►
HCO
3
-
in SBF stimulates corrosion of magnesium by consumption of OH
−. ►
HCO
3
-
suppresses dissolution of magnesium by formation insoluble carbonates. ► The multiple effects are balanced by
HCO
...3
-
concentrations. ► SBF with 27
mmol/L of
HCO
3
-
is more suitable for
in-vitro studies.
Corrosion behaviour of pure magnesium in simulated body fluids (SBF) with HCO
3
− concentrations of 4, 15 and 27
m
mol/L is studied. Magnesium is not sensitive to pitting corrosion in all the SBFs. Higher HCO
3
− concentration effectively slow down the corrosion rates. Uniform and compact corrosion product layer preferentially forms in SBF with HCO
3
− of 27
m
mol/L. Potentiodynamic polarization test indicates that HCO
3
− of 27
m
mol/L dramatically enhance corrosion potential and induce passivation. EIS results further confirm that higher concentration of HCO
3
− induce more effective protection layer, especially in SBF with HCO
3
− of 27
m
mol/L.
•There is a strong orientation dependence of Hall-Petch relation for pure Ti.•Previous model and its parameters predict a reverse trend to the experimental observations.•Orientation mediated ...activation of additional deformation modes mainly accounts for the orientation dependence of Hall-Petch slope.•A new method combining the use of CPFEM and Eshelby's model for stress concentration at grain boundary is proposed to analyze orientation dependence of Hall-Petch relation.
The Hall-Petch slope k represents the magnitude of grain boundary strengthening. For the first time, a strong orientation dependence of the k for pure titanium (Ti) is reported in the present study, namely, a much lower k for the TD-tension of a Ti plate (188 MPa μm1/2) than the k values for the RD-tension of the Ti plate (358 MPa μm1/2), SD-tension (369 MPa μm1/2) and SD-compression (397 MPa μm1/2) of a Ti rod. Here, the RD and TD are respectively the rolling direction and transverse direction of the plate, and SD is the axial direction of the rod, while tension and compression stand for uniaxial tension and compression, respectively. It is found that the mechanisms reported previously cannot explain the orientation dependence of k experimentally observed in the present study. A new mechanism is proposed by combining crystal plasticity finite element modeling and the Eshelby model for the stress concentration at grain boundaries. The results indicate that an orientation-mediated deformation-transfer is the main contributor to this orientation effect on k. More specifically, for the RD-tension, SD-tension and SD-compression, a single slip system is predominant in most grains when plastic deformation-transfer occurs. In contrast, multiple slip systems are activated in the majority of grains in the TD-tension. The activation of additional slip systems remarkably reduces the stress concentration at grain boundaries, leading to a lower k. Afterward, the reasons for the orientation-mediated deformation transfer behavior are discussed.
•Cryogenic MDF is employed to improve the deformation tolerance of Mg alloys.•More twin sequences and interactions under cryogenic condition is promoted.•A synergistic increase in grain refinement ...and dislocation density is achieved.•Superior mechanical properties through a balance between microstructure and dislocations are obtained.
This study reports an approach to improve the mechanical properties of AZ80 magnesium alloy by combining rapid Multi-directional forging (MDF) and pre-cryogenic process. Compared to the tolerable strain of 0.96 with yield strength of 319 MPa, tensile strength of 469 MPa and elongation of 6.4 % obtained under direct room temperature MDF, the alloy achieved a higher cumulative strain of 1.44 and an increased yield stress of 380 MPa, tensile strength of 502 MPa, with moderate elongation of 6.0 % through the pre-cryogenic step. More interactions among twinning variants and the activation of new {1 0 −1 2}–{1 0 −1 2} or tertiary twinning sequences in cryogenic MDF sufficiently coordinated plastic deformation, thus greatly improving the deformation limit. Furthermore, the pronounced twinning interactions enormously facilitated grain size reduction, texture dispersion, and the accumulation of dense dislocations, all of which served to the superior mechanical properties.
Although {101¯2} twinning behavior has been extensively studied, it has rarely been investigated under biaxial tension. In the present research, the mechanical response and {101¯2} twinning behavior ...of a Mg AZ31 sheet under biaxial tension with various stress ratios (σND:σTD) along the normal direction (ND) and transverse direction (TD) were systematically studied. The results indicate that the yield stress along the ND is nearly independent of σND:σTD. Schmid law applied to {101¯2} twinning under biaxial tension yields better efficiency than that for uniaxial tension, and a higher efficiency is observed with a larger stress ratio along the TD; the fraction of experimentally observed twin variants with Schmid factors (SFs) of ranks 1 and 2 is approximately 45% for uniaxial tension, whereas it is approximately 68% for σND:σTD=2:1 and approximately 88% for σND:σTD=1:2. This result contradicts with the previously reported result that Schmid law is more efficient for uniaxial stress states than under multiaxial stress conditions. It is found that biaxial tension will increase the difference in SF for the six twin variants and make Schmid law more efficient. Twin variant selection and volume fraction are highly dependent on σND:σTD. For uniaxial tension, the activities for the six variants are similar, and the (0002) poles of the twins are distributed evenly on a circle approximately 80°–90° away from the ND. In contrast, the variants (01¯12)011¯1 and (011¯2)01¯11 are more favorable under biaxial tension, and more (0002) poles of the twins concentrate around the rolling direction with a higher ratio of σTD. At a similar strain level, the twin fraction under biaxial tension is often different from that under uniaxial tension, and this difference varies with σND:σTD. This mechanism is associated with a stress-ratio-mediated detwinning behavior under biaxial tension. The findings in the present study provide a new and significant understanding of the twinning behavior of Mg alloys.
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•The Schmid law for twinning exhibits a better efficiency under biaxial tension than under uniaxial tension.•The selection of twin variants and related texture evolution are highly dependent on the stress ratio of biaxial tension.•Unlike uniaxial tension, the twin fraction and texture under biaxial tension might be affected by detwinning.
•The planar mechanical anisotropy exhibits a grain size independence.•A new method for quantitative analysis on the planar mechanical anisotropy is proposed.•To reduced planar yield anisotropy, the ...tilting angle and direction of basal poles away from ND are crucial parameters.
A TD-tilted basal texture in Mg alloys often generates a strong planar mechanical anisotropy. Unfortunately, there has been not a quantitative study on this issue. In the present study, the mechanisms for the anisotropy of tensile yield strength in RD-TD plane were quantitatively studied for a Mg-2Zn-1Ca alloy with different grain sizes and textures. Here, RD and TD refer to the rolling direction and transverse direction of the plate. The results show that there is a strong planar yield anisotropy (∼24 ± 5 MPa), regardless of grain size, while this yield anisotropy is absent in texture with a circular distribution of basal poles. The examination of microstructure and X-ray diffraction analysis reveals that, beside basal slip as the predominant mode under both RD-tension and TD-tension, prismatic slip in RD-tension and both prismatic slip and extension twinning in TD-tension are also important. The parameters of Hall-Petch relation are also used to further analyze the mechanism for yield anisotropy. The changeable intercept, σ0, and invariant Hall-Petch slope, k, between RD-tension and TD-tension are rationalized accounting for this anisotropy. The reasons why there are different σ0 and similar k between TD-tension and RD-tension are quantitively discussed. At last, a texture design for decreasing the planar yield anisotropy is given.
Different contents of carbon element (0-3 at.%) was added into CoCrFeMnNi high entropy alloys (HEAs) to prepare carbide-reinforced CoCrFeMnNi matrix composites. The effects of carbon on ...microstructures and mechanical properties were systematically studied. The CoCrFeMnNi HEA sheet without carbon showed fine recrystallized grains with a grain size of approximately 5 μm and contained Cr-rich sigma phase. The CoCrFeMnNiCx HEA sheets with 1.0 at.% and 3.0 at.% C presented fine recrystallized grains and a small fraction of elongated grains. A large number of nano-scaled carbides were observed in the carbon-containing HEA sheets. With the carbon content increasing from 0 at.% to 3.0 at.%, the strengthening of the α-fiber texture is more obvious, and tensile yield strength increased from 371 MPa to 792 MPa, however, the elongation decreased from 54% to 11%, respectively. The CoCrFeMnNiC1 HEA sheet with a volume fraction of 2.9% nano carbides showed excellent balanced mechanical property, with tensile yield strength of 634 MPa and elongation of 38%. The increase of yield strength for the CoCrFeMnNiC1 HEA was mainly ascribed to the combined effects of precipitation Orowan strengthening and dislocations strengthening. Precipitation Orowan strengthening is the primary strength contributor, with a value of approximate 157 MPa.
•Cr-rich sigma phase was observed in C-free CoCrFeMnNi HEA sheet.•Nano-scaled M23C6 carbides were found in C-containing CoCrFeMnNi HEA sheets.•The CoCrFeMnNiC1 HEA sheet showed an excellent balance of strength and ductility.•Nano-scaled M23C6 carbides provided the largest contribution to the increase of YS.
High alloying elements effectively strengthen zinc alloys, but these elements also usually decrease the ductility and corrosion resistance of such alloys. In the present study, energy-saving and ...efficient rotary-die equal-channel angular pressing (ECAP) was successfully applied to a zinc alloy with a low Mg content (0.1 wt%) at room temperature (RT), which facilitated the formation of a microstructure with a fine grain size (1.14 ± 0.22 µm), weak texture, and dynamically precipitated nano-sized Mg-Zn precipitates at the grain interior. The Zn-0.1Mg alloy formed via RT-ECAP exhibited an outstanding performance, with a high ultimate tensile strength of 383 MPa, excellent ductility of 45.6 %, and a suitable biodegradation rate of 0.014 mm/y in Hank’s solution. This alloy is thus a promising candidate for various biodegradable medical applications.
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•Severe plastic deformation of ECAP is realized in a Zn-0.1Mg (wt %) alloy at room temperature.•The room temperature ECAP alloy exhibits both outstanding mechanical properties and biodegradable performance.•The effect of processing temperature on microstructure and property evolutions of ECAP alloy is revealed.
Anisotropy of a 32-pass equal channel angular pressing (ECAP) processed Mg-3.7Al-1.8Ca-0.4Mn (wt%) alloy with synergistically enhanced mechanical properties and corrosion resistance was investigated. ...Compared with coarse α-Mg grains and discontinuous network Al2Ca phase in as-cast alloy, the ECAP alloy exhibits homogeneous fine α-Mg grains (~2.6 µm), but refined Al2Ca particles show different distribution in three planes, which is network in the extrusion plane (EP), uniform in normal plane (NP) and bandlike in transverse plane (TP). Refinement of microstructure after ECAP resulted in the synergistically enhanced mechanical and corrosion performances in three planes (directions). Moreover, due to grain refinement, the protective film was easily produced on the ECAP alloy, resulting in different corrosion mechanisms developed between ECAP and as-cast alloys. The main factors effecting anisotropic properties of ECAP alloy are Al2Ca distribution and texture of α-Mg. Based on the anisotropy investigations, grain size in several microns and high (0001) basal texture density are necessary to realize RE-free Mg-Al-Ca-Mn alloys with synergetic high strength and good corrosion performance.
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•Corrosion and mechanical properties of a Mg-3.7Al-1.8Ca-0.4Mn ECAP alloy were enhanced in three-dimensional space.•Anisotropic microstructural evolution of ECAP alloy were investigated.•The anisotropy of corrosion mechanism of ECAP alloy were illustrated.•Main microstructure characteristics impacting anisotropic properties were expounded.•The way to realize Mg-Al-Ca-Mn alloys with strength-corrosion synergy was concluded.
