32 research outputs found
HighPâTNano-Mechanics of Polycrystalline Nickel
We have conducted highPâTsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volumeâpressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify âmicro/localâ yield due to high stress concentration at the grain-to-grain contacts and âmacro/bulkâ yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highPâTconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse HallâPetch relationship. We explain this abnormal HallâPetch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni
The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by powder metallurgy
The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by consolidation of nanostructured powders and nanopowders is analysed by developing three simple mathematical equations for calculating the α factor for different thermomechanical powder consolidation processes such as hot pressing, high pressure torsion and extrusion. The α factor is the fraction of the area of the powder particle surfaces newly formed during consolidation over the total particle surface area which includes both pre-existing surface area and the newly formed surface area. It is demonstrated that the values of the α factor calculated using these equations can be reasonably used to predict the level of inter-particle atomic bonding that is likely to be achieved through cold-welding by the above mentioned typical thermomechanical powder consolidation processes which also include high energy mechanical milling. Based on this analysis, it is clear that uniaxial hot pressing of a powder compact in a rigid die at low homologous temperatures (<0.5Tm) is unlikely to be capable of achieving a sufficiently high level of inter-particle atomic bonding for producing a high quality consolidated material, while processes involving a large amount of plastic deformation have such capabilities
High strength, ductility, and electrical conductivity of in-situ consolidated nanocrystalline Cu-1%Nb
Nanocrystalline metalsïżœwith grain sizes less than 100 nmïżœ have strengths exceeding those of coarse-grained and even alloyed metals [1,2]. A bulk nanocrystalline Cu-1%Nb alloy was synthesized by an in-situ consolidation mechanical alloying technique. The mechanical behavior of this alloy was investigated by hardness and tensile tests. The nanostructure was investigated by X-ray diffraction and transmission electron microscopy and the fracture surface by scanning electron microscopy. Electrical resistivity was measured using a four-point probe technique. The dilute additives of Nb and the processing conditions induced artifact-free bulk nanocrystalline materials that possess extraordinary high strength, good ductility, and high electrical conductivity. 2017 Elsevier B.V.Financial support by Qatar National Research Funds (QNRF) under grant no. NPRP9-180-2-094 is gratefully acknowledged
Influence of 1%Nb Solute Addition on the Thermal Stability of In Situ Consolidated Nanocrystalline Cu
Nanocrystalline (nc) Cu and Cuâ1% Nb bulk materials are synthesized using a combination of cryogenic and room temperature ball milling. The grain size values of these in situ consolidated Cu and Cuâ1% Nb, determined using transmission electron microscopy, are found to be 22ânm and 18ânm, respectively. In this investigation, isochronal heat treatments are performed for 1âh to establish grain size and microstructural changes as a function of temperature. The annealing of nc Cuâ1% Nb at a temperature of 1073âK reveals a slight increase in the average grain size from 18 to 45ânm. The grain size of nc Cu, however, increases from 22ânm to about 3âÎŒm after annealing at the same conditions. The present results indicate that solute entrapment plays a major role in thermal stability of the high purity contaminantâfree Cu with the addition of only 1âat% Nb after annealing for 1âh up to a homologous temperature of 0.8. Kinetic stabilization via clustering of Nb atoms on the grain boundaries and the triple junctions is also observed after annealing at high temperature for longer times.This publication was made possible by the NPRP award number NPRP9-180-2-094 from the Qatar National Research FundScopu
Mechanical properties of bulk nanocrystalline aluminum-tungsten alloys
The Al-W alloy powders containing up to 4 at. pct W particles in a nanocrystalline Al matrix were synthesized by ball milling and then hot compacted (HC) at 573 K or cold compacted using high-pressure torsion (HPT). Hardness measurements were made to determine the effects on mechanical properties. Based on existing models, the hardening effect of W particles in HC samples was attributed to Orowan-particle strengthening. Composite-type strengthening due to large W particles appeared to be negligible. Both the compressive strain and shear strain in high pressure torsion added to the strengthening effects without producing a change in the grain size