In the present study, a coarse grained AZ31 plate was refined by twin boundaries (TBs) and grain boundaries (GBs), respectively. A comparative study about the different effects of grain refinements ...by GBs and by TBs on tension-compression yield asymmetry was performed. Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths, but to a different degree. TBs are more effective to harden twinning, but yield a lower strengthening against prismatic slip, and a much lower tension-compression yield asymmetry is thus obtained. Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening. The misorientation of TBs provides a lower geometric compatibility factor (a higher hardening) for both prismatic slip and twinning than that of GBs, which in detail is the result of the much higher angle between c-axes of the two sides of TBs (about 86°) than GBs (0-50°). It is found that, for hardening of prismatic slip, boundary coherence plays a more important role than misorientation. With regard to twinning, the different misorientation of TBs from GBs mainly accounts for their different hardening effects.
Development of strontium releasing implants capable of stimulating bone formation and inhibiting bone resorption is a desirable solution for curing osteoporosis. In this work, well-ordered SrTiO3 ...nanotube arrays capable of Sr release at a slow rate and for a long time are successfully fabricated on titanium by simple hydrothermal treatment of anodized titania nanotubes. This surface architecture combines the functions of nanoscaled topography and Sr release to enhance osseointegration while at the same time leaving space for loading of other functional substances. In vitro experiments reveal that the SrTiO3 nanotube arrays possess good biocompatibility and can induce precipitation of hydroxyapatite from simulated body fluids (SBF). This Ti-based implant with SrTiO3 nanotube arrays is an ideal candidate for osteoporotic bone implants. The proposed method can also be extended to load other biologically useful elements such as Mg and Zn.
In magnesium and its alloys,
{
10
1
¯
2
}
tension twinning is an important deformation mode and is highly dependent on the strain path. Although the
{
10
1
¯
2
}
-twinning behavior has been ...extensively modeled, the effects of twinning models on the predicted results has seldom been compared. In this study, two typical twinning models, predominant twin reorientation (PTR) and twinning-detwinning (TDT), were chosen to simulate the
{
10
1
¯
2
}
twinning-predominant deformations of a Mg alloy AZ31 rolled plate, in compression along the transverse direction (TD-c) and in tension along the normal direction (ND-t), and the results were compared with experimental data. In addition to the strain-stress curves in the ND-t and TD-c, six other flow curves were used to determine the material-parameter inputs for the simulations with the elastic visco-plastic self-consistent (EVPSC) model. Compared with the PTR model, the TDT model permits better curve fitting and texture prediction. The PTR model cannot fit the TD-c and ND-t flow stresses simultaneously, whereas the TDT model can. The best-fit parameters for the two models are identical at low strains but diverge somewhat at high strains. The simulated twin volume fractions are similar in the two models, but the predicted textures are significantly different. The PTR model can only reproduce the texture at strains over 5 pct in the TD-c and cannot reproduce the deformed texture in the ND-t. In contrast, the TDT model can reproduce all the experimental textures. To fit both the compression and tension curves well, strong latent hardening of the critical resolved shear stress (CRSS) for
{
10
1
¯
2
}
twinning by other twinning systems (
h
tt
) is necessary. The
h
tt
favors the twin variant with the highest Schmid factor in compression. The
h
tt
increases the CRSS for all
{
10
1
¯
2
}
twinning systems in tension, but the CRSS for the dominant twinning system remains relatively low in compression.
In the present study, microstructure and mechanical behavior of a Mg/Al composite rod with a soft Mg AZ31 sleeve and an ultra hard Al 7050 core were systematically studied, with the aim to disclose ...the influence of Al core on microstructure and texture of Mg sleeve and that of Al fraction on mechanical behavior. Our results show that, compared to extrusion of a monolithic Mg billet, co-extrusion of the Mg/Al bimetal billets does not change the typical extrusion texture of Mg sleeve, but can effectively refine the grains in Mg sleeve. The Mg/Al composite has much higher tensile or compressive yield strengths than the monolithic Mg rod, meanwhile a similar plasticity. With the Al fraction up to 33.8%, the tensile yield strength increases by about 61% and compressive yield strength by about 214%. The Mg/Al rod also possesses a much lower tension-compression yield asymmetry than the monolithic Mg rod. A higher Al fraction further reduces this yield asymmetry. Both the inclusion of a hard Al core and the grain refinement in Mg sleeve contribute to the quite high strength and the much lower yield asymmetry in Mg/Al rod. The Al fraction hardly affects {101¯2} twinning activity in Mg sleeve.
In the present study, a pre-treatment is employed to tailor the recrystallization behavior of a Mg–3Al–1Zn (AZ31) plate that contains {101¯2} twins. When the {101¯2} twins are generated by a ...transverse direction (TD) compression directly from a hot rolled plate, those twins have a quite high thermal stability and can be totally removed only after an annealing at 350°C for 2h. A severe grain coarsening takes place, too. In contrast, if a 5.5% pre-compression along the normal direction (ND) prior to the formation of {101¯2} twins is conducted, the stored energy increases and recrystallization behavior changes. Our results show that the 5.5% ND pre-compression does not change the main deformation mode during the subsequent TD compression and hardly reduces the compressive plasticity along the TD. The stored energy generated by the ND pre-compression results in extensive nucleation at grain boundaries and increases the thermal mobility of {101¯2} twin boundaries. A fully recrystallized structure with the refined grains is achieved after an annealing at 250° for 1h and orientations of the {101¯2} twins are well kept after recrystallization.
The present work reports the effect of annealing treatment on detwinning deformation in Mg alloy AZ31 and pure Mg that have pre-strained twins. It shows that appropriate annealing enhances, rather ...than reduces, the yield strength of the pre-strained AZ31, but it does not cause any strengthening of the pre-strained pure Mg. STEM–EDS mapping shows that both Al and Zn segregate to twin boundaries in the pre-strained AZ31 after the annealing process. It is proposed that it is the pinning of twin boundary by segregated solute atoms that results in an increased activation stress for detwinning deformation and, hence, annealing hardening.
Various electrochemical approaches, including potentiodynamic polarization, open circuit potential evolution and electrochemical impedance spectroscopy (EIS), are employed to investigate the ...degradation behavior of biomedical magnesium alloy under the influence of aggressive ions, such as chloride, phosphate, carbonate and sulfate, in a physiological environment. The synergetic effects and mutual influence of these ions on the degradation behavior of Mg are revealed. Our results demonstrate that chloride ions can induce porous pitting corrosion. In the presence of phosphates, the corrosion rate decreases and the formation of pitting corrosion is significantly delayed due to precipitation of magnesium phosphate. Hydrogen carbonate ions are observed to stimulate the corrosion of magnesium alloy during the early immersion stage but they can also induce rapid passivation on the surface. This surface passivation behavior mainly results from the fast precipitation of magnesium carbonate in the corrosion product layer that can subsequently inhibit pitting corrosion completely. Sulfate ions are also found to stimulate magnesium dissolution. These results improve our understanding on the degradation mechanism of surgical magnesium in the physiological environment.
In this work, the microstructure and texture evolution of an as-cast Mg-2Zn-2Gd alloy subjected to different hot rolling regimes (cross rolling (CR) and unidirectional rolling (UR), CR+UR and UR+UR, ...respectively) and subsequent annealing at 400 °C were systematically investigated. The hot CR helps to develop relatively homogeneous distribution of (0002) poles in a circle inclining about 40° away from the ND in the annealed sample. However, this type of texture cannot be retained after subsequent unidirectional rolling along the RD and annealing, forming a texture with (0002) poles inclining approximately 40° from the ND toward the TD. It is found that the (0002) poles of recrystallized grains generally distribute along the direction perpendicular to the rolling direction, irrespective of the orientations of parent grains. There is a intense in-plane anisotropy of yield strength in plates containing a TD-tilted texture, while the homogeneous distribution of (0002) poles along the TD and RD can completely remove this planar mechanical anisotropy.
The high ductility of Mg-Li alloy has been mainly ascribed to a high activity of pyramidal <c+a> slip to accommodate plastic strain. In the present study, however, a quantitative analysis reveals ...that Li-addition can only slightly stimulate the activation of pyramidal <c+a> slip under compression along the normal direction of a hot-rolled Mg-4.5 wt.% Li plate, with a relative activity of approximately 18%. Although the limited activity of pyramidal <c+a> slip alone cannot accommodate a large plastic strain, it effectively reduces the number of {101¯1}−{101¯2} double twins, which are believed to be favorable sites for crack initiation. The evidently reduced activity of double twins leads to a lower cracking tendency, and therefore improves ductility.
The effects of hybrid aging and solution treatments on the degradation of bio-degradable die-cast AZ63 magnesium alloy in 37
±
1
°C Tyrode's simulated body fluid have been investigated. The heat ...treatment is observed to alter the microstructure of the alloy. The amount of β-Mg
17Al
2 precipitates is larger and their distribution is more homogeneous. The homogeneous microstructure enhances the corrosion resistance of the alloy and the best corrosion rate achieved on the aged sample is approximately 1/2 of that of the untreated alloy. The corrosion mechanism of the aged alloy is dominated by filiform and pitting corrosion. Better understanding and control of surface corrosion will expedite the use of Mg alloys in biomedical implants.
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