Materials Science Forum Vol. 941

Abstract: Microstructure and mechanical property of the Mg-TM (TM=Ni or Cu) -Y alloys were investigated. Results revealed that Mg phase, Long period stacking ordered (LPSO) phase, and Mg₂TM phase were formed in the Mg-TM-Y alloy around the composition ratios of TM and Y are 1:1 or 1:2. Tensile test clearly showed relationship between the mechanical property and microstructure in Mg-Ni-Y cast alloy. The 0.2% proof stress (σ_0.2) of the Mg-Ni-Y cast alloy increase with the increasing solute elements contents, while the elongation decreases. This result indicated that the Mg-TM-Y alloy with the composition ratios of TM and Y are 1:1 or 1:2 have both high proof stress and appropriate elongation. It was suggested that the LPSO phase was appropriate strengthening phase in the Mg-rich region in the Mg-TM-Y alloy system. Basal texture of the LPSO and Mg phases was formed by hot-rolling in the sheet plane and the Mg₂TM phase was dispersed in the Mg phase. The Mg₉₇Cu₁Y₂ rolled sheet showed highσ_0.2 about 350 MPa at room temperature. Furthermore, formation of an oxide film on the Mg-Cu-Y alloy was investigated at 973 K in air. As a result, it was suggested that the Y₂O₃ film was formed on the re-melted Mg-Cu-Y alloy surface as an incombustible oxide film.

Abstract: The article presents the results of an investigation of microstructure, mechanical properties and corrosion resistance of magnesium alloy WE43 processed by rotary swaging. The resulting microstructure is characterized by an average size of structural elements of 0.5 – 0.8 μm. The grain refinement leads to an increase in the strength of the alloy to 393 – 416 MPa while the tensile elongation stays at a level of 7 – 12.5%. The microstructure produced by rotary swaging does not lead to deterioration of the resistance of the alloy to electrochemical and chemical corrosion.

Abstract: FAST (Fast light Alloys Stamping Technology) has recently been developed to efficiently and economically manufacture lightweight, high strength structural components from aluminium alloys sheet. Post-form strength prediction of 6xxx series aluminium alloy (AA6xxx) after FAST and multiple stage heat treatments has been a challenge. This is due to the effect of pre-existing dislocations induced via high temperature plastic deformation in the forming process. In the present research, a new PFS (post-form strength) model has been proposed to predict the age-hardening response of AA6xxx alloys undergoing FAST and subsequent thermal cycles. The model incorporates two sub-models, for simulating viscoplastic flow and predicting strength evolution respectively. The first sub-model incorporates a set of constitutive equations, developed to model the stress-strain curve of AA6xxx during FAST. The second sub-model employs precipitation-hardening and dislocation-hardening theories to simulate the evolution of microstructure and, as a consequence, strength of alloys undergoing artificial ageing cycles. This is calculated by considering the intrinsic resistance of the alloy to dislocation movement due to solute atoms and precipitates. The strength was computed accurately via the internal state variables method, in which dislocation density, volume fraction of precipitates, solute concentration and radii of precipitates were correlated. Furthermore, the model was validated by comparing results with transmission electron microscope (TEM) images as well as hardness measurements. Hence, the model performs as a powerful and comprehensive tool to simulate post-form strength of 6xxx series aluminium alloys that undergo complicated thermomechanical processes including high temperature deformation and post-form heat treatment, with less than 5% deviation between measured and predicted values.

Abstract: We have investigated {332}<113> twinning and detwinning mechanisms in β-Ti alloys. Microstructure-twinning relations were evaluated in a β-Ti-15Mo (wt.%) alloy by statistical analysis of the evolving twin structure upon deformation by in-situ SEM testing and electron backscattering diffraction (EBSD). We find that most of the primary twins (~80%) correspond to the higher stressed variant and follow Schmid’s law with respect to the macroscopic stress. Detwinning mechanism was evaluated in a multilayered β-Ti-10Mo-xFe (x: 1-3 wt.%) by EBSD. We find that the detwinning process consists of two independent events that occur at two different microstructural regions, i.e. twin tips located at grain interiors and grain boundaries. Both detwinning modes can be explained from a thermodynamic standpoint where the boundary dissociation processes minimize the boundary free energy.

Abstract: The present work addresses the microstructure evolution of refractory Me-Si-B (Me = Mo, V) alloys consisting of a refractory metal solid solution phase (Me_SS) and two intermetallic phases Me₅SiB₂ and Me₃Si. The aim of the present study is to find the ternary eutectic composition in such systems which are expected to combine a well-defined eutectic microstructure with properties such as high strength, an excellent creep resistance at high temperatures and acceptable oxidation behavior. Two refractory metal systems based on molybdenum and vanadium are investigated. Various alloy compositions located in different primary solidification areas were produced by arc-melting and analyzed via SEM. The obtained results are discussed in the light of published liquidus projections. By carrying out these experiments, a Me_SS-Me₅SiB₂-Me₃Si ternary eutectic could be determined in both Me-Si-B systems.

Abstract: An equiatomic CrCoNi medium-entropy alloy plate was heavily deformed by conventional cold rolling and subsequently annealed at different temperatures. Microstructure and texture evolution of the deformed and annealed sheets were investigated by electron backscatter diffraction and X-ray diffraction. Heavy cold rolling induces an alloy type α-fibre texture with major brass component. This type of texture is indicative of low stacking fault energy of the CrCoNi alloy. Annealing at 700 °C leads to a homogeneously recrystallized microstructure with ultrafine grains of about 800 nm average size. The volume fraction of different texture components is almost similar after annealing at different temperatures. However, the overall texture intensity after annealing is very weak. Finally, in order to understand the microstructure and texture evolution of the CrCoNi alloy, it is critically compared with other low stacking fault energy FCC materials.

Abstract: Taking three titanium commercial alloys: commercial purity titanium (c.p.Ti, single-phase α), Ti64 (Ti-6(wt.%)Al-4V, two-phase α+β) and TIMETAL-LCB (Ti-1.5Al-4.5Fe-6.8Mo, both two-phase α+β and single-phase β) as program materials, the influence of phase composition, microstructure and deformation rate (V_D, varied from 10^-4 to 10¹ s^-1), and deformation mode (compression and 3-point flexure) on the mechanical behavior was studied and compared with data earlier obtained during tensile tests. The size of the matrix phase (alpha- or beta-grains) size and morphology of α+β intragranular mixture were varied using different treatments. Deformation Energy (U_D) was used for analysis of the mechanical behavior of the materials tested. It was found that the U_D dependencies on deformation rate are different for different methods of loading and are determined by a combination of the phase composition, dispersion, and morphology of the phase constituents. More ductile and less dependent on V_D behavior showed c.p.Ti and Ti64 with globular microstructure on all three testing modes, while other materials had some negative features depending on the certain test conditions. Details of mechanical behavior, peculiarities of pores and cracks nucleation causing in final fracture are discussed basing on the results of detailed microstructure study of tested specimens.

Abstract: In this paper, the laser welding of thin titanium sheet in a butt joint configuration are investigated using a continuous Yb: YAG disk source, with high beam quality and a particular fiber configuration, enable to provide a broad range of beam diameters with different intensity distribution. The thermal efficiency of the laser process is discussed as a function of the fiber type. The weldability results for the CP Ti grade 2 and the Ti-6Al-4V titanium alloy are expressed in terms of full penetration, and correct bead geometry (NF L06-395-2000). Full penetration welds are easily achieved with the core fiber, but the outer fiber produces welds with limited geometric defects. Butt joints microstructure consists of an acicular α phase in the fusion zone for CP Ti, and a martensitic α’ phase for the Ti-6Al-4V alloy. Tensile test results confirm a similar or slightly higher joint strength for the full penetration welds, compared with the parent metal.

Abstract: The high-pressure torsion processing technique was used to consolidate and process magnesium-based chips. Chips were prepared by machining commercially pure magnesium and a magnesium alloy AZ91 separately. Optical microscopy and microhardness measurements showed good consolidation of pure magnesium. The magnesium alloy continued to exhibit the boundaries between the chips even after 5 turns of HPT suggesting poor bonding. The results show that soft chips are easier to consolidate through HPT than harder alloys.

Abstract: In order to investigate the effect of rolling on microstructure and mechanical properties of different initial states, cast and extruded magnesium alloy AZ80 bars were rolled in calibre. The microstructural characterization was done by light microscopy. As a result, the initial grain size of the cast AZ80 (66 μm) clearly differs from the extruded bar (13 μm). After 14 passes of hot rolling in calibre, a significant grain refining effect was achieved resulting in grain sizes of 5 μm for the cast and 3 μm for the extruded material. To investigate the mechanical properties in the initial and rolled state, tensile tests of both conditions were conducted at room temperature. Due to grain refining, the tensile strength (162 MPa) and the elongation (3 %) of cast AZ80 increased remarkably during 14 passes of calibre rolling (360 MPa and 19 %). The strengthening effect was also evident for the rolled extruded AZ80. However, the cast material exhibited cracks during calibre rolling due to its inexpedient microstructure for a high deformation calibre. On the contrary, the extruded AZ80 was easily deformable. This shows the clear impact of initial states on aspired end properties of processed materials. Future investigations will deal with developing a suitable calibration for cast AZ80.