Jing Tao Wang

Continuous Recrystallization Phenomenon in High Purity Copper during Equal Channel Angular Pressing up to High Strain at Room Temperature

Equal channel angular pressing (ECAP) was conducted at room temperature to a high strain level of ~24 in high purity copper. Tensile testing, Transition Electron Microscopy (TEM) and Electron backscatter diffraction (EBSD) were used to characterize the microstructure and property evolution with the increase of ECAP strain. It was found that tensile yield strength and the stored energy increases upon ECAP processing until a peak reached at 8~12 passes of ECAP, and their saturation was observed at higher ECAP passes. Continuous recrystallization phenomenon in microstructure was observed, where dislocation free crystallites with large misorientation to their surrounding matrix and smaller than the nuclei for discontinuous recrystallization were observed embodied in the matrix of deformed structure with high dislocation density. A two-step process was observed for the formation of these small crystallites, first the condensing of dislocation tangles into a narrow boundary, mostly low angle boundary; And second local migration (in sub-micrometer range) of short grain boundaries, in strong contrast to the dramatic migration of long large angle grain boundaries during discontinuous recrystallization to swallow the deformed matrix, was observed leading to vanish of small subgrains.

Historic Retrospection and Present Status of Severe Plastic Deformation in China

Severe plastic deformation (SPD) in solid metals, with an aim to obtaining ultra-fine grained or even nano-structured materials, attracted great efforts all over the world. This article retrospects to the ancient origin of this technique and presents some aspects of recent research activities in China briefly. SPD appeared as “folding and forging” for the fabrication of high quality steel sword as early as ~ BC500, this technique developed and maturated at around AD220~280, with a special term in Chinese “BAI-LIAN” (means Multitudinous Repetitive Processing), the steel fabricated by this process is called BAI-LIAN steel. Inlay gold characters on unearthed ancient steel swords recorded the number of processing repetitions. This ancient technique even resulted in several Chinese idioms very popular until at present time. Current research activities on SPD in China, supported by the National Science Foundation, National high tech program, local government technical programs and enterprises etc., are blooming all over China, in Universities, Chinese Academy of Science, National Research Institute and Enterprises. SPD is becoming a hotspot in materials field in China. Chinese materials scientists expect greatly increasing contribution to the development of SPD.

Mechanical Anisotropy of Extruded Mg-10Gd-2Y-0.5Zr Alloy

Tensile and compressive tests were performed on extruded Mg-10Gd-2Y-0.5Zr (mass fraction, %) alloy specimens with different tilt angles relative to extrusion direction. The microstructures were examined by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Calculations of the orientation factors for basal slip were done for that existed texture. The results show that the alloy doesn’t show tensile-compressive yield strength asymmetry, and the highest flow stress is appeared along the extrusion direction. Meanwhile the extruded textures are much more randomized. The reason may be that the addition of rare-earth is benefit to activate the non-basal slips, especially pyramidal slip. The reduction of c/a ratio is helpful to enhance the symmetry of the Mg crystal, which decreases the critical resolved shear stress of slip.

Influence of HPT Deformation Temperature on Microstructures and Thermal Stability of Ultrafine-Grained Tungsten

In the present work high pressure torsion (HPT) was imposed on commercial purity (CP) tungsten at different temperatures of 450 °С and 490 °С to achieve different microstructures and grain boundary misorientation spectra (GBMS). After HPT at 450°С, ultrafine grained microstructure with an average grain size of ~150 nm was developed in the metal. HPT at 490 °С results in an elongated structures with average width of ~500nm. EBSD investigations showed that over 92% are HAGB type in microstructure HPT-processed at 450°С, and in contrast, over 50% of grain boundaries are LAGB in sample processed at 490°С. Annealing at 900°С for 1h, of the sample with homogeneous UFG structure resulted from HPT at 450°C, leads to only limited decrease (~20%) in microhardness.

An analytic bond-order potential for the Fe–Cu system

An angular-dependent analytic bond-order potential (ABOP) for copper and Fe–Cu system was developed, based on the ABOP of pure iron introduced by Muller et al (2007 J. Phys.: Condens. Matter 19 326220). The potential parameters for the present ABOP model of copper were determined by fitting to the experimental data of the basic properties of fcc Cu and ab initio calculated properties of bcc Cu. The model predicts the vacancy formation energy in good agreement with the experimental result, although no vacancy formation information was used in the fitting of the model parameters. The melting point of Cu is also properly reproduced. The Fe–Cu binary system was described by adding two independent cross parameters in the potential model. The cross parameters were fitted using the ab initio data of the formation energies and lattice parameters of fictitious Fe–Cu alloys. The potential was applied to investigate the point defects and small defect clusters in dilute Fe–Cu alloys. The results were compared with the ab initio data and the values obtained with other potentials.

Contribution of Twins to the Strengthening of Commercial Purity Titanium after Equal-Channel Angular Pressing

Room temperature equal-channel angular pressing (ECAP) was employed on commercial purity titanium in the present work. Mechanical twining was observed in ~90% grains while the grains were not significantly refined (~10 μm) after ECAP. TEM observations showed that the twins observed under OM are usually composed of a serial of parallel twin bands with a width of ~1 μm. Microhardness and tensile tests showed that such a coarse-grained microstructure combined with a high fraction of mechanical twins has a microhardness of ~ 240 Hv, yield strength of ~ 730 MPa, tensile strength of ~ 740 MPa and elongation to failure of ~ 16%. This yield strength is much higher than 620-640 MPa, the yield strength of ultrafine-grained titanium by 8~12 passes of ECAP at 450 oC with a grain size of 200-300 nm, and is close to ~ 790 MPa, the yield strength of commercial Ti-6Al-4V alloys.

Heterogeneity and Anisotropy in Microstructure and Mechanical Properties of Pure Copper Processed by Equal Channel Angular Pressing

Pure copper (99.98%wt) square bars (32x32 mm) were processed by equal channel angular pressing (ECAP) Microstructure evolution was characterized by microscopy. Room temperature mechanical properties were obtained by tensile and micro-hardness tests. With increasing number of ECAP passes and cold rolling reductions, the initial coarse grained structure in the as-received material was transformed gradually into an ultrafine grained microstructure with an average grain size of 0.2~0.3 μm. Subsequent rolling resulted deformation twining in this ultrafine grained microstructure, which gives further strengthening in addition to the strengthening obtained by ECAP. Property anisotropy in three orthogonal directions of samples processed by ECAP was characterized by tensile testing.

An investigation on rolling texture transition in copper preprocessed by equal channel angular pressing

The effects of equal channel angular pressing (ECAP) and subsequent rolling path on the evolutions of rolling texture and flow stress anisotropy in the finally cold-rolled copper sheet was investigated. Copper billets processed by 1, 2, 4, and 8 passes of ECAP were subjected to cold rolling via three different paths: unidirectional rolling along extrusion or transverse direction of ECAP, or cross rolling. The microstructure, texture, and flow stress were characterized by EBSD, TEM, XRD, and tensile testing, respectively. The rolling texture was found transformed from copper-type to brass-type as the initial prerolling microstructure was refined from coarse grained to ultrafine grained (UFG) by multipass ECAP; Cross rolling, which is conventionally considered effective in reducing texture strength and thus mechanical anisotropy in coarse-grained materials, has proven to be ineffective in UFG copper. The flow stress anisotropy in the rolled copper sheet was found mainly controlled by the microstructural-dependent anisotropy of critical resolved shear stress rather than texture strength, and this leads to decrease of flow stress anisotropy in the cold-rolled copper sheet upon the increase in the number of preprocessing ECAP passes.

Microstructures and Mechanical Properties of Fcc Pure Metals with Different Stacking Fault Energies by Equal Channel Angular Pressing

In the present work 99.98% commercial pure copper, 99.5% commercial pure nickel and 99.5% commercial pure aluminum were imposed on high strain levels of ~24, ~8 and ~44 by equal channel angular pressing (ECAP) via route Bc, respectively. Microstructures and mechanical properties are investigated by TEM observations, tensile tests and microhardness tests. It shows that grain sizes of pure copper, pure nickel and pure aluminum has been severed refined from several tens of microns into several hundreds of nanometers after ECAP processing, however, microstructure of copper are mainly consisted of equiaxed (sub) grains with illegible grains/ (sub) grains boundaries after processed by ECAP, while it is featured as lamellar boundaries in that of pure nickel and as elongated grains in that of pure aluminum underwent a same strain level of ECAP. Results of mechanical properties show that yield strength and microhardness increase as strain increase up to a max value in copper, and then begin to decrease slightly, while mechanical properties of the other two increase as strain increases in nickel up to a strain level of ~12, and as in aluminum, yield strength and microhardness increase as strain increase in a relative low strain level, and then reach an saturation value.

Enhanced strength and ductility in hypo-eutectoid Cu-Al alloys through ECAP and annealing

In the present investigation, a bimodal structured alloy with ultrafine-grained (UFG) eutectoid matrix embedded with micrometer-grained pre-eutectoid phase was introduced into the hypo-eutectoid Cu-10.8wt.%Al and Cu-11.3wt.%Al alloys by means of pre-pressing heat-treatment, equal-channel-angular pressing (ECAP) and subsequent annealing. Different size of micrometer grained pre-eutectoid phase was obtained by controlling the cooling rate during pre-pressing heat-treatment of the hypo-eutectoid alloy. The tensile deformation behavior of the developed microstructures is characterized by a maximum tensile yield strength up to 800MPa, which is three times higher than that of the un-treated alloy. It is found that the size of the micrometer grained pre-eutectoid phase is critical to the improvement of the bimodal structured alloy. With larger micrometer grained pre-eutectoid phase, no obvious improvement in plastic elongation was observed with the increase of volume fraction of the pre-eutectoid phase from 20% to 40%, but a decrease in the yield tensile strength was observed. An optimal combination of strength and ductility was obtained particularly in those samples embedded with small-sized micrometer-grained pre-eutectoid phase, which provide extra strain gradient hardening effect.