Production of a nanostructured copper by Spark Plasma Sintering (original) (raw)
Synthesis and mechanical behavior of nanostructured materials via cryomilling
Progress in Materials Science, 2006
Cryomilling, the mechanical attrition of powders within a cryogenic medium, is a method of strengthening materials through grain size refinement and the dispersion of fine, nanometerscale particles. The technique was developed as a means to decrease both the size of these particles and their spacing within a metallic matrix to increase threshold creep stress and intermediate temperature performance. More recent work has been concerned with increasing the strength of lightweight structural materials. In this overview paper, the available literature is reviewed that covers the microstructural evolution during cryomilling, consolidation and processing, the thermal stability of the microstructure, and mechanical properties of consolidated materials. The properties of cryomilled materials are compared to those results for powders and consolidated materials generated by mechanical alloying, milling at ambient temperatures and other means to produce fine grained materials. Cryo Al-7.5Mg [40] Cryo Inconel 625 [39] Al [42] Al [29] AlRu [41] Fe [42] Ni [29]
Processing and behavior of nanostructured metallic alloys and composites by cryomilling
Journal of Materials Science, 2007
Recent interest in nanostructured materials stems, not only from their potential use in a variety of applications, but also from the reported discovery of novel fundamental phenomena. The consolidation of cryomilled powder provides a potential pathway towards large scale manufacturing of nanostructured metallic materials. This approach typically engenders the mechanical attrition of powders in liquid nitrogen, followed by consolidation, using established commercial techniques, such as isostatic pressing and extrusion. In this overview paper, published data are reviewed and discussed with particular emphasis on the following topics: nanostructure evolution mechanisms; primary consolidation and secondary processing methods; thermal stability of cryomilled materials; and mechanical behavior of consolidated materials. Recent mechanical behavior data and the associated mechanisms of cryomilled Al alloys are discussed in an effort to shed light into the fundamental behavior of ultrafine grained and nanostructured materials.
Cryomilled nanostructured materials: Processing and properties
Materials Science and Engineering: A, 2008
Nanostructured (i.e., 1-200 nm grain size) and ultrafine-grained (i.e., 200-500 nm grain size) metals are of interest, not only as a result of their unusual combinations of physical and mechanical properties, but also because they can be readily synthesized using well-developed synthesis techniques. Cryomilling, i.e., mechanical alloying in liquid nitrogen, is representative of a class of synthesis techniques that attain the nanostructured state via severe plastic deformation. In this overview, published data related to cryomilled materials are reviewed and discussed with particular emphasis on cryomilling mechanisms; microstructure and thermal stability of cryomilled powders; primary consolidation and secondary processing methods; microstructural evolution during consolidation; and mechanical response of consolidated materials. The deformation behavior and the underlying mechanisms that govern cryomilled materials are discussed and compared with those of nanostructured materials processed via other methods, in an effort to shed light into the fundamental behavior of ultrafine-grained and nanostructured materials. Published by Elsevier B.V.
Journal of Materials Science, 2011
It has been found difficult to fully densify some mechanically milled pure metal powders by spark plasma sintering (SPS). In this study, the densification behavior of cryomilled, nanostructured (NS) Cu powders during SPS was related to changes to the chemistry of the powders. The results showed that the presence of very small amounts of O and N in the powders, which were introduced during cryomilling and handling, significantly influenced the densification response. Moreover, reduction/removal of O/N via thermal annealing of the powders before SPS led to complete densification of the powders during subsequent SPS. The mechanisms responsible for this behavior were ascertained: O and N existed in the cryomilled powders in the form of thermally unstable compounds, and the subsequent thermal decomposition of these compounds during SPS generated the gaseous species, leading to porosity formation and incomplete densification; annealing of the powders before SPS removed the gases which resulted from thermal decomposition, thereby facilitating complete consolidation during subsequent SPS.
Nanometric grain formation in ductile powders by low-energy ball milling
Nanostructured Materials, 1999
Based on microstructural observations by TEM and in particle size distribution done by sedimentationphotometry, a new grain size refinement mechanism for ductile powders in mechanical alloying is proposed. A 90-95% of the particle population was of submicrometric fragmented particles. These were detected from the beginning of the milling process up to 90 ks. It seems that the fragmentation of the original particles occurred under dynamic conditions to generate those submicrometric ones. Under these conditions the original grain size (100 nm to 350 nm) was preserved and a low level of dislocations was observed at these submicrometric particles. Once these submicrometric particles were deformed, grains smaller than 20 nm were observed. It seems from TEM results that the submicrometric fragmented particles were also deformed under dynamic conditions. This could be a new grain size refinement mechanism present in ductile metallic powder systems where the fragmentation is the dominant stage from the beginning of the milling up to some intermediate milling time. In the Cu-20at%Ni, Cu and Ni systems where the particle coalescence process was the dominant stage during all the milling process, a derivation of the mechanism proposed by Hellstern [3] was identified. In our case, powders were mainly deformed by slip and not by shear. It recognizes that the way to refine the grain size in milled powders is influenced at least by the metallic system used as well as by the equipment and the process conditions employed.
In this study, nanostructured Al 5083 powders, which were prepared via cryomilling, were consolidated using spark plasma sintering (SPS). The influence of processing conditions, e.g., the loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties were studied. Additionally, the mechanisms that govern densification during SPS were discussed also. The results reported herein suggest that the morphology and microstructure of the cryomilled powder resulted in an enhanced densification rate compared with that of atomized powder. The pressure-loading mode had a significant effect on the mechanical properties of the samples consolidated by SPS. The consolidated compact revealed differences in mechanical response when tested along the SPS loading axis and radial directions. Higher sintering pressures improved both the strength and ductility of the samples. The influence of grain size on diffusion was considered on the basis of available diffusion equations, and the results show that densification was attributed primarily to a plastic flow mechanism during the loading pressure period. Once the final pressure was applied, power law creep became the dominant densification mechanism. Higher sintering temperature improved the ductility of the consolidated compact at the expense of strength, whereas samples sintered at lower temperature exhibited brittle behavior. Finally, densification rate was found to be inversely proportional to the particle size.
Localized Defects in Cold Die-Compacted Metal Powders
Journal of Manufacturing and Materials Processing
In powder metallurgy (PM), the compaction step is fundamental to determining the final properties of the sintered components. The deformation and defectiveness introduced in the powder material during uniaxial die compaction can be correlated to the activation and enhancement of the dislocation pipe diffusion, a lattice diffusion mechanism during the sintering process. Its coefficient depends on the dislocation density. The powder particles are mostly deformed along the direction of the compaction (longitudinal direction) rather than along the compaction plane; consequently, the contact areas perpendicular to the direction of the compaction present a higher density of dislocations and lattice defects. This high density intensifies the shrinkage along the direction of compaction. To demonstrate the influence of uniaxial cold compaction on the material’s stress state the powder particles and their contacts were modeled using spheres made of pure copper. These spheres are compacted in ...
Influence of Cryomilling on Crystallite Size of Aluminum Powder and Spark Plasma Sintered Component
Nanomaterials, 2022
The present investigation aims to develop nanocrystalline (NC) pure aluminum powders using cryomilling technique and manufacture bulk components using spark plasma sintering (SPS). The cryomilling was performed on pure Al powders for 2, 6, and 8 h. The cryomilled powders were then consolidated using SPS to produce bulk components. The particle morphology and crystallite size of the powders and the bulk SPS components were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the crystallite size of pure Al powders decreases with increased cryomilling time. The results also showed that the SPS at elevated temperatures resulted in a slight increase in crystallite size, however, the changes were insignificant. The mechanical properties of the bulk components were determined using a Vickers microhardness tester. The hardness of the cryomilled SPS component was determined to be three times higher th...
Cu-Ti alloy, processed by multiaxial forging (MAF) at cryogenic temperature with a cumulative strain up to 1.64, was investigated for microstructure and mechanical properties. The deformed microstructures were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The average grain size of 2 μm was achieved in the deformed sample after 3 cycles of MAF. TEM studies indicated that the shear bands width of the deformed sample after 3 cycles reduced to 1 μm. Tests for mechanical properties indicated an increase in tensile strength and hardness and it was found to be correlated with an increase in dislocation density and grain boundary strengthening mechanism. Ultimate tensile strength (UTS) of 390 MPa, 480 MPa, and 590 MPa was observed in MAF processed samples after 1, 2, and 3 cycles, respectively. Hardness increased from 65 Hv (as-received) to 240 Hv after 3 cycles of MAF. Fractography analysis showed that, with an increase in number of MAF cycles, dimple size reduced up to 1 cycle and percentage elongation increased after 2 cycles of MAF.
Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part I: Microstructure Evolution
Metallurgical and Materials Transactions A, 2012
Aluminum alloys are widely used because they are lightweight and exhibit high strength. In recent years, spark plasma sintering (SPS) technology has emerged as a viable approach to sinter materials due to its application of rapid heating and high pressure. In this study, SPS was chosen to consolidate dense ultrafine-grained (UFG) bulk samples using cryomilled nanostructured Al 5083 alloy (Al-4.5Mg-0.57Mn-0.25Fe, wt pct) powder. Both bimodal microstructure and banded structure were observed through transmission electron microscopy (TEM) investigation. The evolution of such microstructures can be attributed to the starting powder and the process conditions, which are associated with the thermal, electrical, and pressure fields present during SPS. A finite element method (FEM) was also applied to investigate distributions in temperature, current, and stress between metallic powder particles. The FEM results reveal that the localized heating, deformation, and thermal activation occurring at interparticle regions are associated with the formation of the special microstructure.
Acta Materialia, 2013
A bulk nanostructured alloy with the nominal composition Cu-30Zn-0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys ($200-400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)-ON , Al-ON and Al-N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.
Journal of Alloys and Compounds, 2007
The microstructure evolution of Cu-nanostructured powders versus the ball milling conditions was investigated by whole peak profile powder pattern modeling method. This method allows defining in some approach the characteristics of as-milled Cu powder microstructure in terms of crystallite size, type and density of dislocations and twin faults density. It is shown that the change of microstructure characteristics of as-milled Cu powder versus the ball milling conditions (under constant time of the ball milling) depend on only some energy parameters of the milling, for example, average size of crystallite is uniquely defined by energy of the shock, whereas the portion of edge and screw components of dislocation structures depend on a ratio between normal and tangential components of shock.
Powder Metallurgy, 2017
Water atomised copper powders (AT-Cu) have been processed by continuous and interrupted mechanical milling (MM) for different milling times. For continuous cycle the powders are subjected first to a severe flattening process and then to an intense welding phenomenon. In the case of interrupted cycle MM behaviour proceeds with an intense fracturing process. By quantitative X-ray-diffraction analysis the interrupted cycle shows constantly a delay of the microstructure evolution with all the phenomena shifted at longer milling time. For both types of cycle crystalline size decreases down to 20 nm. After 6000 min of interrupted MM the formation of Cu 2 O has been revealed and a strong dependency between oxygen content and microstructural parameter has been attested analysing the variations of lattice parameter and lattice strain. When the interstitial oxygen atoms lose their Cottrell locking action dislocation annihilation occurs leading to a reduction of dislocation density and lattice strain.
Grain size stabilization and strengthening of cryomilled nanostructured Cu 12 at% Al alloy
Journal of Alloys and Compounds, 2017
Nanocrystalline Cu12 at% Al alloy was synthesized by high energy ball milling under cryogenic condition. The alloy was then annealed up to 900 °C (or 0.87 T m). Phase and microstructural evaluation were carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM) and the mechanical property was investigated by microhardness testing. Results show that at elevated temperatures grain growth of Cu 12 at% Al alloy is much less than pure Cu prepared under same condition. The microhardness of the alloy decreases from 2.91GPa to 2.73 GPa (~6% decrease) after annealing at 900 °C. Superior thermal stability at high temperature was ascribed to the grain boundary pinning by nano-scale intermetallic particles. Kinetically stabilized grain size and hardness show good agreement with theoretical predictions.