Characterization of mobile type I and type II twin boundaries in 10M modulated Ni–Mn–Ga martensite by electron backscatter diffraction

Mobile type I and type II twin boundaries mediating the magnetic field-induced strain in five-layered modulated (10M) Ni–Mn–Ga martensite were analyzed by electron backscatter diffraction. Taking into account the slight monoclinic distortion of the pseudo-tetragonal lattice, the electron backscatter diffraction study reveals domains of 0.01–1 mm thickness adjacent to the type I and type II twin boundaries. The domains differing in the modulation direction are {1 0 0) compound twins and their effect on twinning stress is discussed. Detailed analysis of type II twin boundary reveals that the domains are further internally twinned by compound {1 1 0) twins 1–15 μm in size. An additional example of a complex twin microstructure combining type I and type II twin boundaries is presented.     

Three-dimensional analysis of grain topology and interface curvature in a β-titanium alloy

While considerable efforts have been made to model the effects of grain coarsening, there has been little experimental verification of these models. Using serial sectioning techniques, the full 3-D morphology of 2098 β-titanium grains in Ti–21S are analyzed and directly compared to grain coarsening theories. The experimental grain size distribution and the distribution in the number of grain faces are shown to have a close comparison to the predictions of the steady-state size distribution from a number of simulations and analytical theories. The geometric factor of the growth rates is determined by measuring the mean curvature of the grain faces. It is found that, on average, the grains with an average of 15.5 faces have a zero integral mean curvature of the grain faces, higher than the predicted value of 13.4 faces. This difference is suggested to be due to the non-random nearest-neighbor effects within the grain network.     

Thermo-mechanical and metallurgical aspects in friction stir processing of AZ31 Mg alloy—A numerical and experimental investigation

Friction stir processing of AZ31 Mg alloy was investigated by numerical modeling and experiments. A CFD based, fully coupled, 3D, thermo-mechanical model was built to better understand the effect of process parameters on temperature, material flow and strain rate. In order to account for material softening phenomena at elevated temperatures and extremely high strain rates that occur during the FSP process, experimentally measured peak temperatures were utilized to introduce a correction function in the flow stress constitutive relation. The numerical results showed that rotational speed as compared to translational speed had a more dominant effect on temperature field and strain rate. In addition, the asymmetric material flow around the tool axis caused higher peak temperature and strain rate on the advancing side (AS), while the material in the path of tool pin was swept around the retreating side (RS). FSP experiments confirmed peak temperatures measured at sheet surface near shoulder perimeter on AS were always higher than corresponding RS peak temperatures, under the selected range of process parameters. In addition to thermo-mechanical aspects, the metallurgical characteristics of FSP i.e. mainly the grain size evolution was studied by optical and electron microscopy. Experiments revealed that the coarse bimodal microstructure of as-received AZ31 Mg was subdivided into a defect-free, fine grain microstructure at the rotational speed of 1000 rpm, while a defect-free but a relatively coarse and bimodal microstructure evolved in the material at rotational speeds higher than 1000 rpm. Furthermore, in the selected range of process parameters the increases in translational speed resulted in finer grain sizes without the formation of voids or defects.     

Twinning and detwinning during compression–tension loading measured by quasi in situ electron backscatter diffraction tracing in Mg–3Al–Zn rolled sheet

Twinning and detwinning are the important deformation modes in magnesium alloys during cyclic loading at room temperature. To analyze these two deformation mechanism, cyclic compression–tension experiments were performed on Mg–3Al–1Zn rolled sheet along the rolling direction. In these tests, the microstructure evolutions of a series of grains during deformation were traced by using quasi in situ electron backscatter diffraction(EBSD). Important quantities like the Schmid factors of twinning system, the fraction of twinning during compression, and the fraction of twinning after reverse loading were calculated on the basis of measured quantities. The influence of Schmid factor of twinning variants on detwinning upon reverse loading was analyzed. Detwinning would prefer to proceed during reverse loading if the Schmid factor of twinning in the twinning area before reverse loading is sufficiently large.     

Direct observation of dislocation dissociation and Suzuki segregation in a Mg–Zn–Y alloy by aberration-corrected scanning transmission electron microscopy

Crystal defects in a plastically deformed Mg–Zn–Y alloy have been studied on the atomic scale using aberration-corrected scanning transmission electron microscopy, providing important structural data for understanding the material’s deformation behavior and strengthening mechanisms. Atomic scale structures of deformation stacking faults resulting from dissociation of different types of dislocations have been characterized experimentally, and modeled. Suzuki segregation of Zn and Y along stacking faults formed through dislocation dissociation during plastic deformation at 300 °C is confirmed experimentally on the atomic level. The stacking fault energy of the Mg–Zn–Y alloy is evaluated to be in the range of 4.0–10.3 mJ m^?2. The newly formed nanometer-wide stacking faults with their Zn/Y segregation in Mg grains play an important role in the superior strength of this alloy at elevated temperatures.     

Microstructures and mechanical properties of hot rolled AZ31 Mg alloy sheets

The microstructures and mechanical properties of hot rolled AZ31 Mg alloy sheets were studied to understand the microstructure evolution during AZ31 Mg alloy hot rolling process. The roller was heated to 180 ℃ with burning hydrogen, and the extruded plates were rolled at 400 ℃ from 10 to 1 mm with a reduction of 30% in thickness per pass. The result shows that there is no side-cracking of these rolled sheets every pass. The extruded microstructures are greatly refined and mechanical properties are improved. The fine grains of about 4μm were obtained of the final 0.9 mm sheets.     

In situ neutron diffraction and polycrystal plasticity modeling of a Mg–Y–Nd–Zr alloy: Effects of precipitation on individual deformation mechanisms

In situ neutron diffraction compression tests were performed on Mg–Y–Nd–Zr alloy WE43, in the solution heat-treated, peak- and over-aged conditions. The flow curves and internal strain evolutions were modeled using polycrystal plasticity simulation, with the inclusion of an elastic phase to account for the presence of precipitates. The results reveal that prismatic plate-shaped precipitates strongly impede basal slip; the critical resolved shear strength (CRSS) of basal slip increases from 12 to 37 MPa, an increase of over 200%. However, hard deformation modes such as non-basal slip of 〈a〉 dislocations are required for macroscopic yielding. These hard modes are not as strongly affected by aging, with CRSS values which increase from 78 to 92 MPa, an increase of only 18%. The results of the study are consistent with recent modeling of the relative Orowan strengthening of individual deformation modes and the superposition of various strengthening effects (solid solution and precipitation). This finding helps to explain why the age-hardening response of Mg–Y–Nd–Zr alloys is not exceptional. It is concluded that future precipitation-strengthened alloy and process design strategies should focus on promoting high number densities of particles. The effect of aging upon twinning is surprising. The most age-hardened material exhibits more twinning than the solutionized material. To model this behavior using polycrystal plasticity, the critical stress to activate twinning (especially the strain hardening thereof) must be decreased.     

Effect of welding current on strength and microstructure in resistance spot welding of AZ31 Mg alloy

In this paper, resistance spot welding were performed on lmm-thickness magnesium AZ31B plates. The effect of welding current on the microstructure and tensile shear force was investigated. It was found that the welding current governed the nugget growth, and the nugget could not form if current levels were insufficient. The nugget revealed a homogeneous, equiaxed, fine-grained structure, which consisted of non-equilibrium microstructure of α-phase dendrites surrounded by eutectic mixtures of α and β（ Mg17All2 ） in the grain boundaries. With increasing welding current, the size of grains in nugget would be more smaller and uniform, and the width of plastic rings would be larger. Tensile shear tests showed that tensile shear force of the joints increased with increasing welding current when the welding current was smaller than 17 000 A. The maximum tensile shear force was up to 1980 N.     

Grain and grain boundary activities observed in alumina–zirconia–magnesia spinel nanocomposites by in situ nanoindentation using transmission electron microscopy

At room temperature, in situ nanoindentation experiments in a transmission electron microscope reveal the grain and grain boundary activities in fully dense ceramic nanocomposites composed of Al₂O₃:ZrO₂:MgAl₂O₄ (AZM) processed by spark plasma sintering (SPS). The composites have a bi-modal grain size distribution, where the larger grains (500 nm?1 μm in diameter) consist of Al₂O₃ and MgAl₂O₄ grains, and the smaller grains (100–300 nm in diameter) are primarily ZrO₂. In situ dynamic deformation studies show that the AZM nanocomposites undergo the deformation mainly through the grain-boundary sliding and rotation of small grains, i.e. ZrO₂ grains, and some of the large grains, i.e. MgAl₂O₄ grains. We observed both plastic and elastic deformations in different sample regions in these multi-phase ceramic nanocomposites at room temperature.     

Microstructural analysis of modification and grain refinement in a hypoeutectic Al–Si alloy

Abstract

It is well known that in hypoeutectic Al–Si alloys addition of titanium boride refines primary aluminium and addition of strontium modifies eutectic silicon. In this work their individual and simultaneous effect on both features has been investigated quantitatively. On the basis of experimental observations and quantitative analysis, it was concluded that an addition of 0·002 wt-% titanium almost halves grain size of primary aluminium in an ingot of A356, cast in a bar-shaped copper mould. Combining 0·002 wt-% titanium (as Al–3Ti–B) with 0·02 wt-% strontium resulted in a decrease in this grain refinement. However, an addition of 0·02 wt-% strontium has a slight refinement effect on primary aluminium phase. In order to quantify the effect of addition on eutectic silicon readily, as-cast specimens were heat-treated. It was observed that the number of eutectic silicon particles in Sr-modified specimens increased in a higher level compared to a Sr + Ti treated specimen. It was also found that Ti slightly influences the size of eutectic silicon particles.     

The inclination dependence of gold tracer diffusion along a Σ3 twin grain boundary in copper

Grain boundary impurity diffusion of Au in diffusion bonded Σ3〈011〉 Cu bicrystals has been studied by the radiotracer serial-sectioning technique using the ¹⁹⁵Au isotope with high specific activity. The dependence of the grain boundary diffusion parameter sδD_b on the inclination angle Φ of the grain boundary plane with respect to the close-packed (111) plane of the coherent twin boundary has been determined for the temperatures of 673 and 773 K (here s, δ and D_b are the segregation factor, the grain boundary width and the grain boundary diffusion coefficient, respectively). At both temperatures sδD_b exhibits a minimum at Φ=70.5° and a maximum at Φ=82.0°. It is shown that there is no direct correlation between the energy and diffusivity of the grain boundaries in this system and that the periodicity in the grain boundary plane has a significant influence on the grain boundary diffusivity.     

XRD studies of electrochemically hydrided–dehydrided thin films of the alloy AZ31 and AZ31 with Ti

A magnetron sputtered thin films of the AZ31 alloy and AZ31 alloy with Ti capped with Pd were electrochemically hydrogenated and dehydrogenated in a 3 M KOH solution. A phase composition and structure of the films were studied by XRD. It has been determined that the behaviour of magnetron sputtered alloy AZ31 during electrochemical charging with hydrogen was alike that of pure Mg. The shift of the XRD peak Mg (0 0 0 2) to lower angles indicates that a hydrogen solid solution in the AZ31 alloy was formed along with MgH₂. When the AZ31 alloy with 18 at.% of Ti was electrochemically hydrogenated the whole film was transformed into hydride. The minor part of the hydride was in the nanocrystalline state with a structure of rutile and a major part of the hydride was in the amorphous state. After dehydrogenation only a part of the alloy recovered and the rest remained in the state of amorphous hydride. A positive shift of peak Pd (1 1 1) was observed in all of the XRD patterns for hydrogenated films. At least partially the shift should be associated with the compressive stresses in the top-cap layer of Pd, which arose due to the hydrogenation of the AZ31 alloy.     

Influence of magnesium on grain refinement and ductility in a dilute Al–Sc alloy

Experiments were conducted to determine the influence of magnesium additions on grain refinement and tensile ductility in an Al–0.2% Sc alloy processed by equal-channel angular pressing (ECAP). The experiments show ECAP reduces the average grain size to within the range of ∼0.70 to ∼0.20 μm for alloys containing from 0 to 3% Mg but the as-pressed grain size increases to ∼0.3 μm in an alloy with 5% Mg because it is then necessary to use additional annealing treatments during the pressing process. The ultrafine grains introduced by ECAP are stable to high temperatures in the alloys containing from 0 to 3% Mg: in all alloys, the average grain size is <5 μm after annealing for 1 h at temperatures up to ∼750 K. High superplastic ductilities were achieved in the alloy containing 3% Mg but alloys containing 0.5% and 1% Mg exhibited the enhanced ductilities generally associated with conventional Al–Mg alloys. The results suggest the addition of ∼3% Mg is optimum for achieving superplastic elongations at rapid strain rates in the Al–0.2% Sc alloy.     

Improvement of wear resistance of AZ31 and AZ91HP by high current pulsed electron beam treatment

The surface modification of magnesium alloys （AZ31 and AZ91 HP） was studied by a high current pulsed electron beam（HCPEB）. The results show that the cross-sectional microhardness of treated samples increases not only in the heat affected zone（ HAZ）, but also beyond HAZ, reaching over 250μm. This is due to the action of quasi-static thermal stress and the shock thermal stress wave with materials, which result in its fast deformation on the surface layer and so increases microhardness. For the AZ91HP alloy, a nearly complete dissolution of the intermetallic phase Mg17Al12 is observed, and a super-saturated solid solution forms on the re-melted surface, which is due to the solute trapping effect during the fast solidification process. Measurements on sliding wear show that wear resistance is improved by approximately 5.6 and 2.4 times for the AZ31 and AZ91HP respectively, as compared with as-received samples.