Materials Science Forum Vols. 667-669

Abstract: The structurization of a high purity niobium from double electron-beam melted cast microstructure to fine-grained microstructure was completed by equal-channel angular pressing by the Bc route up to a Von Mieses strain of 13.8. In addition, for the viscoplastic behavior study as well as nanostructure and properties improving the hard cyclic viscoplastic deformation, die forging at room temperature and followed heat treatment with low heating rate were conducted. The nanostructure of processed samples was characterized by transmission electron microscopy and X-ray diffraction testing. This paper focuses on several new trends in the study of improved mechanical and physical properties of pure niobium, to what purpose these materials will be used in industry. The crystallite size, microstrains and dislocation density in severe plastic deformed pure niobium were calculated and electric conduction was measured. The nanocrystalline microstructure with minimal crystallite size down to 62 nm as mean in cross-section of sample was received. By this the dislocation density varies from 5.0 E+10 to 2.0 E+11 cm-2 and was maximal for pure niobium which has minimal electrical conductivity, maximal value of hkl-parameter and maximal relative microstresses. The microhardness was maximal for sample after 12 passes by Bc route and for samples with 8 and 10 passes followed heat treatment at 170 and 350°C. The mechanisms answerable for the electronic conduction were discussed according to the microstructure evolution in the different directions and for different strain levels.

Abstract: Commercial pure iron billets having diameter of 60 mm and length of 180 mm were subjected to equal channel angular pressing (ECAP) at 350 °C for 1-4 passes via route BC. Microstructural evolutions on three planes (X, Y, Z planes) were characterized by optical microscopy and transmission electron microscopy (TEM). It was found that after four passes an ultrafine microstructure could be formed on the X plane, but a band structure remained on the Z plane. Accordingly, the mechanical properties exhibited apparent dependence on the orientations. The strength in the X and Y directions was higher than that in the Z direction. The microstructural refinement and mechanical properties were discussed in terms of experimental results.

Abstract: Basic research considering the minimum achievable grain size of severe plastic deformation (SPD) materials is often performed with high purity metals. However, a literature study reveals large discrepancies in the microstructural size of high purity nickel processed by SPD. The focus of this work is the influence of small impurity concentrations on the resulting saturation microstructure. The microstructure of SPD nickel was systematically investigated using different purities ranging from 99.79 wt% up to 99.99 wt%. The materials were deformed by high pressure torsion (HPT) at different temperatures from liquid nitrogen temperature (-196°C) up to 400°C. It was found that carbon concentration is the governing element in achieving the finest microstructures and the highest strength. Therefore, further experiments with well defined carbon doped samples were performed. By changing the carbon content from 0.008 wt% up to 0.06 wt% tensile strength in the saturation regime increases by more than 700 MPa. It will be shown that even small variations (<0.01 wt%) lead to significant changes in ductility and tensile strength values.

Abstract: Among the materials employed for orthopedic implants, Ti-6Al-4V is definitely the best choice due to its excellent properties. However, a more intense use is hindered by high cost and the presence of harmful elements, viz. Al and V. A solution can be found in commercially pure Ti, provided its tensile and fatigue properties can be upgraded by some process of Severe Plastic Deformation. In the present work, Grade 2 Ti was submitted to up to four Equal Channel Angular Pressing passes, followed by cold rolling (30 70 and 90% reduction ratio) Hardness and tensile properties were determined, paying attention to the work hardening behavior. Results show that Severe Plastic Deformation gives a 442 MPa increase over the yield strength of annealed Grade 2 Ti (originally 337 MPa), while elongation reached 21%. Best results were obtained with four passes followed by 70% cold rolling reduction. Finally, the microstructural stability was assessed by hardness measurements.

Abstract: The strengthening of AlCuMg(Li) alloys subjected to high pressure torsion (HPT) deformation with strain reversals was studied by microhardness (Hv) tests and differential scanning calorimetry (DSC). It was found that the strengthening is lower for both cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) as compared to monotonic HPT (m-HPT). The DSC results demonstrate that |HPT influences S phase precipitation. With increasing strain, the maximum heat flow (height of the S peak) and the heat content of S formation peaks increases. There is a larger S heat content reaction in the periphery of HPT processed disks compared with those in the centre. Strain reversal also has a significant influence on the S precipitation. The strengthening during HPT deformation is discussed in terms of the density of statistically stored and geometrically necessary dislocations.

Abstract: Superplasticity and microstructural evolution of a commercial Al-5.4%Mg-0.5%Mn-0.1%Zr alloy subjected to severe plastic deformation through equal-channel angular pressing (ECAP) and subsequent rolling was studied in tension at strain rates ranging from 1.4×10-4 to 5.6×10-2 s-1 in the temperature interval 400-550°C. The alloy had an unrecrystallized microstructure with an average crystallite size less than 5 m. The alloy exhibited the yield strength of ~370 MPa, ultimate strength of ~450 MPa and elongation-to-failure of ~15% at ambient temperature. In spite of small crystallite size the alloy shows moderate superplastic properties. The highest elongation-to-failures of ~450% appeared at a temperature of ~500°C and an initial strain rate of ~1.4×10-3 s-1, where the strain rate sensitivity coefficient, m, is of about 0.57. The relationship between superplastic ductilities and microstructure is discussed.

Abstract: A dispersion-strengthened Cu-0.2 wt.% Zr alloy was subjected to equal-channel angular pressing (ECAP) at room temperature for up to 12 passes through route BC using a die having a channel angle of 90°. The microstructural investigations were performed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Tensile creep tests were conducted at temperature 673 K and at the applied stress in the range from 80 to 180 MPa. The migration of boundaries and subsequent grain growth were restricted by Cu9Zr2 precipitates. The study was performed in order to evaluate the effects of severe plastic deformation and precipitation on creep behaviour and microstructure of the pressed alloy. It was found that creep behaviour is strongly dependent on number of ECAP passes. The pressed alloy after up to 4 ECAP passes exhibited a considerable improvement in creep properties in comparison with the unpressed alloy.

Abstract: The quasi-static and dynamic compression experiments of ultrafine-grained copper fabricated by equal channel angular pressing method were performed at temperatures ranging from 77 to 573K. The influence of temperature on flow stress, strain hardening rate and strain rate sensitivity were investigated. The results show that the flow stress of ultrafine-grained copper shows much larger sensitivity to testing temperature than that of coarse grained copper. However, the temperature sensitivity of ultrafine-grained copper to true strain is comparative weaker than that of coarse grained copper. For the ultrafine-grained copper, both the strain hardening rate and its sensitivity to temperature of ultrafine-grained copper are lower than those of its coarse counterpart. The SRS also displays apparent dependence on temperature. The activation volume for UFG-Cu is estimated to be on the order of ~10b3 in current experiment temperature. It is suggested that the dislocation-grain boundary interactions process might be the dominant thermally activated mechanism for UFG-Cu.

Abstract: In the present investigation, annealed billets of commercially pure Al (1050) with coarse-grained microstructure of 0.6 mm were ECAPed through a die with an internal angle of 90o using two routes A and BC. The samples were processed up to four passes using both routes. The change in the processing route results in the change of the shear plane, and consequently the change in the produced microstructure. The microstructure study was conducted on the extrusion direction and the shear plane. The cell size, misoriention and the fraction of high angle boundaries were determined by using electron back scattered diffraction (EBSD). A study of mechanical behavior was conducted by cutting tensile and compression specimens from the ECAPed specimen in the extrusion direction to study the effect of processing route and the number of passes on the deformation characteristics. Enhanced strength was observed but with anisotropic behavior between tension and compression. Cyclic deformation under load control (HSF) was also performed and the S-N curves were established as a function of number of passes and processing route. The fractography of fractured tensile specimens was also investigated.

Abstract: Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-3%Li-1%Sc alloy for four-passes to study the microstructure uniformity and tensile properties. After ECAP, the microstructure become muddled, contains about 65% of deformed coarse grains with abundant low angle grain boundaries and about 35% of recrystallized small grains. Meanwhile, a strong basal texture is formed in the ECAP sample. The texture type of the recrystallized grains and the deformed grains are the same, however, the texture strength of the recrystallized grains is much lower than the deformed ones. Tensile strength is improved effectively and the elongation is maintained after ECAP. The increment of strength results from the microstructure refinement and residual dislocations produced by ECAP, while the recovery of ductility may be attributed to a shear type texture formed in the alloy during ECAP.