Papers by Keyword: Deformation Mechanism

Abstract: Recovery usually softens strain hardened coarse grained metallic materials but it can increase the strength of ultrafine grained materials. The present work shows evidence that dynamic recovery can produce strain-hardening behavior in ultrafine grained aluminum processed by high pressure torsion. Mechanical testing reveals an increase in flow stress during low strain rate tensile tests and a decrease in strain rate during creep tests. No significant change is observed in the grain size. It is shown that this effect can be used to increase the uniform elongation of these materials.

Abstract: The present study aims to investigate the mechanical behaviour of pure cobalt in plasticity at varying temperatures, through in situ thermomechanical loadings under Electron Backscattered Diffraction (EBSD) in order to gain a deeper understanding of the various mechanisms that occur. EBSD analysis allows for the determination of microstructural parameters at a refined scale including grain size, grain misorientation, crystallographic and morphologic texture, and phase ratios, which can be employed to establish a correlation between microstructural changes and deformation mechanisms with temperature. Analyses were conducted in situ using either a furnace that can reach 1000 °C, or a 10 kN thermomechanical device, which enables simultaneous heating and mechanical loading. Both test types were automated in the Scanning Electron Microscope (SEM) by correlating the stage movement with the region of interest, enabling EBSD mappings to be acquired always at the same location. Mappings were post-processed via the spherical indexing process, which yields high-quality indexation (with a reduced number of points exhibiting a Confidence Index of less than 0.1), even at high strain levels. Such experiments conducted on cobalt demonstrated austenitic and martensitic transformations between hexagonal close packed and face centred cubic phases with temperature. Indeed, approximately 31.6 % of the initial face centred cubic phase has transformed into the hexagonal phase for 8 % strain during an in situ tensile test. This transformation is initiated in plasticity by dislocation motions in basal planes and subsequently accelerated by the concurrent activation of mechanical twinning. Additionally, an in situ thermal treatment in the SEM enabled the accurate determination of the phase transformation temperature: 460 °C during heating and 350 °C during cooling, corresponding to the points where the cubic phase fraction reaches a 50 % relative change.

Abstract: High-entropy alloys (HEAs) have led to breakthroughs in materials science due to their superior properties and the challenge of achieving the high strength and high ductility trade-off. Microstructural evolution during cold and warm compression tests of the single-phase Al₈Cr₁₂Mn₂₅Fe₃₅Ni₂₀ high entropy alloy (Fe-HEA) is investigated in the present work. The current study assesses the effect of temperature on the mechanical properties and deformation mechanism of the face-centered cubic structure Fe-HEA. The arc-melted ingot is homogenized at 1473 K and then directly hot-rolled to break the cast structure of the alloy prior to testing procedures. Fe-HEA is tested through uniaxial compressive testing at three different selected temperatures: 293, 473, and 673 K utilizing a Gleeble thermo-mechanical simulator at a strain rate of 0.001 s^-1. The compressive behavior at 673 K showed a higher strain hardening exponent when compared to 293 and 473 K. The deformed microstructural features of the compressed and quenched specimens, deformation mechanism, and phase revolution are investigated with X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Dislocation densities for the deformed conditions were estimated to be 4.11 × 10¹⁴ and 5.39 × 10¹⁴ m^-2 for the 473 and 673 K deformed conditions, respectively. At a deformation temperature of 673 K, B2 precipitation is observed at the high-angle grain boundaries.

Abstract: Magnesium and its alloys display a non-usual relationship between flow stress and grain size at room temperature. Breaks in the Hall-Petch relationship have been reported in the literature. Inverse Hall-Petch behavior in which flow stress reduces with grain size decreasing has also been reported in pure magnesium and magnesium alloys with ultrafine and nanocrystalline structures. The present overview discusses these effects in terms of controlling deformation mechanisms. The distinct strength observed in pure magnesium and magnesium alloys with ultrafine grained structure is also discussed. It is shown that experimental data for fine and ultrafine grained magnesium alloys agree with a model suggested recently based on the mechanism of grain boundary sliding. It is also exhibited that the stability of the grain structure might control the strength of ultrafine grained samples.

Abstract: High dimensional accuracy is of crucial importance in digital manufacturing to guarantee production capability and product performance. For manufacturing of thin-walled complex extrusions, it is often challenging to meet the tight dimensional tolerance requirements for automated mass production, due to dimensional imperfections and variations accumulated from the thermo-mechanical processing history. Recently, a new calibration technique, called Transverse Stretch and Local Bending, was developed, enabling significant improvement of the dimensional accuracy of thin-walled open profiles at a low cost. However, the deformation mechanisms have not been well understood, which in turn affect the process design for achieving high-precision products. In this study, a through-process finite element model was established and experimentally verified, which is used as a tool to investigate the mechanisms in the calibration process. It is found that the gap opening is mainly reduced in the inserting stage, but the calibration stage plays a key role in achieving high-precision products after unloading. The critical factor to achieve high dimensional accuracy is reducing the through-thickness gradients on both the profile bottom and sidewall. By controlling the total vertical displacement in the transverse stretch and local bending, the stress gradients can be effectively reduced, and the dimensional deviation caused by springback after unloading can be well mitigated. This fundamental study will benefit the industry to obtain high-precision extrusions.

Abstract: The plastic deformation mechanism of Ti-55531 alloy with bimodal microstructure was investigated by compression testing at room temperature. The bimodal microstructure was composed of equiaxed primary α phase (α_p) and transformed β (β_trans) that consisted of acicular secondary α phase (α_s) and residual β phase (β_r). In the initial stage of deformation, the α_p grains first underwent plastic deformation, the dislocations germinated and increased, forming the dislocation loop with the dislocation free zone in α_p at the true stain of 0.083. With the true strain subsequently increasing to 0.105, the dislocation tangle and dislocation pile-up occurred in α_p, and a lot of dislocations were also activated in most of α_s. Moreover, the dislocation density was increasing gradually in β_r with the adding of strain. Finally, the dislocation pile-up and dislocation tangle appeared in α_s and β_r at the true strain of 0.163. The whole deformation process was coordinated by α_p, α_s and β_r. They accommodated mutually and completed deformation together.

Abstract: Aluminum alloys have been attracting significant attention. Especially Al-Mg-Si alloys can exhibit an excellent balance between strength and ductility. Deformation mechanisms and microstructural evolution are still challenging issues. Accordingly, to describe how the type of phase influence mechanical behaviour of Al/Mg/Si alloys, in this paper atomic simulations are performed to investigate the uniaxial compressive behaviour of Al-Mg-Si ternary phases. The compression is at the same strain rate (3.10¹⁰ s−1); using Modified Embedded Atom Method (MEAM) potential to model the deformation behaviour. From these simulations, we get the total radial distribution function; the stress-strain responses to describe the elastic and plastic behaviors of GP-AlMg₄Si₆, U2-Al₄Mg₄Si₄ and β-Al₃Mg₂Si₆ phases. For a Detailed description of which phase influence hardness and ductility of these alloys; the mechanical properties are determined and presented. These stress-strain curves obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. From the results, it was found that GP-AlMg₄Si₆ & U2-Al₄Mg₄Si₄ phases are brittle under uniaxial compressive loading while β-Al₃Mg₂Si₆ phase is very ductile under the same compressive loading. The engineering stress-strain relationship suggests that β-Al₃Mg₂Si₆ phase have high elasticity limit, ability to resist deformation and have the advantage of being highly malleable. Molecular dynamics software LAMMPS was used to simulate and build the Al-Mg-Si ternary system.

Abstract: Aluminum alloys development always exit in the manufacturing process. Al/Mg alloys have been attracted significant attention because of their excellent mechanical properties. The microstructural evolution and deformation mechanisms are still challenging issues, and it is hard to observe directly by experimental methods. Accordingly, in this paper atomic simulations are performed to investigate the uniaxial compressive behavior of Al/Mg phases; with different ratio of Mg ranging from 31% to 56%. The compression is at the same strain rate (3.10¹⁰ s⁻¹), at the same temperature (300K) and pressure, using embedded atom method (EAM) potential to model the interactions and the deformation behavior between Al and Mg.From these simulations, we get the radial distribution function; the stress–strain responses to describe the elastic and plastic behaviors of β-Al₃Mg₂, ε-Al₃₀Mg₂₃, Al₁Mg₁ and γ-Al₁₂Mg₁₇ phases with 31, 41, 50 and 56% of Mg added to pure aluminum, respectively. The mechanical properties, such as Young’s modulus, elasticity limit and rupture pressure, are determined and presented. The engineering equation was used to plot the stress-strain curve for each phase.From the results obtained, the chemical composition has a significant effect on the properties of these phases. The stress-strain behavior comprised elastic, yield, strain softening and strain hardening regions that were qualitatively in agreement with previous simulations and experimental results. These stress-strain diagrams obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. Under compression, the deformation behavior of β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases is slightly similar. From the results, it was found that ε-Al₃₀Mg₂₃ phase are brittle under uniaxial compressive loading and γ-Al₁₂Mg₁₇ phase is very ductile under the same compressive loading.The engineering stress-strain relationship suggests that β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases have high elasticity limit, ability to resist deformation and also have the advantage of being highly malleable. From this simulation, we also find that the mechanical properties under compressive load of ε-Al₃₀Mg₂₃ phase are evidently less than other phases, which makes it the weakest phase. The obtained results were compared with the previous experimental studies, and generally, there is a good correlation.The Al-Mg system was built and simulated using molecular dynamics (MD) software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).

Abstract: Single Point Incremental Forming (SPIF) is a die-less forming process with advantages of high-flexibility, low-cost and short lead time. The high local strains that are applied to the metal sheet, often exceeding the conventional formability limit. This paper is focused on comparison of predicted forming limit curves with measured experimental data on Hot-Dip Zinc-Coated Cold-Rolled sheet, with 0.20 mm thick is studied in single point incremental forming. Truncated square pyramid and cone are formed to study the formability of blank sheets at room temperature. It was found that both Formulation of plastic instability criteria and Keeler’s formula gives the lowest FLC. FLDs have predicted failures in forming process consistently with the real experiments. The experimentally obtained cracking limit differ from analytical one and empirical one by about 3.398 and 2.135 true strain respectively at FLD₀, the corresponding plane strain values.

Abstract: By means of creep properties measurement, microstructure observation and contrast analysis of dislocation configuration, the creep behavior of a 4.5%Re/3.0%Ru-containing single crystal nickel-based superalloy at elevated temperature is investigated. Results show that the creep life of the alloy at 1040°C/160MPa is measured to be 725h to exhibit a better creep resistance at high temperature. In the primary stage of creep at high temperature, the γ phase in alloy has transformed into the N-type rafted structure along the direction vertical to the stress axis, the deformation mechanism of alloy during steady state creep is dislocations slipping in γ matrix and climbing over the rafted γ phase. In the latter period of creep, the deformation mechanism of alloy is dislocations slipping in γ matrix and shearing into the rafted γ phase. Wherein the dislocations shearing into the γ phase may cross-slip from {111} to {100} planes for forming the K-W locks to restrain the slipping and cross-slipping on {111} plane, which is thought to be the main reason of the alloy having a better creep resistance. As the creep goes on, the alternate slipping of dislocations results in the twisted of the rafted γ phase to promote the initiation and propagation of the cracks along the interfaces of γ/γ phase up to creep fracture, which is thought to be the damage and fracture mechanism of alloy during creep at high temperature.