Grain Refinement Of Cu Research Articles

Swirl-like Cu-Sn phase formation and the effects on the ultrasonic spot welded joint of Sn-coated Cu plates

Previous studies have shown that sharp physical and chemical changes may induce the formation of swirl-like reaction phases in the ultrasonic spot welding (USW) of different materials. However, the mechanisms have not been well understood. In particular, swirl-like structures have seldom been discovered or discussed in the USW of Cu/Sn. In this study, swirl-like phases were observed in the USW of a Sn-coated T2 Cu plate with different welding energies (300−400 J). The evolution of the microstructure from the flat region to the swirl region was studied, the diffusion coefficient of Cu in Sn and the peak temperature of the interface during welding were theoretically calculated and experimentally verified, the formation mechanism of the swirl-like phases was discussed, and the relationship between the joint properties and welding energies was investigated. The results show that the content of Cu3Sn increases, while that of Cu6Sn5 decreases gradually from the flat region to the swirl region and that grain refinement of Cu and IMCs occurs near the swirl regions. The greatly enhanced diffusivity shows that intense atomic diffusion takes place during the welding process. When the welding energy exceeds 300 J, the experimentally measured peak temperatures in the welding processes are higher than the theoretical values. The formation of the swirl-like phase is attributed to the uneven distribution of interfacial stress and the collapse of cavitation bubbles. When the welding energy reaches 400 J, the peak lap shear load of the welded specimen reaches 1008 N, and the associated fractures indicate the combined characteristics of ductility and brittleness.

Atomic-scale investigation on the structural evolution and deformation behaviors of Cu–Cr nanocrystalline alloys processed by high-pressure torsion

Abstract A systematic investigation based on high-resolution transmission electron microscopy (HRTEM) and quantitative analysis was carried out to gain insights into the structural evolution and deformation behaviors of 57 wt%Cu - 43 wt%Cr dual-phase alloys deformed by high pressure torsion (HPT). Nanometer-sized grains were obtained, with Cu and Cr grain size being reduced from 33.7 nm to 17.4 nm and 35.6 nm to 18.6 nm respectively after deformation with 5 rotations and 25 rotations. During the early deformation stage, in a way, grain refinement in Cu was controlled by the interaction between Cr solutes (or clusters) that was forced to dissolve into Cu matrix and extended dislocations motion. At the higher strain condition, the deformation mode became more complicated and the grain refinement of Cu and Cr can be affected by solute effects and constraint effect during severe plastic deformation in this dual-phase system. In addition, inverse grain-size effect on stacking fault (SF) and twinning tendencies was observed in Cu under different deformation conditions. A critical grain size was calculated to describe the required shear stress for partial dislocation nucleation. HRTEM investigations and statistical analysis on twins observed in 25 rotations-strained sample reveal that the twins are associated with the grain boundary activities and the recovery procedure in the extremely fine grain size region. These atomic-scale observations provide new views in understanding the deformation mode, especially twinning behavior in the extremely fine grain size range below the critical grain size that are hardly obtained in experiments.

Plastic strain-induced grain refinement at the nanometer scale in copper

Microstructural evolution and grain refinement in pure Cu subjected to surface mechanical attrition treatment (SMAT) were investigated by means of systematic transmission electron microscope observations. Two different mechanisms for plastic strain-induced grain refinement in Cu were identified, corresponding to different levels of strain rate. In the subsurface layer of the SMAT Cu samples with low strain rates, grains are refined via formation of dislocation cells (DCs), transformation of DC walls into sub-boundaries with small misorientations, and evolution of sub-boundaries into highly misoriented grain boundaries. The minimum size of refined grains via this process is about 100 nm. In the top surface layer (thickness <25 μm) with a high strain rate, the grain refinement includes: (i) formation of high-density, nanometer-thick twins dividing the original coarse grains into twin–matrix (T–M) lamellae; (ii) development of dislocation walls that further subdivide the T–M lamellae into equiaxed nano-sized blocks; (iii) evolution of these preferentially oriented blocks into randomly oriented nanosized grains. The minimum size of such refined grains is about 10 nm. The present study demonstrates the critical role of strain rate on the plastic strain-induced grain refinement processes and on the minimum grain size obtainable via plastic deformation.

Dynamic Processes for Nanostructure Development in Cu after Severe Cryogenic Rolling Deformation

The dynamic grain refinement behavior upon severe plastic deformation has been systematically studied in commercial purity copper that was heavily cold rolled to large deformations at cryogenic temperatures. The low-temperature rolling allows the accumulation of extraordinarily high densities of dislocations in Cu, enabling an investigation of the various dynamic recrystallization phenomena upon further deformation. The eventual steady-state grain sizes achieved, as well as the dynamic recrystallization mechanisms, are studied using controlled deformation tests combined with transmission electron microscopy. The dominant mechanisms observed to contribute to grain refinement in Cu include the classical migration dynamic recrystallization process, the deformation twinning, as well as the continuous dynamic recrystallization via progressive lattice rotation upon deformation to extremely large strains. The strain rate and deformation temperature have strong effects on the operative mechanisms and the grain sizes achieved in the ultrafine and nanocrystalline regimes.

In-situ formation of Cu–Al 2O 3 nano-scale composites by chemical routes and studies on their microstructures

Copper–alumina composite briquettes have been prepared by hydrogen (pH 2=0.1 atm) reduction of a homogeneous mixture of finely divided CuO and Al 2O 3 formed by two chemical processes. In these processes, the Al 2O 3 of the desired pct. has been formed in-situ by (1) addition of Al(NO 3) 3 solution to CuO; or (2) by addition of stoichiometric amount of ammonia to a slurry of CuO in Al(NO 3) 3 solution, followed by filtration, and then heating the mixture in both the cases, at 850°C for 2 h. The Cu–Al 2O 3 composites thus formed, are sintered at 975°C for 2 h in H 2 atmosphere (composites C-1 and C-2). Composite samples have also been prepared by H 2 reduction of the briquette, made out of; (3) the mixture of Al 2O 3 and CuO powders; or (4) by direct addition of Al 2O 3 to Cu powder, followed by sintering at 975°C for 2 h (composites C-3 and C-4), in order to compare their homogeneity and microstructures with those (C-1 and C-2), made by the above two chemical processes. The characterization of the composite samples has been done by metallography, SEM and TEM techniques. The distribution of Al 2O 3 particles in the Cu matrix, is found to be improved when the Cu is produced in-situ by reduction of CuO by H 2 and very significant improvements in this regard, are noticed in case of composites prepared by the chemical methods, particularly the one through Al(NO 3) 3 decomposition. In this case (C-1), besides the grain refinement of Cu (50–250 nm) and homogeneous distribution of Al 2O 3 in it (about 10 nm in size), an appreciable amount of a third phase is found to appear. The existence of the third phase has been indicated by SEM, TEM and EDS analyses. The third phase is suggested to be possibly CuAlO 2.

Grain refinement of Cu–Zn–Al and Cu–Al–Ni by Ti addition

AbstractTo obtain fine grained Cu based shape memory alloys after thermomechanical processing, Ti is added to β-Cu–Zn–Al or β-Cu–Al–Ni as a particle forming element. This work consists of a study of the mechanism that controls the grain growth limiting effect during the final annealing treatment. A critical evaluation of the grain growth models in particle containing materials and comparison with the experimental results lead to the conclusion that the grain growth inhibition is mainly attributable to the effect of the second phase particles but also to the influence of Ti atoms in solid solution.MST/678

Grain refinement in CuIn alloys through thermal cycling

A thermal cycling technique which allows grain refinement of Cu  In alloys from 400 μm to less than 10 μm has been developed. The process consists of annealing in the α phase field of a fully precipitated alloy. The time and temperature dependence of the grain size indicated a value of 0.45 for the exponent n in the general equation for the grain growth law, D = kt n . The value of n was found to be slightly dependent on temperature in a 50 deg C interval. The temperature dependence of the grain growth rate suggested an apparent activation energy Qg = 21.5 k cal/mole for the grain boundary migration. This value lies in the range of 0.4 to 0.5 of the activation energy for the diffusion of In in pure Cu, which is approximately the same value required for boundary migration. A small change in the composition from 15 wt.% In to 16.2 wt.% In balance Cu does not seem to have an appreciable effect on either the exponent n or the absolute value of the grain diameter. The apparent activation energy was also found to be independent of composition. A change in the thickness of the alloy resulted in smaller grain size but the effect was noticeable only in specimens solution annealed for longer durations. The value of the exponent decreased from 0.45 to 0.42. It has been observed that during normal grain growth in these alloys, growth continued to larger grain diameters when using single anneals than during multi anneals. The effect was noticeable only after the fourth cycle. The results were found to be in reasonable agreement with a recently proposed single boundary migration theory.