1,720,991 research outputs found
Ultra-fine grained degradable magnesium for biomedical applications
No full textProperties of commercially available purity magnesium and wrought ZM21 Mg alloy were investigated in view of their
biodegradable applications. In particular, the opportunities offered by grain size refinement down to the ultra-fine scale achieved by
equal channel angular pressing (ECAP) and warm extrusion were discussed and material properties were analyzed. Results show that
the grain refinement will lead to a significant improvement in compression strength. The tension strength of the coarse grained alloy
is always significantly higher than that measured in compression due to the sharp texture of the starting wrought alloy. ECAP also
causes an attenuation of the above texture effects, promoting marked changes in plastic flow behavior. The corrosion behavior of the
investigated materials are affected by a combination of microstructural effects such as chemistry, grain size and the extent of lattice
distortion inherited from previous processing stages. ECAP leads to refinement of grain size and to increased lattice defect density
which apparently produce counterbalancing effects on corrosion performance. The improved dispersion of second-phase particles
brings positive effects on development of pitting
Formability enhancement of Al 6060 sheets through fiber laser heat treatment
Full text linkDue to the continuous weight reduction effort in the automotive sector, formability enhancement of aluminum alloys in forming and hydroforming processes is gathering much attention from research institutes and industries. During sheet forming processes, large deformations are desired to obtain complex shapes but these are limited by the appearance of defects such as wrinkling and cracks. To avoid these issues, intermediate annealing heat treatments are often applied as a possible solution. Nevertheless in large components where small details have to be created, local heat treatment through lasers can be cost effective over the furnace treatment of the whole part and it would limit possible geometrical distortion in large components. The following article presents fiber laser process parameters definition on deformed sheets made of Al6060 alloy. Grain structure variation and hardness decrease were studied to correctly select process parameters (laser power, feeding speed and overlapping among subsequent passes) to increase material formability. In addition, a systematic comparison between fiber laser and furnace heat treatment was assessed proving the equivalence of the two methods in terms of achieved mechanical proprieties
Grain size and texture dependence on mechanical properties, asymmetric behavior and low temperature superplasticity of ZK60 Mg alloy
Full text linkThe mechanical properties of ZK60 alloy were characterized as function of texture and grain size. An experimental methodology was designed by exploiting ECAP and annealing treatments on an extruded alloy, through which the individual effects of the mentioned parameters could be investigated. Microstructural observations revealed a significant grain refinement through ECAP processing, leading to an equiaxed ultrafine grain (UFG) structure with an average size of 500 nm. The initial extrusion fiber texture was gradually evolved into a new and more intense one featuring the preferential alignment of the basal planes along the ECAP shear planes with considerably higher intensity and Schmid factor. After annealing, the UFG structure was replaced by a structure close to that of the extruded condition. However, the basal texture orientation was only slightly modified. Tensile and compression tests at room temperature showed that regardless of grain size, fracture elongation and yield asymmetry were strongly influenced by basal texture orientation. Nevertheless, tensile tests at 200 degrees C showed that the flow stress was considerably texture dependent, whereas the fracture elongation was mainly dictated by the grain size
Microstructure, mechanical behavior and low temperature superplasticity of ECAP processed ZM21 Mg alloy
Full text linkIn this study, ultra-fine grained ZM21 Mg alloy was obtained through two-stage equal channel angular pressing process (ECAP) at temperatures of 200 and 150 degrees C. For each stage four passes were used. Plastic behavior, mechanical asymmetry and low temperature superplasticity of ultra-fine grained ZM21 alloy were investigated as a function of processing condition with particular attention to microstructural and texture evolution. Microstructural observations showed that after the first stage of ECAP an equiaxed ultra-fine grain (UFG) structure with average size of 700 nm was obtained. Additional stage did not cause any further grain refinement. However, Electron Backscattered Diffraction analysis showed that the original extrusion fiber texture evolved into a new one featuring a favorable alignment of the basal planes along ECAP shear planes. Such a preferential alignment provided a considerably higher Schmid factor value of 0.32, resulting in a remarkable loss in tensile yield stress, from 212 to 110 MPa and an improvement of the tensile fracture elongation, from 24% to 40%. Tensile and compression tests at room temperature revealed that yielding asymmetry could be alleviated by either weakening of basal plane fiber texture or by grain refinement. Tensile tests at 150 degrees C showed that texture supplies a significant contribution to plastic flow and elongation, making dislocation slip the dominant mechanism for deformation, while grain boundary sliding was not actively operated at this temperature. However, at 200 degrees C the effect of texture on fracture elongation of UFG alloys was subtle and the impact of grain size became more important. Hence, UFG samples exhibited maximum elongation values exceeding 370% at a strain rate of 5.0 x 10(-4) s(-1), confirming that the flow stress has notable texture dependence, while superplastic ductility was strongly influenced by grain size, being detectable only in UFG samples
Influence of ECAP process on mechanical and corrosion properties of pure Mg and ZK60 magnesium alloy for biodegradable stent applications
Full text linkEqual channel angular pressing (ECAP) was performed on ZK60 alloy and pure Mg in the temperature range 150-250 °C. A significant grain refinement was detected after ECAP, leading to an ultrafine grain size (UFG) and enhanced formability during extrusion process. Comparing to conventional coarse grained samples, fracture elongation of pure Mg and ZK60 alloy were significantly improved by 130% and 100%, respectively, while the tensile strength remained at high level. Extrusion was performed on ECAP processed billets to produce small tubes (with outer/inner diameter of 4/2.5 mm) as precursors for biodegradable stents. Studies on extruded tubes revealed that even after extrusion the microstructure and microhardness of the UFG ZK60 alloy were almost stable. Furthermore, pure Mg tubes showed an additional improvement in terms of grain refining and mechanical properties after extrusion. Electrochemical analyses and microstructural assessments after corrosion tests demonstrated two major influential factors in corrosion behavior of the investigated materials. The presence of Zn and Zr as alloying elements simultaneously increases the nobility by formation of a protective film and increase the local corrosion damage by amplifying the pitting development. ECAP treatment decreases the size of the second phase particles thus improving microstructure homogeneity, thereby decreasing the localized corrosion effects
Effects of texture and grain size on mechanical properties of AZ80 magnesium alloys at lower temperatures
Full text linkIn order to investigate the effect of texture and grain size on mechanical properties of AZ80 magnesium alloy at lower temperatures, ECAP was conducted for 1, 2 and 4 passes at 523 K. Tensile and compressive tests were carried out on the ECAP processed samples at roomtemperature, 373 K and 423 K, respectively. The results showed that a significant grain refinement took place and the original extrusion fiber texture evolved into a new preferred crystal orientation, featuring a favorable alignment of the basal planes along shear planes after ECAP process.
At room temperature grain refinement strengthening played an important role, leading to an improvement of mechanical properties with increasing number of ECAP passes. However, at higher temperature, texture and grain boundary sliding (GBS) mechanisms controlling deformation behaviors, resulting in a softening effect and considerable fracture elongation improvement, as well as yield asymmetry reduction
The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes
Full text linkIn order to investigate the effect of twinning–detwinning on the mechanical properties of AZ31 extruded
magnesium alloy pre-compression and pre-stretch deformation were conducted along extrusion direction
(ED) at 1%, 3%, 5% strain levels. After pre-strain, the strain-path was inverted by performing tensile
or compressive tests at room temperature. Results showed that the detwinning behavior occurred during
the inverse tension after the pre-compression. Although due to the aforementioned effect the tensile
yield strength decreased, by increasing the pre-compressive levels both fracture elongation and peak
strength improved. In the inverse compressive tests after pre-stretch the {1012} twinning was
restrained and the volume fraction of twins decreased, leading to the improvement of yield strength
by increasing in pre-stretching levels
Formability and anisotropy of the mechanical properties in commercially pure titanium after various routes normal and different speed rolling
No full textVarious routes (unidirectional, cross, and three directions) normal and different speed rolling (DSR) are conducted on pure titanium sheet at 673 K and sequent 933 K annealing is followed. The results show that transverse direction (TD)-split double peak texture is kept during unidirectional rolling and a fiber basal texture is formed after cross and three-direction rolling. However, TD-split texture is preserved and rotates about 45° while the fiber basal texture is generated after cross and three direction rolling combining (DSTDR) DSR, respectively. This may
be related to the changed strain path and induced shear deformation as well as thermal activation. Due to rotation of grains, the anisotropy of mechanical properties of Ti sheets decreases, especially in various DSR routes. Erichsen value is improved greatly in DSTDR specimens
- …
