 0 1) MgB₂ peak presence in the Mg-2B mixture processed using both crystalline and amorphous boron after CMA under hydrogen as well as subsequent ED assisted alloying but this evidence is rather ambiguous. We postulate here that it is highly likely that a certain critical Mg nanograin size must be achieved before a successful reaction to form nanocrystalline MgB₂ is going to be completed. Following recent report by Gümbel et al. [Appl. Phys. Lett. 80 (2002) 2725] this critical value can be roughly estimated at 15 nm or less. Calculations of the Mg nanograin size in the present work show that only three Mg-2B powders ball milled under hydrogen meet this critical nanograin size criterion for the Mg phase. However, a massive formation of the β-MgH₂ hydride in these powders consumes the available Mg in the reaction with hydrogen which may leave inadequate concentration of Mg to form MgB₂ even though the nanograin size of Mg is sufficiently refined, say below 15 nm.     

Preparation of W–Al–Mo ternary alloys by mechanical alloying

Single phase W_XAl₅₀Mo_50−X (X = 40, 30, 20 and 10) powders have been synthesized directly by mechanical alloying (MA). The structural evolutions during MA and subsequent as-milled powders by annealing at 1400 °C have been analyzed using X-ray diffraction (XRD). Different from the Mo₅₀Al₅₀ alloy, W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys were stable at 1400 °C under vacuum. The results of high-pressure sintering indicated that the microhardnesses of two compositions, namely W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys have higher values compared with W₅₀Al₅₀ alloy.     

Correlation between microstructure and mechanical properties of severely deformed metals

There is a correlation between the microstructure and the mechanical behavior of ultrafine-grained face centered cubic (fcc) metals processed by equal-channel angular pressing (ECAP). It is shown that the saturation yield strength is related to the maximum dislocation density according to the Taylor equation and, in addition, the value of the parameter α in the Taylor equation is strongly affected by the stacking fault energy because of different geometrical arrangements of dislocations within the grains. It is also demonstrated that the ductility of Cu processed by ECAP decreases with increasing strain but at extremely high strains the ductility is partially restored due to the recovery of the microstructure.     

Al-V合金预磨状态对Al-Fe-V-Si-Nd机械合金化过程中显微组织演化的影响

采用高能球磨法制备Al89.5Fe6 .4V0 .7Si2 .4Nd合金粉末 ,并用X射线衍射技术研究了球磨过程中的组成。发现经 60h高能球磨 ,合金粉末的微观组织由Al非晶和Al3V相组成 ;Al V合金的预磨状态影响Al89.5Fe6 .4 V0 .7Si2 .4Nd合金机械合金化过程中显微组织演化。     

On the general outline of physical properties of amorphous-nanocrystalline Fe-Cr-Mn-N alloy powders prepared by mechanical alloying under nitrogen

This paper reviews last findings about physical properties of Fe-Cr-Mn-N powders synthesized by mechanical alloying under nitrogen. Their thermal, magnetic, indentation, and grain growth behaviors and nitrogen distribution in their amorphous-nanocrystalline structure are regarded as a function of milling time. Particularly, the role of nitrogen in the aforementioned phenomena is reviewed in detail.     

添加W对机械合金化TiAl合金显微组织和性能的影响

采用机械合金化结合粉末冶金技术制备了Ti-44．7A1-xW（at％）合金材料。采用透射电镜、扫描电镜和金相显微镜研究不同W添加量对机械合金化TiAl基合金的显微组织和高温抗氧化性能的影响，并对合金的力学性能进行测试。研究表明，通过机械合金化在TiAl基合金系统中添加微量W元素会形成新的固溶体相，这种新成分相大大提高TiAl基合金的抗弯强度σb当W添加量为1．0at％时，σb达到峰值；随后随着W含量的增加，抗弯强度降低。W元素的添加有效的制约了合金基体的内部氧化，使TiAl合金的高温抗氧化性能明显提高。     

Phase evolution of Mg–Al–Zr nanophase alloys prepared by mechanical alloying

Al–Mg and Al–Mg–Zr alloys were processed by mechanical alloying. The phase constitution of the powders was strongly dependent on the composition of the starting mixture. In as-milled powders, an Al(Mg) solid solution was formed with up to 40 at% Al, which after annealing transformed to the equilibrium β-Al₃Mg₂ phase. For high Mg concentrations (60–90 at%) the dominant phase was γ-Al₁₂Mg₁₇ in accordance with the equilibrium phase diagram. The addition of Zr led to the appearance of Zr–Al intermetallics causing Mg to precipitate out of the Al(Mg) solution. The effect of zirconium was also to refine the structure and to retard grain growth.     

X-ray diffraction study on Ni---Al---Se amorphization by mechanical alloying

In this paper, the amorphization process in mechanical alloyed Ni---Al---Se powders has been investigated by X-ray diffraction. The influence of Se on the amorphization of Ni---Al alloys, and the influence of milling time on the powder structure, microcrystallites size and phases lattice distortion are presented. Also, the Fe and Cr content from the milling medium, after 400 h, was determined.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.     

Nanocrystalline Al3Ni2 alloy with high hardness produced by mechanical alloying and high-pressure hot-pressing consolidation

An elemental powder mixture corresponding to Al₃Ni₂ phase stoichiometry was subjected to mechanical alloying. A metastable nanocrystalline AlNi intermetallic phase with the mean crystallite size of 12 nm was formed upon milling. Heating of the synthesised powder in a calorimeter up to 720 °C caused phase transformation into an equilibrium Al₃Ni₂ intermetallic phase with the mean crystallite size of 41 nm. The product of mechanical alloying was consolidated at 1000 °C under the pressure of 5 GPa and 7.7 GPa. During consolidation, a phase transformation analogous with the one observed in the course of heating in the calorimeter took place. Both bulk materials have nanocrystalline structure with mean crystallite size of 67 nm and 58 nm, the smaller one in the sample consolidated under the higher pressure. The hardness of the produced Al₃Ni₂ intermetallic is 8.81 GPa (898 HV1) and 8.72 GPa (887 HV1), while the specific yield strength, estimated using the Tabor relation, is 624 kNm/kg and 617 kNm/kg for the sample hot-pressed under 5 GPa and 7.7 GPa respectively. On the basis of the obtained results, we can assume that the quality of consolidation with preserving a nanocrystalline structure is satisfactory and the hardness as well as the estimated specific yield strength of the produced materials are relatively high.     

Nanocrystalline τ2 phase obtained by mechanical alloying of Al60Fe15Si15Ti10 powder mixture followed by consolidation

In the present work an elemental powder mixture of Al60Fe15Si15Ti10 (at.%) was mechanically alloyed in a high-energy ball mill. A part of the milling product was examined in a calorimeter, while another portion was subjected to consolidation by hot-pressing at 1000 °C for 180 s under a pressure of 7.7 GPa. The results obtained show that a nanocrystalline cubic phase with the lattice parameter a₀ = 11.645 Å, isomorphous with the τ₂ (Al₂FeTi) phase, is formed during mechanical alloying process. Heating of the milling product in the calorimeter up to 720 °C causes limited growth of grains, however the τ₂ phase remains nanocrystalline with the mean crystallite size of 28 nm. Grain growth takes place during consolidation of the milling product as well, although the τ₂ phase remains nanocrystalline with the mean crystallite size of 34 nm. The microhardness of the bulk nanocrystalline sample is 1013 HV0.2 and its open porosity is 0.3%. The results obtained show that the quality of compaction with preserving nanometric grain size of the τ₂ phase is satisfactory and its microhardness is relatively high.