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- MakaleAçık Erişim2018Scopus SJRQ3Effect of long-term natural aging on microstructure and room temperature superplastic behavior of UFG/FG Zn-Al alloys processed by ECAP
Demirtas, M.; Yanar, H.; Purcek, G.
Two potential superplastic compositions of Zn-Al alloy systems, Zn-22Al and Zn-0.3Al alloys, were chosen and processed by equal-channel angular pressing/extrusion (ECAP/E) in order to achieve high strain rate (HSR) superplasticity at room temperature (RT). ECAP-processed samples of both alloys were then subjected to long-term natural aging up to 1100 days to evaluate the effect of long-term natural aging on their microstructures and superplastic behaviors. Before natural aging, the maximum elongations to failure at RT were 400% for ultrafine-grained (UFG) Zn-22Al at the strain rate of 5 × 10 −2 s −1 and 1000% for fine-grained (FG) Zn-0.3Al at the strain rate of 1 × 10 −4 s −1 . Long-term natural aging did not cause a significant change in the elongation of UFG Zn-22Al alloy with 355% maximum elongation. However, optimum strain rate giving the maximum elongation decreased to 3 × 10 −3 s −1 . On the other hand, Zn-0.3Al alloy lost more than half of its superplastic elongation and showed an elongation to failure of 435% at the end of the natural aging period of 1100 days. Microstructural analyses show that grain boundary corrosion occurred in dilute Zn-0.3Al alloy during the natural aging process. Corroded grain boundaries resulted in cavity nucleation during the tensile tests and some of these cavities attained large sizes and caused premature failure.
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İncele - MakaleYalnız künye2018Scopus SJRQ1Influence of high pressure torsion-induced grain refinement and subsequent aging on tribological properties of Cu-Cr-Zr alloy
Purcek, G.; Yanar, H.; Shangina, D.V.; Demirtas, M.; Bochvar, N.R.; Dobatkin, S.V.
Effects of ultrafine grain formation via high pressure torsion (HPT) and precipitation during aging on the microstructure, mechanical and tribological properties of a Cu-Cr-Zr alloy have been investigated systematically. HPT results in the formation of ultrafine-grained (UFG) structure in the alloy with an average grain/subgrain size of 155 nm which leads to remarkable improvement in its hardness and strength along with a reduction in elongation to failure. Aging of UFG alloy brings about further strengthening due to the precipitation. UFG formation by HPT increases substantially the wear resistance of Cu-Cr-Zr alloy and reduces the friction coefficient. The highest wear resistance and the lowest friction coefficient are obtained on the sample processed by HPT and subsequent aging. The dominant wear mechanism of the alloy varies depending on the applied processes. Adhesion with smearing is the predominant wear mechanism for the initial (warm extruded) samples having coarse-grained (CG) structure. UFG samples show less adhesional effect with less smearing, and a higher tendency to the formation of cracks, abrasion and delamination seem to be dominant in those samples. Oxidative wear mechanism is also operative in both CG and UFG alloy samples. It may be concluded from this study that a combined process including UFG formation by HPT and subsequent precipitation by artificial aging provides a simple and effective processing procedure for improving the strength, hardness and wear resistance of Cu–Cr–Zr alloys without modification of the chemical composition.
AtıfScopus96WoS91OpenAlex99İncele - MakaleYalnız künye2018Scopus SJRQ1High temperature superplasticity and deformation behavior of naturally aged Zn-Al alloys with different phase compositions
Demirtas, M.; Kawasaki, M.; Yanar, H.; Purcek, G.
Two Zn-Al alloys, eutectoid Zn-22Al and dilute Zn-0.3Al, were processed through equal-channel angular pressing (ECAP). The ECAP-processed alloys were subjected to long-term natural aging at room temperature for 120 days in order to investigate the effect of long-term natural aging on high-temperature superplasticity and the deformation behavior of these alloys. The grain size of the Zn-22Al alloy increased from 200 nm after ECAP to 400 nm after the natural aging process. The Zn-0.3Al alloy with a grain size of 1.3 µm after ECAP demonstrated a reasonably stable microstructure with the grain size of ~ 1.7 µm after long-term natural aging. The naturally aged ultrafine-grained (UFG) Zn-22Al alloy showed an excellent superplastic elongation of about 1800% at 200 °C. Grain boundary sliding was determined as the dominant deformation mechanism in this alloy. On the contrary, a considerably low elongation to failure of 230% was recorded in the naturally aged fine-grained (FG) Zn-0.3Al alloy at 150 °C, and dislocation creep was evaluated as the main deformation mechanism for this alloy. Detailed microstructural analyses showed that the FG Zn-0.3Al alloy was exposed to grain boundary corrosion during the natural aging process due to the presence of Al precipitates at grain boundaries whereas such formation was not observed in the UFG Zn-22% Al alloy. Grain boundary sliding was hindered by grain boundary corrosion and the cavity nucleation at the corroded grain boundaries caused a premature failure leading to such a low elongation in the Zn-0.3Al alloy.
AtıfScopus30WoS30OpenAlex33İncele - MakaleAçık Erişim2017Scopus SJRQ1Enhancing the Damping Behavior of Dilute Zn-0.3Al Alloy by Equal Channel Angular Pressing
Demirtas, M.; Atli, K.C.; Yanar, H.; Purcek, G.
The effect of grain size on the damping capacity of a dilute Zn-0.3Al alloy was investigated. It was found that there was a critical strain value (≈1 × 10−4) below and above which damping of Zn-0.3Al showed dynamic and static/dynamic hysteresis behavior, respectively. In the dynamic hysteresis region, damping resulted from viscous sliding of phase/grain boundaries, and decreasing grain size increased the damping capacity. While the quenched sample with 100 to 250 µm grain size showed very limited damping capacity with a loss factor tanδ of less than 0.007, decreasing grain size down to 2 µm by equal channel angular pressing (ECAP) increased tanδ to 0.100 in this region. Dynamic recrystallization due to microplasticity at the sample surface was proposed as the damping mechanism for the first time in the region where the alloy showed the combined aspects of dynamic and static hysteresis damping. In this region, tanδ increased with increasing strain amplitude, and ECAPed sample showed a tanδ value of 0.256 at a strain amplitude of 2 × 10−3, the highest recorded so far in the damping capacity-related studies on ZA alloys.
AtıfScopus8WoS7OpenAlex7İncele - BildiriAçık Erişim2017Optimization of RT superplasticity of UFG Zn-22Al alloy by applying ECAP at different temperatures and phase regions
Demirtas, M.; Yanar, H.; Purcek, G.; Estrin, Y.; Liao, X.
Zn-22Al alloy was subjected to either one-step or two-step equal channel pressing (ECAP) to investigate the effect of processing temperature on its microstructure and room temperature (RT) superplasticity. In one-step ECAP processes, 4 passes ECAP were applied to the alloy at different temperatures: RT, 100°C and 250°C in two-phase region below eutectoid temperature and 350°C in single-phase region above eutectoid temperature. In two-step ECAP processes, one-step ECAP-processed samples were subjected to four more passes ECAP at RT. Considering the one-step ECAP processing, RT superplasticity increased with decreasing ECAP temperature as expected, and the highest RT superplasticity was achieved as 350% after 4 passes ECAP at RT. On the other hand, application of 4 more passes ECAP at RT to the sample showing the lowest superplastic elongation after one-step ECAP (the sample processed at 350°C) resulted in the maximum RT elongation of 400% at a high strain rate of 5×10-2 s-1. These results suggest that first step temperature of two-step ECAP process is needed to increase above the eutectoid point of Zn-22Al alloy to achieve high RT superplasticity. These results were attributed to the changes in microstructure inside the single-phase and two-phase regions during the processes.
AtıfScopus12WoS10OpenAlex9İncele - MakaleYalnız künye2016Scopus SJRQ1Optimization of strength, ductility and electrical conductivity of Cu-Cr-Zr alloy by combining multi-route ECAP and aging
Purcek, G.; Yanar, H.; Demirtas, M.; Alemdag, Y.; Shangina, D.V.; Dobatkin, S.V.
Properties of Cu-Cr-Zr alloy with ultrafine-grained (UFG) structure produced by equal-channel angular pressing (ECAP) via different routes have been investigated. Special attention was paid to the optimization of multi-functional structural, thermal, electrical and mechanical properties of the alloy by aging of UFG one. Multi-pass ECAP via different routes gives rise to the formation of a deformation-induced submicrocrystalline structure with the grain (subgrain) sizes in the range of 200-300. nm depending on applied routes which leads to high hardness and strength in the Cu-Cr-Zr alloy with reduced ductility. Amongst the applied routes, route-Bc was found to be the best processing path for achieving the lowest grain size, the highest hardness and strength. Aging of 8Bc-processed UFG samples increases the hardness and strength of Cu-Cr-Zr alloy while retaining an electrical conductivity comparable to that of aged coarse-grained (CG) one. A satisfactory electrical conductivity of 71%IACS without considerable loss of peak hardness was achieved after aging of 8Bc-processed UFG alloy at 425. °C for 240. min. The precipitation strengthened UFG alloy remains its stable behavior at elevated temperatures up to 450. °C.
AtıfScopus164WoS147OpenAlex151İncele - MakaleAçık Erişim2016Scopus SJRQ1Effects of grain size on room temperature deformation behavior of Zn–22Al alloy under uniaxial and biaxial loading conditions
Cetin, M.E.; Demirtas, M.; Sofuoglu, H.; Cora, Ö.N.; Purcek, G.
Effects of grain size on room temperature (RT) deformation behavior of superplastic Zn–22Al alloy under uniaxial and biaxial loading conditions were investigated. Two–step equal channel angular pressing (ECAP) and subsequent annealing processes were applied to the alloy in order to obtain microstructures with various grain sizes ranging from submicron to micron sizes. Grain size of 200 nm was achieved after ECAP of the alloy, and it was increased up to ~2.60 µm by annealing at 250 °C for different time periods. Changes in deformation behaviors of the alloy with various grain sizes were found to be in good agreement under the uniaxial tensile and biaxial Erichsen test conditions. Increasing grain size decreased both the maximum elongation under uniaxial deformation and limiting dome height (LDH) under biaxial deformation. The high elongation to failure and LDH values for the samples with submicron grain size were attributed to their high strain rate sensitivity.
AtıfScopus20WoS19OpenAlex21İncele - MakaleAçık Erişim2016Scopus SJRQ1Effect of different processes on lamellar-free ultrafine grain formation, room temperature superplasticity and fracture mode of Zn-22Al alloy
Demirtas, M.; Purcek, G.; Yanar, H.; Zhang, Z.J.; Zhang, Z.F.
The Zn-22 wt.%Al was processed using five different selected routes including thermal, thermo-mechanical and severe plastic deformation (SPD) techniques in order to produce ultrafine grained (UFG) microstructure for achieving room temperature (RT) and high strain rate (HSR) superplasticity, and to compare the same set of the results. After all routes, the microstructural evolutions and RT uniaxial tensile tests at different strain rates ranging from 1 × 10-3 to 1 × 100 s-1 were investigated. The smallest grain size was achieved to be 300 nm after the thermo-mechanical process including conventional hot and cold rolling steps. However, the lamellar microstructure of the alloy was not completely eliminated during that process. On the other hand, equal-channel angular pressing (ECAP) as one of the SPD techniques decreased the grain size down to 400 nm with a lamellar-free microstructure. The maximum elongation value was achieved to be 330% with the sample tested at 1 × 10-3 s-1 after a thermo-mechanical process including conventional hot and cold rolling, and an aging stage between these rolling steps. It was found that lamellar-free microstructure causes higher superplastic elongation even if it has higher grain size compared to the sample having partially lamellar structure.
AtıfScopus40WoS39İncele - MakaleYalnız künye2015Scopus SJRQ3Effect of Chemical Composition and Grain Size on RT Superplasticity of Zn-Al alloys processed by ECAP
Demirtas, M.; Purcek, G.; Yanar, H.; Zhang, Z. J.; Zhang, Z. F.
Dilute Zn-0.3Al, eutectic Zn-5Al and eutectoid Zn-22Al alloys were processed by multi-pass equal channel angular pressing (ECAP) in order to achieve fine grained (FG) or ultrafine-grained (UFG) microstructure and room temperature (RT) superplasticity. ECAP refined the microstructure of Zn-0.3Al and resulted in a FG Zn-rich.-matrix with an average grain size of 2 mu m and homogeneously distributed nano-sized Al-rich a-particles with the grain sizes in the range of 50-200 nm. A bi-modal microstructure was achieved in Zn-5Al alloy with UFG Al-rich a-and FG Zn-rich.-phases having mean grain sizes of 110 nm and 540 nm, respectively. ECAP brought about an agglomerate-free UFG microstructure in Zn-22Al alloy with an average grain size of 200 nm which is the lowest one obtained so far for this alloy after ECAP processing. The maximum RT superplastic elongations of 1000%, 520% and 400% were achieved for Zn-0.3Al, Zn-5Al and Zn-22Al alloys, respectively. Considering the RT superplasticity in Zn-Al alloys, it was found that lower Al content results in higher superplastic elongations even if the alloy has relatively larger grain size. Grain boundary sliding (GBS) was found to be the main deformation mechanism in region-II as the optimum superplastic region during RT deformation for all three Zn-Al alloys with the strain rate sensitivity factor ranging between 0.25-0.31.
AtıfWoS19İncele - MakaleAçık Erişim2015Scopus SJRQ1Improvement of high strain rate and room temperature superplasticity in Zn-22Al alloy by two-step equal-channel angular pressing
Demirtas, M.; Purcek, G.; Yanar, H.; Zhang, Z.J.; Zhang, Z.F.
The Zn-22Al alloy was subjected to equal-channel angular pressing (ECAP) to improve its high strain rate (HSR) superplasticity at room temperature (RT). A well-designed two-step ECAP process formed an ultrafine-grained (UFG) microstructure with an average grain size of 200nm as the lowest one obtained so far after ECAP processing of this alloy. Also, agglomerate- and texture-free microstructure with UFG Al-rich α- and Zn-rich η-grains separated mostly by high-angle grain boundaries (HAGBs) was produced by this process. The maximum RT elongation was achieved to be 400% with a strain rate sensitivity of 0.30 at a very high strain rate of 5×10-2s-1 after the two-step ECAP process. This elongation value is the highest one obtained at RT and at all strain rates for this alloy up to now. The current results demonstrate that such an improvement in superplasticity of Zn-22Al alloy after the two-step ECAP process can enhance its applications where RT and HSR superplasticity are strongly needed.
AtıfScopus48OpenAlex48İncele - MakaleYalnız künye2015Scopus SJRQ1Effect of equal-channel angular pressing on room temperature superplasticity of quasi-single phase Zn-0.3Al alloy
Demirtas, M.; Purcek, G.; Yanar, H.; Zhang, Z.J.; Zhang, Z.F.
Quasi-single phase (dilute) Zn-0.3Al alloy was subjected to severe plastic deformation via equal-channel angular extrusion/pressing (ECAE/P), and the effects of ECAP on its room temperature (RT) and high strain rate (HSR) superplasticity and deformation mechanism were investigated. Multi-pass ECAP may refine the coarse-grained microstructure into the fine grained (FG) one. The grain size of Zn-matrix phase decreased down to 2.0m after ECAP. Many spherical Al-rich precipitates decomposed and homogeneously distributed inside the matrix phase. They are ultrafine grained (UFG) α-particles with the grain sizes ranging from 50nm to ~200nm. This special microstructure having FG and UFG micro-constituents brought about an improvement in RT superplasticity even at HSRs. While multi-pass ECAP decreased flow stress of the alloy, its elongation to failure increased substantially depending on the initial strain rates. The maximum elongation was 1000% at a low strain rate of 10-4s-1, and 350% elongation was achieved at a high strain rate of 10-2s-1. Grain boundary sliding (GBS) was found to be the main deformation mechanism in region-II as the optimum superplastic region.
AtıfScopus60WoS59OpenAlex55İncele - MakaleAçık Erişim2015Scopus SJRQ1Achieving room temperature superplasticity in Zn-5Al alloy at high strain rates by equal-channel angular extrusion
Demirtas, M.; Purcek, G.; Yanar, H.; Zhang, Z.J.; Zhang, Z.F.
Multi-pass equal-channel angular extrusion/pressing (ECAE/P) was applied to the eutectic Zn-5Al alloy to achieve high strain-rate (HSR) superplasticity in that alloy at room temperature (RT) by producing ultrafine-grained (UFG) microstructure. ECAE processing transformed the coarse-grained lamellar/spherical microstructure into a unique bimodal structure having equiaxed Zn-rich η-phase with a mean grain size of 540 nm and spherical Al-rich α-phase with an average grain size of 110 nm. The α-phase particles accumulated mainly along the η-phase boundaries. This unique microstructure brought about an extraordinary improvement in HSR superplasticity of the alloy even at RT. While the strength values decreased after ECAE, the elongation to failure increased substantially. The maximum elongation was 520% at the strain rate of 10-3 s-1, still high elongation of about 400% was achieved at a high strain rate of 10-2 s-1. This extraordinary improvement in HSR superplasticity of Zn-5Al alloy was attributed to the morphologically unique bimodal microstructure in UFG regime formed after ECAE. The grain boundary sliding (GBS) was found to be the main deformation mechanism for this alloy in superplastic regime.
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