Influence of different deformation processing on the AZ31 magnesium alloy sheets

AZ31 magnesium alloy sheets were processed by normal rolling (NR), one-pass equal channel angular rolling (1P-ECAR), and cross equal channel angular rolling (C-ECAR) at 400 °C on a die with 105 ° channel angle. The microstructure, texture, and tensile properties of sheets were measured. The results show that ECAR processing can weaken the basal plane texture, thus obviously improve the mechanical properties. The yield ratio σ_s/σ_b decreases and strain hardening exponent n increases along rolling direction (RD) during ECAR, which means that the uniform plastic formability is enhanced. After C-ECAR, the mechanical properties along both the RD and transverse direction (TD) are improved. Different twinning types, fine contraction twinning in the NRed sheets and coarse extension twinning in the ECARed sheets, were observed. The easier activation of twinning and basal 〈a〉 slip leads to the lower yield strength of the ECARed sheets. Dynamic recrystallization (DRX) during the rolling process has great effect on the microstructure of the as-deformed and annealed sheets. The annealed C-ECARed sheets have significant finer and homogenous grains than the annealed NRed sheets, which is attributed to the rarely DRX process during ECAR. The average grain sizes of the annealed C-ECARed samples and NRed samples are 14 and 24 μm, respectively.     

Biocorrosion and biodegradation behavior of ultralight Mg–4Li–1Ca (LC41) alloy in simulated body fluid for degradable implant applications

Biocorrosion and biodegradation behavior of Mg–4Li–1Ca alloy were investigated for industrially important end product conditions, namely the homogenized, rolled, and rolled + annealed ones. Among the three, homogenized material showed the highest corrosion rate (27.2 mm/year) in a simulated body fluid (SBF) owing to its coarse grain structure containing long dumbbell-shaped eutectic phase. Rolled + annealed material exhibited the lowest corrosion rate (0.94 mm/year) corresponding to the highest corrosion resistance (1.854 kΩ cm²) in SBF. This higher corrosion resistance is associated with a uniform distribution of corrosion sites and a lower occurrence of twins in the microstructure. However, the rolled material showed a greater corrosion rate due to an appreciable volume fraction of {10\( \bar{1} \)1} compression twins, {10\( \bar{1} \)2} tension twins, and {10\( \bar{1} \)1}–{10\( \bar{1} \)2} double twins, which form galvanic couples with the adjacent grains that enhances localized corrosion. A mechanism of biodegradation at the alloy/SBF interface is proposed. It involves the formation of bone-like hydroxyapatite and metastable octa calcium phosphate, along with other degradation products, such as magnesium hydroxide and lithium hydroxide.     

An analytical approach and experimental confirmation of dislocation–twin boundary interactions in titanium

The morphology of $ \{ 10\overline{1} 2\} \left\langle {\overline{1} 011} \right\rangle $ { 10 1 ¯ 2 } 〈 1 ¯ 011 〉 deformation twins formed in commercial purity titanium during an initial pass of equal-channel angular pressing was studied by transmission electron microscopy (TEM). The corresponding diffraction patterns show a symmetry line splitting of $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin boundaries (TB) which is related to the presence of interfacial defects. A simple modeling for the interaction between non-screw a-slip lattice dislocations (Burgers vector b =  $ \frac{1}{3}[\overline{1} \overline{1} 20] $ 1 3 [ 1 ¯ 1 ¯ 20 ] ) and the $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin plane is used according to crystallographic geometry and vector conservation. The results show that dislocation dissociation into different Frank partial dislocations on the interfacial plane is more favorable than its transmission to the other side of the interface. The formation of the Frank partials at the TB can produce a small change in the TB misorientation angle and this is consistent with the symmetry line splitting of the $ (10\overline{1} 2) $ ( 10 1 ¯ 2 ) twin boundaries observed by TEM.     

Vorhersage der thermischen Strahlung großer Kohlenwasserstoff- und Peroxid-Poolfeuer mit CFD Simulation

Flame temperatures (T), surface emissive powers (SEP) and irradiances (E) of large-scale JP-4 pool fires (d=2, 8, 16, 25 m) and di-tert-butyl peroxide (DTBP) pool fires (d=1.12 m, 3.4 m) are investigated experimentally and by CFD simulation. As experimental methods an infrared thermographic camera system with video-mixing unit is used for the determination of T, SEP and an ellipsoidal radiometer for the determination of E. The maximum frequency of time-averaged emission temperatures for JP-4 pool fire (d=16 m) are in a range of $ 793\,\mathrm{K} < \overline{T} < 1033$ and for DTBP pool fire (d=1.12 m) are a range of $ 1040\,\mathrm{K} < \overline{T} < 1480\,\mathrm{K}$ . For DTBP pool fire (d=1.12 m), the measurements result in $ \overline{\text{SEP}}\approx 130\,\mathrm{kW/m^{2}}$ which is up to a factor of ≈6 larger in comparison to hydrocarbon pool fires (d≈1 m). In a case of DTBP pool fire (d=3.4 m) with $ \overline{\text{SEP}} \approx 250\,\mathrm{kW/m^{2}}$ this factor is ≈5 compared to $ \overline{\text{SEP}} \approx 50\,\mathrm{kW/m^{2}}$ of LNG pool fire (d=4 m). By increasing the relative distance ?y/d from the pool rim, measured time averaged irradiances $ \overline{E}$ (?y/d) decrease and agree well with CFD predicted $ \overline{E}_{\text{CFD}}$ (?y/d). Also, there is a good agreement between the measured time averaged $ \overline{T}$ and $ \overline{\text{SEP}}$ of hydrocarbons and DTBP pool fires, with the predicted $ \overline{T}_{\text{CFD}}$ and $ \overline{\text{SEP}}_{\text{CFD}}$ values. The possibilities and nowadays limitations of CFD simulation of large pool fires are discussed. This study has shown that the risk potential of accidental pool fires referring to thermal radiation can be predicted much better than in the past.     

Grain boundary motion and grain growth in zinc in a high magnetic field

The motion of grain boundaries in zinc bicrystals (99.995 %) driven by the “magnetic” driving force was measured. An in situ technique for observations and continuous recording the boundary migration was applied. Planar symmetrical and asymmetrical $ \left\langle {10\overline{1} 0} \right\rangle $ tilt grain boundaries with rotation angles in the range between 60° and 90° were studied. The boundary migration was measured in the temperature regime between 330 and 415 °C. The mobility of $ \left\langle {10\overline{1} 0} \right\rangle $ tilt boundaries in zinc and its temperature dependence were found to depend on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90 %) zinc–1.1 % aluminum alloy sheet specimens substantially affected the texture and microstructure evolution. This effect is attributed to the additional magnetic driving force for grain growth arising due to the magnetic anisotropy of zinc.     

The Ideal Strengths of Superconducting MgCNi<Subscript>3</Subscript> and CdCNi<Subscript>3</Subscript>

The stress-strain curves under tensile deformation in the 〈100〉, 〈110〉, and 〈111〉 directions and under shear deformation in the (001)〈110〉, \((110)\langle \overline {1}10\rangle \), \((111)\langle 1\overline {1}0\rangle \), and \((111)\langle 11\overline {2}\rangle \) slip systems have been systematically calculated by first-principles method to study the ideal strengths of superconducting MgCNi₃ and CdCNi₃. The ideal strengths in the three tensile directions are found to be reduced in the order of 〈100〉 → 〈110〉 → 〈111〉 and those for the four shear slip systems in the order of \((110)\langle \overline {1}10\rangle \rightarrow (111)\langle 11\overline {2}\rangle \rightarrow (111)\langle 1\overline {1}0\rangle \rightarrow (001)\langle 110\rangle \) for both superconductors. Their lowest ideal tensile strengths are found to be larger than the corresponding highest ideal shear strengths, which could explain why both superconductors have the ductility. The obtained lattice constants and elastic properties coincide well with the the available experimental and theoretical values.     

Scaling behavior of dynamic hysteresis in Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

The scaling behavior of dynamic hysteresis was investigated in Bi_3.15Nd_0.85Ti₃O₁₂ bulk ceramics at a frequency of 1–1000 Hz and an external electric field amplitude of 79–221 kV/cm. The scaling behavior at low amplitude (E ₀ ≤ 114 kV/cm) takes the form of \(\langle A \rangle \propto f^{ - 0.013} E_{0}^{0.7}\) for low frequency (f ≤ 200 Hz) and \(\langle A \rangle \propto f^{ - 0.013} E_{0}^{0.22}\) for high frequency (f > 200 Hz), where \(\langle A \rangle\) is the area of hysteresis loop and f and E ₀ are frequency and amplitude of external electric field, respectively. At high amplitude (E ₀ > 114 kV/cm), we obtain \(\langle A \rangle \propto f^{0.011} E_{0}^{1.163}\) at low frequency and \(\langle A \rangle \propto f^{ - 0.015} E_{0}^{0.7}\) at high frequency. At low E ₀, the contribution to the scaling relation mainly results from reversible domain switching, while at high E ₀ reversible and irreversible domain switching concurrently contribute to the scaling relation.     

Ni–YSZ(111) solid–solid interfacial energy

Solid-state dewetting of continuous Ni films deposited on the (111) surface of yttrium stabilized zirconia (YSZ) was used to produce equilibrated Ni particles, and the solid–solid interface energy was determined using Winterbottom analysis. The ~150 nm thick Ni films were dewetted (annealed) at 1350 °C in Ar + H₂ (99.9999 %) at an oxygen partial pressure of 10^?20 atm for 6 h. Transmission electron microscopy of equilibrated particles was conducted, and two low-energy low-index orientation relationships were found: $ {\text{Ni[1}}\overline{ 1} 0 ] ( 1 1 1 )\left\| {{\text{YSZ[1}}\overline{ 1} 0 ]} \right. ( 1 1 1 ) $ and $ {\text{Ni[}}\overline{ 1} 1 0 ] ( 1 1 1 )\left\| {{\text{YSZ[1}}\overline{ 1} 0 ]} \right. ( 1 1 1 ) $ , and the interface energies were measured to be 1.8 ± 0.1 and 2.1 ± 0.1 J/m², respectively. A model including grain growth concurrent with dewetting is used to explain the formation of the higher energy orientation relationship.     

Dislocation structure at a \{ {\overline{1} 2\overline{1} 0} \}/ {\langle} 10\overline{1} 0 {\rangle} low-angle tilt grain boundary in LiNbO3

LiNbO₃ is a ferroelectric material with a rhombohedral R3c structure at room temperature. A LiNbO₃ bicrystal with a $ \{ {\overline{1} 2\overline{1} 0} \}/ {\langle}10\overline{1} 0{\rangle}$ 1° low-angle tilt grain boundary was successfully fabricated by diffusion bonding. The resultant boundary was then investigated using high-resolution TEM. The boundary composed a periodic array of dislocations with $ b = { 1}/ 3{\langle} \overline{1} 2\overline{1} 0{\rangle} $ . They dissociated into two partial dislocations by climb. A crystallographic consideration suggests that the Burgers vectors of the partial dislocations should be $ 1/ 3{\langle}01\overline{1} 0{\rangle} $ and $ 1/ 3{\langle}\overline{1} 100{\rangle} $ , and a stacking fault on $ \{ {\overline{1} 2\overline{1} 0} \} $ is formed between the two partial dislocations. From the separation distance of a partial dislocation pair, a stacking fault energy on $ \{ {\overline{1} 2\overline{1} 0} \} $ was estimated to be 0.25?J/m² on the basis of isotropic elasticity theory.     

Detwining in Mg alloy with a high density of twin boundaries

To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.     

The Elastic Properties, Generalized Stacking Fault Energy and Dissociated Dislocations in MgB2 Under Different Pressure

The $\langle11\overline{2}0\rangle$ perfect dislocation in MgB₂ is suggested to dissociate into two partial dislocations in an energy favorable way $\langle11\overline{2}0\rangle\rightarrow\frac{1}{2}\langle11\overline{2}0\rangle +\mathrm{SF}+\frac{1}{2}\langle11\overline{2}0\rangle$ . This dissociation style is a correction of the previous dissociation $\langle1000\rangle\rightarrow\frac{1}{3}\langle1\overline{1}00\rangle+\mathrm{SF}+\frac{1}{3}\langle2100\rangle$ proposed by Zhu et al. to model the partial dislocations and stacking fault observed by transmission electron microscopy. The latter dissociation results in a maximal stacking fault energy rather than a minimal one according to the generalized stacking fault energy calculated from first-principles methods. Furthermore, the elastic constants and anisotropy of MgB₂ under different pressure are investigated. The core structures and mobilities of the $\langle11\overline{2}0\rangle$ dissociated dislocations are studied within the modified Peierls–Nabarro (P–N) dislocation theory. The variational method is used to solve the modified P–N dislocation equation and the Peierls stress is also determined under different pressure. High pressure effects on elastic anisotropy, core structure and Peierls stress are also presented.     

The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes

In 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 {1 0 −1 2} twinning was restrained and the volume fraction of twins decreased, leading to the improvement of yield strength by increasing in pre-stretching levels.     

AZ31镁合金的变形织构和协调变形机理

进行变形速率可控的单向拉伸试验,研究了变形织构与滑移和孪生等协调变形机理对AZ31镁合金综合性能的影响。结果表明:在沿挤压方向拉伸过程中,变形织构使{0002}晶面Schmid因子较低,基面滑移难以开动,屈服强度高。在沿45°拉伸过程中,变形织构使柱面取向晶粒处于发生{0002}滑移的最佳位置,基面取向晶粒的棱柱面滑移也处于最佳位置,屈服强度低而延伸率高。沿横向拉伸的力学性能主要受孪晶影响,由于大量孪晶诱发裂纹,延伸率最低。试样在45°和横向拉伸时产生的大量拉伸孪晶,是出现{0002}双峰织构的诱因。     

Finite element simulations of notch tip fields in magnesium single crystals

Recent experiments using three point bend specimens of Mg single crystals have revealed that tensile twins of \(\{10\bar{1}2\}\) -type form profusely near a notch tip and enhance the fracture toughness through large plastic dissipation. In this work, 3D finite element simulations of these experiments are carried out using a crystal plasticity framework which includes slip and twinning to gain insights on the mechanics of fracture. The predicted load–displacement curves, slip and tensile twinning activities from finite element analysis corroborate well with the experimental observations. The numerical results are used to explore the 3D nature of the crack tip stress, plastic slip and twin volume fraction distributions near the notch root. The occurrence of tensile twinning is rationalized from the variation of normal stress ahead of the notch tip. Further, deflection of the crack path at twin–twin intersections observed in the experiments is examined from an energy standpoint by modeling discrete twins close to the notch root.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Emission of Gas and $$\hbox {Al}_{2}\hbox {O}_{3}$$ Smoke in Gas–Al Particle Deflagration: Experiments and Emission Modeling for Explosive Fireballs

Emission of gas and \(\hbox {Al}_{2}\hbox {O}_{3}\) smoke within the deflagration of \(\hbox {H}_{2}{-}\hbox {O}_{2}\)–{\(\hbox {N}_{2}{-}\hbox {CO}_{2}\)}–Al particles has been studied in a closed combustion chamber at pressures of up to 18 bar and at gas temperatures of up to 3700 K. Measurements of radiance intensity were taken using a five wavelength pyrometer (0.660 \(\upmu \hbox {m}\), 0.850 \(\upmu \hbox {m}\), 1.083 \(\upmu \hbox {m}\), 1.260 \(\upmu \hbox {m}\), 1.481 \(\upmu \hbox {m}\)) and a grating spectrometer in the range (4.10 \(\upmu \hbox {m}\) to 4.30 \(\upmu \hbox {m}\)). In order to characterize the aluminum oxide smoke size and temperature, an inversion method has been developed based on the radiation transfer equation and using pyrometer measurements and thermochemical calculations of \(\hbox {Al}_{2}\hbox {O}_{3}\) smoke volume fractions. Temperatures in combustion gas have been determined using a method based on the assumed blackbody head of the 4.26 \(\upmu \hbox {m}\) \(\hbox {CO}_{2}\) emission line and on its spectral shift with pressure and temperature. For validation purpose, this method has been applied to measurements obtained when calibrated alumina particles are injected in a combustion chamber prior to gaseous deflagrations. This mathematical inversion method was developed to investigate explosive fireballs.     

Choosing a proper loading rate for bonded-particle model of intact rock

Bonded-particle model (BPM) is widely used to model geomaterials, in which calibration against the results from uniaxial/triaxial compressive tests, Brazilian tensile tests and shear tests have been commonly conducted. However, since different loading rates were used, it is difficult to assess the numerical results of these studies if the effects of the loading rate are ignored. This paper discusses the loading mechanisms associated with different loading rates in the BPM and examines the numerical outputs under these different rates. The results indicate that the time step is an important factor controlling the loading rate of the BPM and should be considered in addition to the velocity of the loading platen. The strain rate, which is usually employed to describe the loading rate in a physical test, cannot be used for the direct comparison of different numerical tests in \(\hbox {PFC}^{\mathrm{2D}}\) due to the time step. A proposed “step strain rate”, which considers the time step, is found to be more appropriate for comparing the loading velocity on specimens of varying sizes. Six different loading rates (0.005, 0.01, 0.02, 0.08, 0.2 and 0.6 m/s) are employed in uniaxial compressive tests and Brazilian tests during this study. After comprehensive examinations, a maximum step strain rate of \(1.1 \times 10^{-8}\, \hbox {step}^{-1}\) is considered to be appropriate for quasi-static uniaxial compressive tests and Brazilian tests using the BPM.     

De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed(hcp)metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction(ND) of pre-compressed samples along the extrusion direction(ED). {1012} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain,and the complete de-twinning occurred at a strain of ~7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {1012} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {1012} twinning corresponded to the formation of new texture components C{1210}<0001> and D{0110}<0001> and a decrease in the initial texture components A{0001}<2110> and B{0001}<1010>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{1210}<0001> and D{0110}<0001> and an increase in the components A{0001}<2110> and B{0001}<1010>.     

High energy impact techniques application for surface grain refinement in AZ91D magnesium alloy

A nanostructured surface layer was fabricated on magnesium alloy AZ91D by using the high-energy impact technique (HEIT). With the help of transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM), the microstructure features of the surface layer were systematically observed and characterized in different stages of microstructure evolution. The result revealed the mechanism of grain refinement and strain accommodation. The process of grain refinement, accompanied by an increase in strain in the surface layer, resulted from several processes. The onset of deformation twinning and the intersection with twins system are one of them. The operation of basal slip and pyramidal slip led to the formation of dislocation cells and low-angle dislocation boundaries. The successive subdivision of grains to a finer scale resulted in the formation of highly disoriented nanocrystalline grains. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization. The minimum size of such refined grains was about 40 nm.     

Thermal Conductivity of Pyroclastic Soil (<Emphasis Type="BoldItalic">Pozzolana</Emphasis>) from the Environs of Rome

The paper reveals the experimental procedure and thermo-physical characteristics of a coarse pyroclastic soil (Pozzolana), from the neighborhoods of Rome, Italy. The tested samples are comprised of 70.7 % sand, 25.9 % silt, and 3.4 % clay. Their mineral composition contained 38 % pyroxene, 33 % analcime, 20 % leucite, 6 % illite/muscovite, 3 % magnetite, and no quartz content was noted. The effective thermal conductivity of minerals was assessed to be about \(2.14\,\hbox {W}{\cdot } \hbox {m}^{-1}{\cdot } \hbox {K}^{-1}\). A transient thermal probe method was applied to measure the thermal conductivity (\(\lambda \)) over a full range of the degree of saturation \((S_{\mathrm{r}})\), at two porosities (n) of 0.44 and 0.50, and at room temperature of about \(25\,^{\circ }\hbox {C}\). The \(\lambda \) data obtained were consistent between tests and showed an increasing trend with increasing \(S_{\mathrm{r}}\) and decreasing n. At full saturation (\(S_{\mathrm{r}}=1\)), a nearly quintuple \(\lambda \) increase was observed with respect to full dryness (\(S_{\mathrm{r}}=0\)). In general, the measured data closely followed the natural trend of \(\lambda \) versus \(S_{\mathrm{r}}\) exhibited by published data at room temperature for other unsaturated soils and sands. The measured \(\lambda \) data had an average root-mean-squared error (RMSE) of \(0.007\,\hbox {W}{\cdot } \hbox {m}^{-1}{\cdot } \hbox {K}^{-1}\) and \(0.008\,\hbox {W}{\cdot } \hbox {m}^{-1}{\cdot } \hbox {K}^{-1}\) for n of 0.50 and 0.44, respectively, as well as an average relative standard deviation of the mean at the 95 % confidence level \((\hbox {RSDM}_{0.95})\) of 2.21 % and 2.72  % for n of 0.50 and 0.44, respectively.