Determination of lower-bound ductility for AZ31 magnesium alloy by use of the bulge specimens

Despite the high demand for industrial applications of magnesium, the forming technology for wrought magnesium alloys is not fully developed due to the limited ductility and high sensitivity to the processing parameters. The processing window for magnesium alloys could be significantly widened if the lower-bound ductility (LBD) for a range of stresses, temperature, and strain rates was known. LBD is the critical strain at the moment of fracture as a function of stress state and temperature. Measurements of LBD are normally performed by testing in a hyperbaric chamber, which is highly specialized, complex, and rare equipment. In this paper an alternative approach to determine LBD is demonstrated using wrought magnesium alloy AZ31 as an example. A series of compression tests of bulge specimens combined with finite element simulation of the tests were performed. The LBD diagram was then deduced by backward calculation.

Twinning and the ductility of magnesium alloys Part II. “Contraction” twins

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work presents part II of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile and compression tests results are reported for common wrought alloys: AZ31, ZK60 and ZM20. These data are combined with EBSD analysis and simple flow stress models to argue the following: (i) that “contraction” double twinning (which enables contraction along the c axis) can decrease the uniform elongation, and (ii) that compression double twinning can also account for shear failure at low strains. The last of these is described as a combined consequence of strain softening of the continuum and the local generation of twin sized voids.

Twinning and the ductility of magnesium alloys Part I: “Tension” twins

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work is part I of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile test results are reported for the common wrought alloy AZ31. These data are employed in conjunction with a simple constitutive model to argue that View the MathML source twinning (which gives extension along the c-axis) can increase the uniform elongation in tensile tests. This effect appears to be similar to that seen in Ti, Zr and Cu–Si and in the so called TWIP phenomenon in steel.

The origin of "Rare Earth" texture development in extruded Mg-based alloys and its effect on tensile ductility

The extrusion behaviour, texture and tensile ductility of five binary Mg-based alloys have been examined and compared to pure Mg. The five alloying additions examined were Al, Sn, Ca, La and Gd. When these alloys are compared at equivalent grain size, the La- and Gd-containing alloys show the best ductilities. This has been attributed to a weaker extrusion texture. These two alloying additions, La and Gd, were found to also produce a new texture peak with View the MathML source parallel to the extrusion direction. This “rare earth texture” component was found to be suppressed at high extrusion temperatures. It is proposed that the View the MathML source texture component arises from oriented nucleation at shear bands.

Enhanced tensile ductility of an ultra-fine-grained aluminium alloy

Aluminum alloy 6082 was subjected to equal-channel angular pressing (ECAP), which resulted in an ultra-fine-grained (UFG) microstructure with an average grain size of 0.2–0.4 μm. There was a pronounced effect of the grain refinement on the strain-rate sensitivity and tensile ductility. The Hart criterion of tensile necking fails to explain the observed ductility of the UFG material at low strain rates. A correlation between the observed stronger-than-expected ductility and a tendency to microshear band formation at low strain rates was established.

Some issues relating to the ductility of wrought magnesium alloys

The present work is concerned with gaining a better understanding of the factors that control the ductility of wrought magnesium alloys. The ultimate aim is to develop alloys with vastly improved room temperature formability. It is shown that 3D tomography of fractured tensile specimens reveals disk shaped voids aligned more or less at 45 deg. to the tensile axis. These voids are consistent with twin induced void formation. It is also shown that the double twins that produce such voids form in contradiction to Schmid predictions. Finally, it is demonstrated that low levels of rare-earth additions leads to vastly improved texture and ductility in extrusions, as they do in rolled sheet.

Effect of cold drawing on subsequent surface strain path and ductility during upsetting

Cold drawing is a process that sizes and smooths the surface of steel before it is cold headed to produce bolts. The effect of the changes in the mechanical properties due to cold drawing on the surface strain and ductility during the upsetting process was analysed showing that the stress and strain state can be more readily altered by changes in the process conditions (friction and height-to-diameter ratio) to cause greater increase in the failure strains than can be achieved by pre-drawing.

Simultaneously enhanced strength and ductility of titanium via multimodal grain structure

Commercial Ti with a multimodal grain structure was successfully produced using cryorolling, followed by low-temperature annealing. This multimodal grain structure Ti exhibited a combination of high yield strength (926 MPa), a uniform elongation of 11% and a failure elongation of 23%. The strength enhancement was mainly derived from the ultrafine equiaxed grains, while the improved ductility originated from the large fraction of high-angle grain boundaries and the multimodal grain structure.

A Novel Mo and Nb Microalloyed Medium Mn TRIP Steel with Maximal Ultimate Strength and Moderate Ductility

The multi-phase, metastable, and multi-scale (M3) constitution of a novel transformation-induced plasticity (TRIP) steel (Fe-0.17C-6.5Mn-1.1Al-0.22Mo-0.05Nb, wt pct) was designed through thermodynamic calculations combined with experimental analysis. In this study, Mo and Nb microalloying was used to control the fraction of retained austenite and its mechanical stability during tensile deformation and to improve the yield strength. Thermodynamic calculations were developed to determine the critical annealing temperature, at which a large fraction of retained austenite (~38 pct) would be obtained through the effects of solute enrichment. The experimental observation was in good agreement with the predicted results. According to the critical annealing temperature, such an ultrafine (<200 nm) M3, microstructure with optimum mechanical stability was successfully achieved. The results of this work demonstrated the superior performance with improved yield strength of 1020 to 1140 MPa and excellent ductility (>30 pct), as compared with other TRIP steels. Both angle-selective backscatter and electron backscatter diffraction techniques were employed to interpret the transformation from the deformed martensitic laths to the ultrafine austenite and ferrite duplex structure.

Enhanced ductility and reduced asymmetry of Mg-2Al-1Zn alloy plate processed by torsion and annealing

The ductility and plastic asymmetry of an as-annealed magnesium alloy plate were studied in compression through combined process of torsion and subsequent annealing by optical microscope and EBSD. The yield strength (YS) and ultimate compression strength (UCS) as well as the compression ductility (CD) were simultaneously raised by prior torsion at room temperature. The CD was further enhanced by subsequent annealing. Also, the torqued sample followed by annealing experienced a rising CD with the increase in prior strain, leading to the maximum true strain of 0.279, which is twice that of the as-annealed original one. The sample showed a largely reduced tension-compression yield asymmetry by subjecting to pre-torsion alone or combined with a subsequent annealing. The enhanced ductility and reduced asymmetry are attributed to the development of a gradient microstructure with refined grains, and also randomization of the weakened texture due to torsion and subsequent annealing.

Experimental study on strength and ductility of steel tubular stub columns filled with geopolymeric recycled concrete

Geopolymeric recycled concrete (GRC) is a new construction material which takes environmentalsustainability into account, by using alkali solution and fly ash to completely substitute Portland cementas well as by replacing natural coarse aggregate with recycled coarse aggregate. GRC could be used togetherwith steel hollow sections to form composite section. There is very limited study on such GRC filledtubular sections. This paper presents an experimental study on GRC filled tubular stub columns. A total of 12specimens were tested. The main parameters varied in the tests are: (1) two section sizes of square hollow sections(B × t) with 200mm×6mm and 150mm×5mm; (2) different concrete types: GRC and recycled aggregateconcrete (RAC); (3) different recycled aggregate (RA) replacement ratios of 0%, 50% and 100%. The relationshipof load versus axial strain was recorded and analysed to compare the ultimate strength and failuremechanism. Meanwhile, the ductility of the columns was investigated by a ductility index (DI). The resultsshow that the ultimate strength decreased with increasing RA contents for both GRC and RAC filled columns.The influence of RA content on the strength was greater in GRC than that in RAC. The effect of RA contenton the ductility of the columns was further investigated. Simulation method for predicting load versus strainrelationship is discussed for RAC and GRC filled steel tubular columns with different RA replacement ratios.