A study of annealing stages in commercial pure Cu using mechanical measurements and positron annihilation lifetime technique (original) (raw)

Correlation of thermo-mechanical properties with microstructure in commercial pure Cu and Cu–Zn alloys studied by positron annihilation

Materials Science and Engineering: A, 2004

Isochronal annealing of cold-rolled commercial pure copper (99.97%), Cu-28% Zn, and Cu-32% Zn has been investigated between room temperature and 1173 K. The annealing stages have been identified as recovery, partial recrystallization, complete recrystallization and grain growth using Vickers microhardness, positron annihilation lifetime, and doppler broadening of annihilation radiation measurements. The retardation of complete recovery in Cu-Zn alloys is due to the presence of vacancy-Zn complexes.

Microstructural stability of Cu processed by different routes of severe plastic deformation

Materials Science and Engineering: A, 2011

The thermal stability of ultrafine-grained (UFG) microstructure in Cu processed by different routes of severe plastic deformation (SPD) was studied at both high and room temperatures (RT). It was found that the microstructures produced by multi-directional forging or twist extrusion were more stable than those obtained by equal-channel angular pressing (ECAP) or high-pressure torsion (HPT). During storage of the ECAP-processed specimen at RT for 4 years the vacancy concentration reduced significantly while the dislocation density and the crystallite size remained unchanged. In the case of the HPT-processed sample both grain-growth and reduction of the dislocation density were observed.

Mechanical Properties and Microstructure Development of Ultrafine-Grained Cu Processed by ECAP

Materials Science Forum, 2008

Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal channel angular pressing. The results of mechanical tests and the microstructure characterization by various experimental techniques are presented. The yield stress as well as the strength were shown to increase with increasing strain and exceed the respective values of a coarse-grained material. The microstructure development and its fragmentation after equal channel angular pressing (ECAP) was investigated by the transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The proportion of high angle grain boundaries was found to increase with increasing strain reaching the value of 90% after 8 ECAP passes. Two kinds of defects were identified in ECAP specimens by positron annihilation spectroscopy (PAS): (a) dislocations which represent the dominant kind of defects, and (b) small vacancy clusters (so called microvoids). The main increase of defect density was found to occur during the first ECAP pass. PAS analysis indicated that in the specimens subjected to one ECAP pass the mean dislocation density ρ D and the concentration of microvoids c ν exceeded the values of 10 14 m -2 and 10 -4 at. -1 , respectively. After 4 passes, the number of defects becomes saturated and practically does not change with increasing strain.

Grain boundary excess volume and defect annealing of copper after high-pressure torsion

Acta Materialia, 2014

The release of excess volume upon recrystallization of ultrafine-grained Cu deformed by high-pressure torsion (HPT) was studied by means of the direct technique of high-precision difference dilatometry in combination with differential scanning calorimetry (DSC) and scanning electron microscopy. From the length change associated with the removal of grain boundaries in the wake of crystallite growth, a structural key quantity of grain boundaries, the grain boundary excess volume or expansion e GB ¼ ð0:46 AE 0:11Þ Â 10 À10 m was directly determined. The value is quite similar to that measured by dilatometry for grain boundaries in HPT-deformed Ni. Activation energies for crystallite growth of 0:99 AE 0:11 and 0:96 AE 0:06 eV are derived by Kissinger analysis from dilatometry and DSC data, respectively. In contrast to Ni, substantial length change proceeds in Cu at elevated temperatures beyond the regime of dominant crystallite growth. In the light of recent findings from tracer diffusion and permeation experiments, this is associated with the shrinkage of nanovoids at high temperatures.

Thermal Stability of Ultra Fine Grained Copper Prepared by High Pressure Torsion Using Various Pressures

Journal of Metastable and Nanocrystalline Materials, 2003

Thermal stability of ultrafine grained ͑mean grain size 150 nm͒ copper prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy correlated with transmission electron microscopy. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high number of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Calculations of the lifetime of a positron trapped at a microvoid as a function of its size were performed to obtain information about sizes of the microvoids. Abnormal grain growth, when isolated recrystallized grains grow inside the deformed matrix, takes place from 160°C. From 280 to 400°C recrystallization occurs. Strongly inhomogeneous spatial distribution of defects does not allow application of the simple trapping model. Therefore a model of positron behavior in the ultrafine grained materials was developed in the present work. The model takes into account inhomogeneous spatial distribution of defects and allows for determination of dislocation density, concentration of microvoids, linear size of coherent domains, and volume fraction of the distorted regions. Moreover using this model it was possible to determine the activation energy of the recrystallization.

Positron Annihilation Studies of Microstructure of Ultra Fine Grained Metals Prepared by Severe Plastic Deformation

Materials Science Forum, 2005

In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al 2 O 3 nanocomposite (GlidCop) is compared.

The role of annealing twins during recrystallization of Cu

Acta materialia, 2007

The texture and grain boundary structure of recrystallized materials are dependent upon the character of the deformed matrix and the selective nucleation and growth of crystallites from the deformation structure. Annealing twin boundary formation in materials of low to medium stacking fault energy is not only a product of the recrystallized structure, but also plays an important role in the recrystallization process itself. In situ and ex situ recrystallization experiments were performed on pure copper (99.99% pure) previously deformed by equal channel angular extrusion. Intermittent characterization of the structure on the surface of bulk specimens was accomplished using electron backscatter diffraction. The character of the structure where nucleation preferentially occurs is presumed to be in heavily deformed regions as nuclei were first observed in such microstructures as viewed from the specimen surface. Grain growth is observed to be heavily dependent upon twinning processes at the low temperatures used for in situ experiments, with twinning occurring to aid the recrystallization process. It is shown at these temperatures that the slowest growing grains obtain the highest fraction of twin boundaries as the new twin orientations presumably increase the boundary energy at positions where there is insufficient driving force to continue growth.

Influence of cold working on microstructure and properties of annealing alloyed copper

2012

Purpose: The aim of this study was to investigate the effect of cold plastic deformation of the supersaturation on the structure and properties of the CuTi4 alloy after aging. Design/methodology/approach: CuTi4 alloy of supersaturation temperature of 900°C after heating for 1 hour. After solutioning alloy was processed in two ways: first aged in the temperature range 450-600°C and the second stage rolling reduction with Z=50%, and then aged in the temperature range 450-600°C. Findings: The results confirmed that the temperature within the range 500-600ºC, the hardness increases with increasing aging time until reaching the maximum, but then with increasing aging time the hardness decreases. By using methods of electron microscopy (SEM, EDS, EBSD, TEM) after aging at 550°C after 1 minute of modulated microstructure was observed -characteristic for the spinodal transformation and lamellar, formed by nucleation and growth. Research limitations/implications: A widely used method for increasing the strength properties of metal alloys, in addition to cold plastic deformation, is the strengthening of new phases separated particles during aging. The effect of cold rolling operation between solutioning and aging on microstructure and properties of alloyed copper CuTi4. Further examination also included the effect of time and aging temperature. Practical implications: On the basis of conductivity, the influence of cold plastic deformation and subsequent aging on the hardness and electrical conductivity of the alloy CuTi4. It was found that with increasing aging time and with increasing aging temperature increases electrical conductivity of the alloy. On the basis of X-rays can be concluded that in alloyed copper containing 4% Ti and precipitation hardening metastable phase β'-Cu4Ti is separated, which occurs both in the previously deformed and undeformed cold worked alloy. Originality/value: