Papers by Keyword: Tensile Ductility

Abstract: The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1.1Al (High-Al) with different N contents of 10 and 30 ppm were selected for this study. In addition, two other DQP steels with compositions: Fe-0.3C-1.0Si (High-Si) and Fe-0.3C-0.5Si-0.5Al (Al-Si), both containing about 30 ppm nitrogen, were also included in the study to compare the properties. Detailed metallographic studies using FESEM-EDS, TEM, EPMA and XRD combined with tensile testing and fractographic analysis indicated that already 30 ppm of nitrogen could impair tensile ductility of TMR-DQP processed High-Al steel in comparison to that with 10 ppm nitrogen. Similarly, the effect was adverse also in Al-Si steel (30 ppm N) despite its reduced Al content (0.5 wt.%), but High-Si steel (Al < 0.002 wt.%, N 30 ppm) did not show any such detrimental effect on tensile ductility. Extensive material characterization verified that even 30 ppm of nitrogen could impair ductility of Al-bearing steels, essentially due to the presence of AlN inclusions, despite that TMR-DQP processing enabled stabilization of 6–10% retained austenite (RA) in the steels. The capacity of RA in promoting improved ductility and strain hardening capacity was impaired by the presence of these inclusions. In contrast, impact toughness transition temperature T_28J was not clearly affected with Al-Si when compared to low-N High-Al steel, although excessive splitting in Al-Si caused pronounced scatter in the results and increase in upper shelf impact toughness.

Abstract: An ECAP (equal channel angular pressing) processed UFG Al-5Cu alloy was characterized by electron backscatter diffraction (EBSD). It is revealed that a bimodal grain structure, i.e. ultrafine grains accompanied by micron-sized grains was developed after 4 passes. A high strength (~501 MPa) and a relatively large elongation to failure (~28%) with ~5% uniform elongation were achieved simultaneously after 4 passes of ECAP. The high strength is due to a combination of strengthening by solute, high density of dislocations and ultrafine grains. The enhancement of uniform elongation is primarily due to the enhanced work hardening resulted from the solute Cu content and the bimodal grain structure. The large post-uniform elongation is attributed to the high strain rate sensitivity of the UFG Al-5Cu alloy. More importantly, the present work revealed that during ECAP high solid solution content of Cu and coarse secondary phase particles can introduce inhomogeneous deformation resulting in a desirable bimodal grain structure, which can be utilized as a strategy to gain both high strength and relatively good ductility.

Abstract: Ultrafine-grained (UFG) binary Al-xMg (x=1, 5 and 7 wt %) alloys were processed by equal channel angular pressing (ECAP) at room temperature via route B_c combined with inter-pass annealing. The effects of Mg content, grain size and strain rate on mechanical properties and dynamic strain aging (DSA) behaviour of the Al-Mg alloys upon tensile testing at room temperature were studied. An increase in Mg content from 5 to 7 wt % leads to a pronounced increase in strength and uniform elongation in both the as-homogenized and as-ECAP Al-Mg alloys. Thereby, the Al-7Mg alloy, either prior to or after ECAP processing, possess significantly higher strength and comparable or even higher uniform elongation than the more dilute Al-Mg alloys. However, the as-ECAP Al-Mg alloys exhibit significantly higher strength but little work hardening and hence rather limited uniform elongation. In general, decreasing grain size leads to significant increase in strength while dramatic decrease in ductility. Moreover, DSA serration amplitudes increase with reducing grain size in the micrometer range. However, the UFG Al-Mg alloys exhibit much less DSA effect than the micrometer scaled grain size counterparts, i.e. probably due to the high dislocation densities and special grain boundary features in the UFG materials. Also, the Al-Mg alloys, especially those with a UFG structure, exhibit higher strength and ductility at lower strain rate than at higher strain rate, due mainly to enhanced DSA effect and hence work hardening at a lower strain rate.

Abstract: Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10^-2 s^-1 and 10^-3 s^-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Abstract: In this paper, ultra-fine grained copper fabricated by equal channel angular pressing method and annealed coarse grained copper were tensioned under both quasi-static and dynamic loading conditions using an electronic universal testing machine and the split Hopkinson tension bar respectively. The rapture surface of specimen was also observed via a Scanning Electron Microscope (SEM). The experimental results show that the ductility of polycrystalline copper decreases remarkably due to the grain refinement. However, with the increase of applied strain rate, ductility of the UFG-Cu is enhanced. The fracture morphologies also give the evidence of enhanced ductility of UFG-Cu at high strain rate. It is believed the enhanced ductility of UFG materials at high strain rate can be attributed to the restrained dislocation dynamic recovery.

Abstract: Tensile properties (mainly the ductility and fracture mode) of two-phase Zn-12Al alloy subjected to severe plastic deformation (SPD) via multi-pass equal-channel angular extrusion (ECAE) following route-Bc were investigated. As a result of ECAE processing, elongation to failure (as a ductility) of the alloy increased substantially and continuously with increasing the number of ECAE passes. However, the majority of the tensile strains are obtained in the state of plastic instability and therefore the uniform strains achieved along the gage length are very limited for this alloy. On the other hand, the strength of the alloy increased with increasing the number of passes up to 2, above which it decreased. The alloy sample after four ECAE passes exhibited 168% total elongation to failure at room temperature, which was 26 times higher than that of the as-cast one. This result indicates that multi-pass ECAE is effective on improving the tensile ductility of binary Zn-Al alloys. The fracture mode of the as-cast alloy samples completely changed after multi-pass ECAE and the brittle fracture behavior of the as-cast alloy was transformed into the ductile mode after processing.

Abstract: Zr and/or Nb added Fe3Al based intermetallic alloys (i.e., Fe3Al-Zr, Fe3Al-Nb and Fe3Al-Zr-Nb) were arc-melted, homogenized, hot-rolled and then annealed to evaluate microstructure and tensile property at room temperature as well as at a high temperature (873K). After annealing, the rolled alloys exhibited a recrystallized microstructure containing coarse second phase particles, except for the Nb-added alloy with a minor content of Nb. Relatively high tensile elongation as well as high tensile strength was observed at room temperature in the Zr-added alloys with a minor amount of Zr. Also, these alloys showed relatively high tensile strength and elongation at high temperature (873K). The results suggest that tensile ductility as well as strength of Fe3Al-based alloys can be improved by introduction of the second phase dispersions.