Room Temperature Milling Research Articles

Effect of cryogenic milling on the mechanical and corrosion properties of ODS Hastelloy-N

This study entails the fabrication of two oxide-dispersion strengthened (ODS) Hastelloy-N (HN) alloys utilizing divergent methods. The first alloy was synthesized using cryogenic attritor milling coupled with spark plasma sintering (SPS), while the second was produced via room temperature attritor milling and SPS. The ODS HN alloy derived from cryogenic milling demonstrated superior strength relative to its commercial-grade counterpart. Conversely, the alloy produced through room temperature milling exhibited lower ultimate tensile strength (UTS), attributed to manufacturing defects and the precipitation of Zr at grain boundaries. Corrosion resistance in molten FLiNaK for both ODS samples was found to be inferior compared to commercial HN. Particularly, in the room temperature-milled ODS HN, Zr present at grain boundaries appeared to dissolve more readily than in cryogenic or commercial samples, facilitating enhanced penetration by molten salt. The cryogenically-milled ODS HN contained Zr, yet it was not segregated to grain boundaries. Although the homogeneously dispersed Mo-based compound in the cryogenically-milled ODS HN augmented mechanical properties, it also accelerated corrosion propagation beyond that of the commercial-grade alloy.

The Effect of the Milling Vial Shape on the In-Situ Consolidation of a Nanocrystalline Al-Li-GNPs Nanocomposite Synthesized by Room Temperature Ball-Milling

Several studies investigating the ball-milling of ductile face-centered cubic metals have reported a so-called in-situ consolidation phenomenon where the milled powder is also consolidated during the milling process. Thus, instead of refined powders or agglomerated particles, the formation of spherical bulk particles of the milled material is reported using a combination of cryomilling and room temperature milling processes. In this study, we studied the effect of the milling vial shape on the in-situ consolidation of a graphene nanoplatelets (GNPs) reinforced aluminum-lithium (Al-Li) matrix nanocomposite for the first time. An in-situ consolidated nanometric Al-Li-GNPs nanocomposite with an average grain size of 48 nm and high hardness of 1.48 GPa was attained after only 8 h of room-temperature milling. The results presented suggest that dense nanostructured composites can be prepared by in-situ consolidation during a one-step milling process and subsequently investigated in order to analyze their mechanical behavior. This allows for the intrinsic mechanical behavior of the synthesized material to be examined without the interference of subsequent high-temperature consolidation processes, thus avoiding unwanted structural changes such as grain growth and second phase formations.

Artifact-free bulk nanocrystalline Al-Li alloys with multiple deformation mechanisms and improved tensile properties

An artifact-free bulk nanocrystalline (NC) aluminum (Al)-2% lithium (Li) alloy was synthesized in this study using in-situ consolidation via a combination of cryogenic and room temperature milling. The mechanical behavior of this alloy was investigated by tensile testing and microhardness measurements, and it was compared with coarse-grained (CG) Al alloys and a commercially pure NC Al synthesized using the same method. The transmission electron microscopy (TEM) analysis revealed that the grain size of the NC Al-2% Li alloy and NC pure Al are 18 nm and 29 nm, respectively. The NC Al-Li alloy showed extremely high yield and ultimate tensile strength values of 440 MPa and 556 MPa, respectively. In addition, a high tensile ductility of 14% was achieved in the NC Al-Li alloy along with a relatively high strain hardening exponent (0.11). The high-resolution TEM investigations indicate a dependency of these extraordinarily tensile properties on multiple deformation mechanisms such as dislocation slip and pile-up and deformation twinning.

A study of microstructural evolution of Fe-18Cr-8Ni, Fe-17Cr-12Ni, and Fe-20Cr-25Ni stainless steels after mechanical alloying and annealing

In this study, high-energy mechanical alloying technique was used to produce nanocrystalline stainless steels of three different compositions from elemental powders. The microstructural evolution (grain growth and phase transformation) as a function of alloy compositions and annealing temperatures were investigated by room and high temperature x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The results revealed that stainless steels with low nickel content (i.e., Fe-18Cr-8Ni) underwent a deformation-induced martensitic transformation during room temperature mechanical alloying. Deformation-induced martensitic transformation with increasing nickel content (i.e., Fe-17Cr-12Ni and Fe-20Cr-25Ni) would not be possible by room temperature milling but was created by high strain rate cryogenic processing, the degree to which was compositional dependent. Post process annealing induced the reverse transformation from martensite-to-austenite the ratio of which was found to be a factor of alloy composition and annealing temperature. The real time in-situ x-ray studies showed that the martensite-to-austenite reverse transformation was completed at around 600 °C and 800 °C for Fe-18Cr-8Ni and Fe-20Cr-25Ni steels, respectively. Microscopy studies revealed a significant enhancement in the resistance to grain growth for Fe-17Cr-12Ni steel over other compositions at elevated temperatures as high as 1200 °C. As such, cryogenic processing following by reverse martensitic transformation of high Ni containing alloys provides a pathway for developing higher heat resistant stainless steel alloys.

Effect of milling temperature on structure and reactivity of Al–Ni composites

Reactive Al–Ni composites are of interest for combustion synthesis, for joining, and as components of energetic formulations. Mechanical milling is one of the most practical techniques used for their preparation. This study investigated the effect of ball milling at cryogenic temperatures on the reactivity and structure of Al–Ni composites. Stoichiometric mixtures of Al and Ni powders were milled both at room temperature and cooled by liquid nitrogen. Products were characterized using scanning electron microscopy, differential scanning calorimetry, and x-ray diffraction. Aluminum and nickel were mixed on a finer scale in cryogenically milled powders compared to those prepared at room temperature. DSC traces of the cryogenically milled powder showed three exothermic peaks, while only two clearly resolved peaks were observed for the powders milled at room temperature. A low-temperature reaction preceding the first peak was detected only for the cryomilled material. Apparent activation energies, obtained by model-free analysis, were consistent with previous work on nanofoils. Ignition temperatures of powders prepared at room temperature decreased with increasing structural refinement. This effect was even stronger for the cryogenically milled powder. Energetic characteristics of the prepared materials were correlated with their respective structural refinement. These correlations suggest that the cryomilled material has a narrower zone separating Al and Ni in which the elements are mixed together compared to the similar material prepared by room temperature milling.

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y2O3 composites. Powder blends having the composition of W-1wt% Ni-2wt% ZrC-1wt% Y2O3 were milled at room temperature for 12h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400MPa and green bodies were sintered at 1400°C for 1h under Ar/H2 atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23GPa) and higher wear volume loss value (149.42µm3) than that of the sintered sample after room temperature milling (6.66±0.39GPa; 102.50µm3). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59GPa and the lowest wear volume loss value of 66.0µm3.

Effects of milling time on the development of porosity in Cu by the reduction of CuO

Microscale and nanoscale CuO was dispersed in Cu using room temperature high-energy ball milling over time intervals of 5, 30, 60, 120, and 240 min. These samples were then annealed under a reducing atmosphere for 1 h at temperatures of 400, 600, 800 and 1000 °C to create porosity by the reduction of the entrained oxides. Increases in porosity exceeding 40% were achieved using intermediate milling times and annealing temperatures. When considered cumulatively, the most effective processing conditions were a milling time of 30 min and expansion at 800 °C, but variations exist within each sample type. The complex relationship between milling time and annealing temperature is investigated in terms of particle size, morphology and microstructure. The findings indicate that room temperature milling is more efficient at producing porosity than comparable cryogenic methods, and this may enable industrial scaling of the process.

The Diametrically Loaded Cylinder for the Study of Nanostructured Aluminum–Graphene and Aluminum–Alumina Nanocomposites Using Digital Image Correlation

Non-contact methods for characterization of metal matrix composites have the potential to accelerate the development and study of advanced composite materials. In this study, diametrical compression of small disk specimens was used to understand the mechanical properties of metal matrix micro and nano composites. Analysis was performed using an inverse method that couples digital image correlation and the analytical closed form formulation. This technique was capable of extracting the tension and compression modulus values in the metal matrix nanocomposite disk specimens. Specimens of aluminum and aluminum reinforced with either Al2O3 nanoparticles or graphene nanoplatelets (GNP) were synthesized using a powder metallurgy approach that involved room temperature milling in ethanol, and low temperature drying followed by single action compaction. The elastic and failure properties of MMNC materials prepared using the procedure above are presented.

Strengthening mechanisms of graphene- and Al2O3-reinforced aluminum nanocomposites synthesized by room temperature milling

Powder metallurgy (PM) is a widely used processing method to synthesize ultrafine and nanometric grain-sized alloys and composites. While most previous work on synthesizing nanocrystalline (NC) metal-matrix nanocomposites (MMNCs) has used cryomilling technique with liquid nitrogen and stearic acid followed by high temperature/high vacuum drying, we employed an advanced room temperature method that involves milling in ethanol followed by low temperature atmospheric drying – a much less expensive process. To understand the strengthening mechanisms of MMNCs, pure NC Al was reinforced with varying concentrations of Al2O3 nanoparticles (NPs) and graphene nanoplatelets (GNPs). The results show that i) room temperature milling in ethanol followed by a relatively low temperature drying treatment can produce NC Al and NC Al MMNCs with grain sizes comparable to materials produced by cryomilling, ii) grain boundary strengthening as described by the Hall–Petch relation accounts for the strength of Al‐Al2O3 MMNCs, and iii) grain boundary strengthening and solute strengthening seem to account for the strength in Al‐GNP MMNCs.

B13-P-07Evaluation of cross-sections of low melting point metals prepared by an FIB equipped with a cooling stage

A sample preparation technique, that is a front end of electron microscopy, has become highly important, and thus a choice of it affects a result of the evaluation. Sample preparation of low melting point metals is very difficult because they are soft and heat sensitive materials. The electron microscopic evaluation of them has been always discussed. In this study, a cooling stage was attached to an FIB-SEM for applying to the low melting point metals. Figure 1 shows comparison of cross-sections of lead solder prepared by ion milling at room temperature and low temperature [ 1 , 2 ]. By the room temperature milling, many cracks (arrows) are induced at grain boundaries. The low temperature milling gives a smooth surface without any cracks which produces a clear channeling contrast. Ion milling at a low temperature was confirmed to be very useful for sample preparation to achieve a high quality cross-section of low melting point metals.

Mechanism of nanostructure formation in ball-milled Cu and Cu–3wt%Zn studied by X-ray diffraction line profile analysis

The mechanism of nanostructure formation during cryogenic and room-temperature milling of Cu and Cu–3wt%Zn was investigated using X-ray diffraction line profile analysis. For that, the whole powder pattern modeling approach (WPPM) was used to analyze the evolution of microstructural features including coherently scattering domain size, dislocation density, and density of planar faults. It was found that for all sets of experiments, structural decomposition is the dominant mechanism of nanostructure formation during cryomilling. During subsequent RT-milling, grain refinement still occurs by structural decomposition for pure copper. On the other hand, discontinuous dynamic recrystallization is responsible for nanostructure formation during RT-milling of Cu–3wt%Zn. This is attributed to lower stacking-fault energy of Cu–3wt%Zn compared to pure copper. Finally, room temperature milling reveals the occurrence of a detwinning phenomenon.

Cryomilling effect on the mechanical alloying behaviour of ferritic oxide dispersion strengthened powder with Y2O3

Abstract Cryogenic cooling effect on mechanical alloying of the mixture of Fe–14Cr–3W–0.1Ti and Y 2 O 3 powders was investigated. The powder mixtures were ball milled for 40 h at room-temperature and −150 °C. Cryomilling produced much finer particle/grain size than conventional room-temperature milling. XRD diffraction peak intensity was much lower under cryomilling conditions due to formation of nano-size grains and increased residual microstrain. Absorption amounts of interstitial elements were considerably higher under cryomilling conditions.

Structure and property of tetrafluoroethylene‐propylene elastomer‐OVPOSS composites

Tetrafluoroethylene‐propylene elastomer‐octavinyl‐polyhedral oligomeric silsesquioxane (TFE/P‐OVPOSS) composites containing various percentages of OVPOSS are prepared via room temperature milling and heat vulcanization at 170°C. The composites are characterized by FTIR, 13C‐NMR, 29Si‐NMR, XRD, DSC, SEM, tensile test, DMA, and TGA. The crosslink bond formation between TFE/P and OVPOSS conforms to the structural characterization of the composites. The OVPOSS cages aggregate when the OVPOSS dosage is more than 6 as observed from the XRD curves and SEM images. The swelling test shows that the experimental crosslink density is lower than the calculated crosslink density, indicating that not all reacted vinyl groups are converted into crosslink bonds. Meanwhile, the incorporation of OVPOSS significantly enhances the mechanical property and elevates the glass temperature of the composites. However, the thermal property is only improved slightly. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1281‐1288, 2013

Synthesis and Characterization of NanocrystallineMg-7.4%Al Powders Produced by Mechanical Alloying

Nanocrystalline Mg-7.4%Al powder was prepared by mechanical alloying using a high-energy mill. The evolution of the various phases and their microstructure, including size and morphology of the powder particles in the course of milling and during subsequent annealing, were investigated in detail. Room temperature milling leads to a rather heterogeneous microstructure consisting of two distinct regions: Al-free Mg cores and Mg-Al intermixed areas. As a result, the material is mechanically heterogeneous with the Mg cores displaying low hardness (40–50 HV) and the Mg-Al intermixed regions showing high hardness of about 170 HV. The Mg cores disappear and the microstructure becomes (also mechanically) homogeneous after subsequent cryo-milling. Rietveld structure refinement reveals that the crystallite size of the milled powders decreases with increasing the milling time reaching a minimum value of about 30 nm. This is corroborated by transmission electron microscopy confirming an average grain size of ~25 nm.

Preparation of Freestanding Zn Nanocrystallites by Combined Milling at Cryogenic and Room Temperatures

The present investigation reports the preparation of freestanding nanocrystalline Zn by combined mechanical milling at cryogenic and room temperatures. The cryomilling is used as an effective means of rapid fracturing. The detailed scanning electron microscopy and transmission electron microscopy observations indicate that the minimum crystallite size is 6 ± 2 nm after 3 hours of cryomilling. The crystallite size increases to 30 ± 2 nm after 3 hours of room temperature milling of the cryomilled powder due to deformation-induced sintering. Detailed theoretical analysis allows us to obtain a diagram of size of the nanoparticles formed vs temperature to explain the experimental findings.

Study on Bi—Fe3O4 nanocomposite prepared via mechanochemical processing

In this work Bi-Fe3O4 nanocomposite was synthesized by room temperature milling of Bi2O3 and Fe powders using a planetary ball mill in air. The synthesis reaction proceeds with increase in milling time and is finished by about 4 h. The XRD pattern of the as-milled powder shows that the main phases are Bi and Fe3O4 without any extra phases. The average crystallite sizes of the constituents have been determined by Scherrer’s formula and they were 22 and 18 nm for Bi and Fe3O4 respectively. This was also confirmed by Transmission Electron Microscopy (TEM). Magnetic hysteresis loops at room temperature were recorded using a vibrating sample magnetometer (VSM). A tow-probe method was used to measure resistivity variation of the nanocomposite as a function of magnetic filed and temperature. We have observed a room temperature magnetoresistance (ρ0 — ρH)/ρ0 as large as 17% in a magnetic field of 1 T.

Synthesis, characterization and mechanical behaviour of an in situ consolidated nanocrystalline FeCrNi alloy

IntroductionCurrently, nanocrystalline materials are being widelyinvestigated due to their unique properties. This interest hasspawned the development of various techniques to synthe-size materials exhibiting grain sizes (\20 nm) wherephysical properties are the most exotic. Such processesinclude: high-energy ball milling [1, 2], pulsed electrode-position [3], chemical vapour condensation [4], inert gascondensation [5] etc. However, nanocrystalline samplesproduced from these techniques are either in the formof powders or thin films. Successful consolidation to arte-fact free, fully dense material is required if structuralapplications are ever to be adopted. Many methods toconsolidate these materials (i.e., hot compaction [6],explosive consolidation [7], hot isostatic pressing [8], pre-annealing–compaction–sintering [9] etc.) have been repor-ted in the literature with partial success. In general, thesuccess of these techniques for consolidation hinges on theapplication of high temperatures and pressures, which leadto explosive grain growth in nanocrystalline materials.In situ consolidation technique as developed by Youssefet al. [10] seems promising for the synthesis of consolidatednanocrystalline materials as the step of sintering to bulkdensity is no longer a need and can be eliminated. However,this technique has not been reported widely in the literatureand has been limited to nanocrystalline Zn, Al, Al alloys,copper and copper-based alloys [10, 11]. This study showsthat in situ consolidation of a FeCrNi alloy is possible usingcombination of room temperature (RT) milling and liquidnitrogen temperature milling.Nanocrystalline materials produced by high-energy ballmilling have not been well characterized and the micro-structure of these materials is not well understood. Theprimary challenge in characterization is TEM samplepreparation from ball-milled powders. Nanocrystallinepowders as produced by ball milling are too coarse to beexamined under TEM and the sample preparation for TEManalysis on such powders is not a trivial task. In general,two methods are used for the preparation of conventionalmetallic samples for TEM analysis: electro jet polishingand dimpling followed by ion milling. Nanocrystallinematerials produced by consolidation of ball-milled pow-ders can be problematic for these preparation techniques asthe consolidated samples must first be mechanically thin-ned to \100 lm before being chemically thinned to thefinal thickness. If the material exhibits poor inter-particlebonding, the sample may not withstand the final thinning

Preparation of ultrafine CsCl crystallites by combined cryogenic and room temperature ball milling

The present investigation reports the preparation and microstructural characterization of ultrafine CsCl crystallites using combined cryogenic and room temperature (RT) mechanical milling. The milling has been performed in evacuated WC vials under high purity argon atmosphere. The low temperature milling has been utilized as an effective means of rapid fracturing of the CsCl crystallites. This was followed by RT milling for different time durations. The final crystallite size obtained is 10 ± 6 nm for sample cryo-milled for 11 h and subsequently RT milled for 35 h. The experimental findings indicate the strong effect of duration of cryo-milling on the final size of the crystallites. The prolonged room temperature milling leads to increase of the crystallite size due to deformation-induced sintering. The results have been discussed in the light of currently available literature.

Effect of stacking fault energy on mechanical behavior of bulk nanocrystalline Cu and Cu alloys

Twinning and dislocation slip are two major and competing modes of plastic deformation in metals and alloys. In addition to controlling the dislocation substructure in coarse grained materials, stacking fault energy (SFE) also affects the propensity to form deformation twins. However, the influence of SFE has not been fully explored in nanocrystalline materials. Here the role of SFE in deformation twinning and work hardening was systematically studied in bulk artifact-free, nanocrystalline (nc) Cu (SFE 55 mJ m −2), and a nc Cu–12.1 at.% Al–4.1 at.% Zn alloy (SFE 7 mJ m −2). The nc Cu (23 nm) and nc Cu alloy (22 nm) were synthesized using in situ consolidation during cryo and room temperature milling. Both materials showed ultra-high tensile strength, significant strain hardening, and good ductility. The nc Cu alloy exhibits a higher yield strength and lower uniform elongation (1067 ± 20 MPa, 6.5%) than that of nc Cu (790 ± 12 MPa, 14%). The SFE variation played a significant role in strengthening the nc Cu alloy. High resolution transmission electron microscopy analyses revealed that the low SFE of the nc Cu alloy alters the deformation mechanism from a dislocation-controlled deformation, which allows for the higher strain hardening observed in the nc Cu, to a twin-controlled deformation.

Amorphous trehalose dihydrate by cryogenic milling

Trehalose dihydrate is a safe, naturally occurring disaccharide used as a food ingredient and pharmaceutical excipient. It has been reported that room temperature milling does not lead to the formation of amorphous trehalose dihydrate. This paper reports the behaviour of trehalose dihydrate upon milling at cryogenic temperatures as studied by DSC, TGA, XRPD and Raman spectroscopy. We have demonstrated that the crystal to glass transformation for trehalose dihydrate is possible using cryogenic milling. This is the first reported example of cryogenic milling (a mild and widely applicable technique) applied to generating amorphous hydrates.