In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of -component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy

Change of Deformation Mechanisms Leading to High Strength and Large Ductility in Mg-Zn-Zr-Ca Alloy with Fully Recrystallized Ultrafine Grained Microstructures

Recently, we have found that fully recrystallized ultrafine-grained (UFG) microstructures could be realized in a commercial precipitation-hardened Magnesium (Mg) alloy. The UFG specimens exhibited high strength and large ductility under tensile test, but underlying mechanisms for good mechanical properties remained unclear. In this study, we have carried out systematic observations of deformation microstructures for revealing the influence of grain size on the change of dominant deformation modes. We found that plastic deformation of conventionally coarse-grained specimen was predominated by {0001} slip and {10–12} twinning, and the quick decrease of work-hardening rate was mainly due to the early saturation of deformation twins. For the UFG specimens, {10–12} twinning was dramatically suppressed, while non-basal slip systems containing component of Burgers vector were activated, which contributed significantly to the enhanced work-hardening rate leading to high strength and large ductility. It was clarified by this study that limited ductility of hexagonal Mg alloys could be overcome by activating unusual slip systems ( dislocations) in fully recrystallized UFG microstructures

Enhanced mechanical properties in β-Ti alloy aged from recrystallized ultrafine β grains

Ultrafine β grain structures with recrystallized morphologies were fabricated by severe plastic deformation and subsequent annealing in Ti-10Mo-8 V-1Fe-3.5Al alloy. The minimum mean β grain size of 480 nm was obtained for the first time as a recrystallized structure in Ti alloys. Precipitation behavior of α in subsequent aging significantly changed with decreasing the recrystallized β grain size. Both tensile strength and total ductility of the aged Ti-alloy were increased by the β grain refinement. Tensile strength of 1.6 GPa and total elongation of 9.1% were achieved in the aged specimen having the prior β grain size of 480 nm, which was attributed to its finer and more homogeneous precipitated microstructure having a mixture of nanoscale thin-plate α and globular α without side α plates along β grain boundaries

Achieving large super-elasticity through changing relative easiness of deformation modes in Ti-Nb-Mo alloy by ultra-grain refinement

Large super-elasticity approaching its theoretically expected value was achieved in Ti-13.3Nb-4.6Mo alloy having an ultrafine-grained β-phase. In-situ synchrotron radiation X-ray diffraction analysis revealed that the dominant yielding mechanism changed from dislocation slip to martensitic transformation by decreasing the β-grain size down to sub-micrometer. Different grain size dependence of the critical stress to initiate dislocation slip and martensitic transformation, which was reflected by the transition of yielding behavior, was considered to be the main reason for the large super-elasticity in the ultrafine-grained specimen. The present study clarified that ultra-grain refinement down to sub-mirometer scale made dislocation slips more difficult than martensitic transformation, leading to an excellent super-elasticity close to the theoretical limit in the β-Ti alloy

Transition of dominant deformation mode in bulk polycrystalline pure Mg by ultra-grain refinement down to sub-micrometer

Magnesium (Mg) and its alloys usually show relatively low strength and poor ductility at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure that provides a limited number of independent slip systems. Here we report that unique combinations of strength and ductility can be realized in bulk polycrystalline pure Mg by tuning the predominant deformation mode. We succeeded in obtaining the fully recrystallized specimens of pure Mg having a wide range of average grain sizes, of which minimum grain size was 650 nm, and clarified mechanical properties and deformation mechanisms at room temperature systematically as a function of the grain size. Deformation twinning and basal slip governed plastic deformation in the conventional coarse-grained region, but twinning was suppressed when the grain size was refined down to several micro-meters. Eventually, grain boundary mediated plasticity, i.e., grain boundary sliding became dominant in the ultrafine-grained (UFG) specimen having a mean grain size smaller than 1 μm. The transition of the deformation modes led to a significant increase of tensile elongation and breakdown of Hall-Petch relationship. It was quantitatively confirmed by detailed microstructural observation and theoretical calculation that the change in strength and ductility arose from the distinct grain size dependence of the critical shear stress for activating different deformation modes

Investigation on the Microstructure and Mechanical Properties of Ti-1.0Fe Alloy with Equiaxed α + β Microstructures

© 2020, The Minerals, Metals & Materials Society and ASM International. In this study, the microstructural characteristics and mechanical properties of Ti-1.0Fe alloy with equiaxed α + β microstructures were investigated in detail. Four different equiaxed α + β microstructures with β phase fraction ranging from 3 to 26 pct were obtained by hot deforming the martensite initial microstructure in α + β two-phase region with different deformation temperatures and cooling rates. The average nano-hardness of β grains was found to be much larger than that of α grains, which was attributed to the higher Fe concentration as well as nano-sized athermal ω precipitates inside the β grains. As a result, plastic strain partitioning occurred between the two phases during the tensile deformation, where the plastic strain within the soft α grains was much larger. With the increase of the β phase fraction, both yield and tensile strength of the samples increased, while at the same time, the total elongation gradually decreased. Most of the micro-cracks formed at the α/β interphase boundaries and propagated across the narrowest part of β phase. In the sample with the largest β phase fraction (26 pct), strain-induced β to α′ phase transformation occurred at the expense of initial athermal ω precipitates during the tensile deformation. This resulted into local nano-hardness variations between transformed and un-transformed β areas. Consequently, much more micro-cracks formed in this sample either at the boundaries between transformed and un-transformed β areas, or around the strain-induced α′ phase with a plate-like morphology. This explained the premature fracture shortly after necking in the sample with the largest β phase fraction

Change of deformation mechanisms in ultrafine grained Mg-Zn-Zr-Ca alloy

In this study, a fully recrystallized ultrafine grained (UFG) Mg-Zn-Zr-Ca alloy was successfully fabricated by a process including high pressure torsion (HPT) and subsequent rapid annealing. Room temperature tensile test revealed that the UFG Mg alloy with a mean grain size of 0.98 μm exhibited simultaneously enhanced strength and ductility compared to those of the coarse grained counterpart (grain size 57 μm). Observation of deformation microstructures revealed that {10-12} deformation twinning and basal slip were the dominant deformation mechanisms in the coarse grained specimen, while deformation twinning was significantly inhibited but non-basal slip systems seemed activated in the UFG specimen. The reason for the enhanced mechanical properties in the UFG specimen was discussed based on the change of deformation mechanisms observed

Influence of Fe addition in CP titanium on phase transformation, microstructure and mechanical properties during high pressure torsion

2019 Elsevier B.V. In the present study, commercially pure (CP) titanium with four different Fe additions varying from 0.04 to 1.0 wt% were specifically deformed by high pressure torsion (HPT) process at room temperature. Significant influence of Fe content on the phase transformation, microstructure evolution, and mechanical properties were observed based on systematic evaluations. This study provided the first experimental evidence to support the previously reported theoretical calculation which predicted that the onset pressure of α→ω phase transformation in pure titanium decreased with increasing Fe addition. It has been found that ω phase only formed in CP titanium when it had Fe content higher than 0.25 wt% if the HPT process was conducted under 4.0 GPa. By comparison, ω phase was observed for all four Fe additions if the CP titanium was deformed under pressure of ≥6.0 GPa, with its volume fraction increasing significantly before 1 HPT rotation and then saturating after 10 rotations. Besides, the results also indicate that the Fe addition is very effective in enhancing the grain refinement and improving the mechanical properties of CP titanium by HPT deformation

Ligand-Free Iron-Catalyzed Regiodivergent Hydroboration of Unactivated Terminal Alkenes

The control of regioselectivities has been recognized as the elementary issue for alkene hydroboration. Despite considerable progress, the specificity of alkene substrates or the adjustment of ligands were necessary for specific regioselectivities, which restrict the universality and practicability. Herein, we report a ligand-free iron-catalyzed regiodivergent hydroboration of unactivated terminal alkenes that obtains both Markovnikov and anti-Markovnikov hydroboration products in excellent regioselectivities. Notably, solvents and bases were shown to be crucial factors influencing the regioselectivities and further studies suggested the iron-boron alkoxide ate complex is the key intermediate that determines the unusual Markovnikov regioselectivity. Terminal alkenes with diverse structures (mono-substituted and 1,1-disubstituted, open-chain and exocyclic) underwent the transformation smoothly. The reaction does not require the addition of auxiliary ligands and it can be performed on a gram scale, thus providing an efficient and sustainable method for the synthesis of primary, secondary, and tertiary alkyl borates