First Equal Channel Angular Pressing Pass Research Articles

Modeling grain fragmentation and deformation textures for titanium using a combined approach of the viscoplastic self-consistent model and a shear fluctuation model

A new grain fragmentation model is proposed for the formation of grain substructures in polycrystalline materials during plastic deformation. Micro-shear bands and twins are assumed to originate from grain boundaries due to the local stress concentrations caused by the interaction of grain boundaries and slip bands. These stress concentrations will impose shear fluctuations on the neighboring grains, which can be relaxed by the formation of corresponding substructures, i.e., micro-shear bands and twins, in these neighboring grains. Texture simulations for cold rolling and equal channel angular pressing (ECAP) of commercially pure titanium are performed using a combined approach of the viscoplastic self-consistent (VPSC) model and the shear fluctuation model. The texture update scheme in the combined approach is presented. Experimental textures at different thickness reductions of cold rolling at room temperature (300 K) and after the first pass of ECAP at 473 K are measured and compared with simulated textures obtained by the standard VPSC approach and the combined approach. In comparison with the standard VPSC approach, the combined approach gives better texture predictions for the two cases: The missing components at low to intermediate rolling reductions in the standard VPSC approach are reproduced; at high rolling reductions and after ECAP, the dominating fibers are also essentially smoothened in the combined approach, in better agreements with the experimental textures.

Effect of ECAP processing on microstructure evolution and dynamic compressive behavior at different temperatures in an Al-Zn-Mg alloy

An artificially aged Al-Zn-Mg alloy with a grain size of ~1.3µm was processed by equal-channel angular pressing (ECAP) and then subjected to dynamic compression at a strain rate of 4000s−1 in the range from room temperature to 673K. The results show the η′ phase is refined and a η phase is formed during the first pass of ECAP and after further processing 8 passes the GP zones are removed. An ultrafine-grained (UFG) structure with an average grain size of ~200nm was obtained after 4 passes. It is shown that dynamic compressive deformation assists the precipitation process through precipitate coalescence and by changing the precipitate orientations. The dynamic compressive yield strengths and flow stresses decrease gradually to different degrees with increasing temperature except after ECAP processing for 4 passes where there is thermal stability up to 473K. The ECAP processing significantly improves the strength of the alloy at elevated temperatures by comparison with the as-received material.

The microstructural evolution and thermo-mechanical behavior of UFG Ti processed via equal channel angular pressing

Equal channel angular pressing (ECAP) was applied to grade 1 Ti for up to four passes and the microstructure and thermo-mechanical behavior were determined. ECAP was performed with a channel angle of 90°, at a pressing temperature of 275°C, and using route BC. Electron backscatter diffraction (EBSD) was used to characterize the microstructure. After the first pass of ECAP, {1 0 1¯ 1} twins were present, but significant slip had also occurred. The microstructure was highly anisotropic, consisting of relatively undeformed grains to grains less than a micrometer. Increasing the number of passes decreased the grain anisotropy, and by the fourth pass a relatively homogeneous microstructure was present. After all numbers of passes, there was evidence for continuous dynamic recrystallization as the mechanism of grain refinement, even when twinning had occurred. This refinement accelerated as the number of passes increased. The response of UFG Ti was determined at −196, 22, 200 and 375°C, and at strain rates from 10−4 to 1100s−1. It was found that the ECAP process greatly increases the yield strength versus the as-received condition, and UFG Ti has similar work hardening as the as-received material. The work hardening does not decrease much as the number of passes is increased. Additionally, the strain rate sensitivity tends to increase with the temperature and decrease as the number of passes is increased.

Evaluating the Room Temperature ECAP Processing of a NiTi Alloy via Simulation and Experiments

The NiTi alloys are generally considered hard‐to‐deform materials when processing by equal‐channel angular pressing (ECAP) at room temperature. Nevertheless, it is shown by experiment that a NiTi alloy may be successfully processed by ECAP for up to two passes using a core–sheath configuration. Three‐dimensional finite element simulations were used to more fully evaluate the flow behavior after the first and second pass of ECAP. The values of the microhardness were recorded across transverse directions within the cores and the results show that the imposed strain both decreases by comparison with the processing of conventional sheath‐free NiTi billet and increases by increasing the dimensions of the core. The lower areas of the cores undergo less deformation than the upper areas after successful ECAP processing but the imposed strain, and thus the microhardness values, become homogeneously distributed on the longitudinal planes after two passes of ECAP.

Investigation of texture and mechanical properties of copper processed by new route of equal channel angular pressing

The evolution of crystallographic texture and the mechanical properties of copper subjected to severe plastic deformation (SPD) using equal channel angular pressing (ECAP) were investigated. Samples were subjected to ECAP under two different processing routes: B60 and BC. As the cross sections of the samples were circular, a new route with a rotation angle of 60° in the same direction between consecutive passes was introduced. The material exhibited texture development similar to the simple shear texture in both routes and the most significant changes in texture strength in both processing routes took place after the second pass. Microstructure of ECAP processed samples were investigated using electron backscatter diffraction (EBSD) analysis. Comparison of the EBSD data with optical micrograph of the initial sample confirmed that ECAP process has led to a significant decrease in grain size. Significant increases in hardness and tensile strength were observed after the first pass of ECAP. Variations of tensile strength as a function of the number of passes were related to the dislocation densities and the average boundary spacing.

Quantitative analysis of grain refinement in titanium during equal channel angular pressing

Strain-driven grain subdivision of commercial pure titanium during 1–8 passes of equal channel angular pressing (ECAP) at 723 K is investigated quantitatively by electron backscattering diffraction. The grain refinement mainly takes place in the first pass and the efficiency gradually decreases from 1 to 4 passes, reaching saturation with further increasing strain (6–8 passes). {1 0 1 ¯ 2} tensile twins are firstly observed in the first pass of ECAP. Misorientation gradients and the driving force of dynamic recrystallization are discussed.

Effect of ECAP on microstructure and mechanical properties of cast AZ91 magnesium alloy

An as-cast AZ91 magnesium alloy was processed by Equal Channel Angular Pressing (ECAP) at 320°C. The microstructure and mechanical properties were studied. It has been found that ECAP refines both the grains and precipitates, thus modifies the strength and ductility of the processed alloy. After the first pass of ECAP, the yield stress improves significantly from 71 MPa to 140 MPa.

Microstructure and mechanical properties of an as-cast AZ91 magnesium alloy processed by equal channel angular pressing

An as-cast AZ91 magnesium alloy was processed by equal channel angular pressing (ECAP) at 593K and its microstructure and mechanical properties were studied using electron microscopy and room temperature tensile tests, respectively. It has been found that after the first pass of ECAP, the grain size of the alloy shows a bi-modal distribution, containing fine grains of about 14 μm and large dendrite structure. The dendritic structure completely disappeared after two passes of ECAP. The average grain size of the alloy after six passes of ECAP becomes less than 10 μm. The yield stress of the alloy has significantly increased from 65 MPa of the as-cast alloy to 135 MPa after the first pass of ECAP, but does not show much change with further ECAP. However, the elongation to failure measured from the alloy processed by the first pass of ECAP is similar to that measured from the as-cast alloy. A noticeable increase of the elongation to failure has been observed after the second pass of ECAP, which then remains at the similar level with further ECAP process. The fractography of the tensile tested samples have been studied using scanning electron microscope (SEM) and focused ion beam (FIB) microscope. The facture surface of the as-cast alloy is predominated by cleavages. Although not predominantly, cleavage has also been frequently observed in the alloy processed by one pass of ECAP. With further ECAP process, the facture surface becomes profuse in dimples, characteristic of ductile facture, consistent with the ductility change observed. FIB observation suggests that the cracking is mainly initiated at the blocky particles.

Effect of equal channel angular pressing on the strength and ductility of an AZ80 alloy

Equal channel angular pressing (ECAP) was applied to an extruded AZ80 Mg alloy in order to refine microstructure and enhance mechanical properties. The results show that the initial grain size of 20 μm of the as-extruded samples has been reduced by 75% after the first pass of ECAP, down to about 5 μm. Also as a result of the severe plastic deformation, the intermetallic compound Mg17Al12, which was distributed along grain boundaries and forms networks before the ECAP, has been re-arranged into a more homogeneous distribution and with a more spherical shape after the ECAP. Although no strengthening effect has been observed, the grain refinement and precipitate fragmentation caused by the ECAP has resulted in a 28% increase in the elongation to failure in tensile tests. Fractography observation and texture measurement have also been carried out.

Deformed Microstructures and Mechanical Properties of CP-Ti Processed by Multi-Pass ECAP at Room Temperature

The deformed microstructures and mechanical properties of commercial pure titanium (CP-Ti) with an initial grain size of about 28 μm was investigated using equal channel angular pressing (ECAP). ECAP was conducted at room temperature adopting a die with a channel angle of 120° via route BC. Special attention was paid on the microstructure evolution and mechanical properties of the ECAP samples. Deformation twins were found in most grains after the first and second pass of ECAP. After four ECAP passes, the original grains were refined from 28 μm to about 250 nm, and the ultimate strength and microhardness were significantly enhanced to 773 MPa and 2486 MPa, respectively. Meanwhile good ductility of 16.8% elongation still remained.

Microstructure and corrosion properties of ultrafine-grained interstitial free steel

The microstructure and the corrosion behaviour of ultrafine-grained interstitial free (IF) ferritic steel processed by equal channel angular pressing (ECAP) at room temperature following route C were investigated. Already after the first pass of ECAP, the microstructure was refined by a factor of approximately 200. On further passes, ECAP was found to lead to continuous grain refinement, while elongated grain structure produced in the first pass was retained throughout all processing cycles. After 8 passes, the microstructure comprised bands of elongated grains of average length of 500–1000 nm and average width of 200–300 nm. Corrosion characteristics of a non-deformed coarse grained specimen and an ultrafine-grained specimen that had undergone 8 passes of ECAP were investigated using electrochemical potentiodynamic tests. ECAP induced grain refinement does not appear to influence the electrochemical characteristics in neutral NaCl solutions. This encouraging finding demonstrates that ECAP, while enhancing mechanical characteristics, does not compromise corrosion resistance of IF steel. A protective oxidic layer is formed in alkaline NaCl solution, which reduces the corrosion rate significantly.

Anisotropic compressive properties of iron subjected to single-pass equal-channel angular pressing

The anisotropic compressive properties and shear deformation mechanism of iron subjected to equal-channel angular pressing (ECAP) with single-pass have been investigated. It was found that the anisotropic compressive properties can be attributed to the effect of the ECAP shear plane. It is suggested that the ECAP shear plane induced by the first pass of ECAP is a relatively weak plane in terms of resisting subsequent shear deformation.

Deformation behavior in Zr702 processed by equal-channel angular pressing at room temperature

The microstructural evolution of commercial parity zirconium Zr702 with an initial grain size of ∼40 μm during equal channel angular pressing (ECAP) was investigated. ECAP was carried out at room temperature via route B C, in which the sample was rotated 90° in the same sense in each pass. After four ECAP passes, the microstructure of Zr702 revealed that the grain size decreased to ∼0.5 μm without fracture. Deformation twins were found in all the grains after the first pass of ECAP. Transmission electron micrographs of the ECAP-processed Zr702 revealed that the strain imposed during ECAP is accommodated mainly by { 1 1 2 ¯ 1 } deformation twinning.