Major Deformation Mode Research Articles

On microstructural homogenization and mechanical properties optimization of biomedical Co-Cr-Mo alloy additively manufactured by using electron beam melting

The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM) technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal γ-fcc → ε-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant ε or predominant γ phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The γ → ε strain-induced martensitic transformation is the major deformation mode of the γ phase, meanwhile the formation of stripped ε phase at {111}γ habit planes contributed to a good combination of strength and ductility. Nevertheless, the ε phase was deformed mainly by (0001)ε <11 2¯0 >ε basal and {1 1¯00}ε <11 2¯0 >ε prismatic slip systems, exhibiting very limited ductility and strength. In addition, the ε grains act as secondary hardening factor in the samples consisting of dual γ/ε phase, leading to a non-uniform deformation behavior.

Structure and shear response of tilt grain boundary in titanium nitride

The equilibrated structures and shear responses of <001> titanium nitride (TiN) symmetric tilt grain boundaries (GBs) are investigated using molecular dynamics simulation. The equilibrated GBs are only composed of the kite-shaped structures connected with edge dislocation cores. From the shear response analysis of TiN GBs, we find that there exist two major deformation modes, i.e., GB sliding and shear deformation coupled with GB motion (shear-coupling). GB sliding may finally lead to the GB fracture and the shuffling of GB atoms. The shear-coupling could be further divided into the ideal and non-ideal ones. For the ideal shear-coupling, the shear deformation coupling with GB motion can be exactly described by a factor solely dependent on the GB tilt angle. However, it is not if the additional shear deformations caused by GB sliding or vacancy emission occur during the shear-coupling. Based on the geometrical analysis of atomic deformation mechanism in GB, we propose two theoretical coupling factors to describe the shear-coupling with the additional GB shear deformations. In addition, the deformation mode of GBs could be changed by elevating the temperatures and adding vacancies into GBs.

High temperature quasistatic and dynamic mechanical behavior of interconnected 3D carbon nanotube structures

Carbon nanotubes (CNTs) are one of the most appealing materials in recent history for both research and commercial interest because of their outstanding physical, chemical, and electrical properties. This is particularly true for 3D arrangements of CNTs which enable their use in larger scale devices and structures. In this paper, the effect of temperature on the quasistatic and dynamic deformation behavior of 3D CNT structures is presented for the first time. An in situ high-temperature nanomechanical instrument was used inside an SEM at high vacuum to investigate mechanical properties of covalently interconnected CNT porous structures in a wide range of temperature. An irreversible bucking at the base of pillar samples was found as a major mode of deformation at room and elevated temperatures. It has been observed that elastic modulus and critical load to first buckle formation decrease progressively with increasing temperature from 25 °C to 750 °C. To understand fatigue resistance, pillars made from this unique structure were compressed to 100 cycles at room temperature and 750 °C. While the structure showed remarkable resistance to fatigue at room temperature, high temperature significantly lowers fatigue resistance. Molecular dynamics (MD) simulation of compression highlights the critical role played by covalent interconnections which prevent localized bending and improve mechanical properties.

Establishing the microstructure-strengthening correlation in severely deformed surface of titanium

ABSTRACTSurface nanostructuring of engineering materials can be utilised to enhance materials performance for various applications. The aim of this work was to investigate the evolution of microstructure and its correlation with strengthening mechanisms in nanocrystalline commercially pure titanium (cp-Ti) produced by surface mechanical attrition treatment (SMAT). The individual contributions of dislocation slip and twining as the deformation mechanisms during SMAT have been quantified using X-ray line profile analysis and corroborated with transmission electron microscopy and electron backscattered diffraction techniques. It is found that twining is operative only in the early stages of deformation. The absence of twin–twin intersections suggests that twining is not directly responsible for the initial refinement of grain size. Dislocation slip is the major deformation mode, which leads to the refinement of the microstructure by forming low-angle lamellar boundaries. Continuous dynamic recrystallisation is demonstrated to be the mechanism of nanocrystallisation in cp-Ti using detailed microscopic analysis. In contrast to previous studies, which have neglected the contribution of Taylor strengthening, it is observed that a combination of Hall–Petch and Taylor relationships can explain the strength only if separate set of parameters K (Hall–Petch constant) and α (geometrical factor in Taylor relationship) are used for the nanocrystalline surface and severely deformed sub-surface of cp-Ti. Taken together, this work provides new insights into the underlying mechanisms for engineering nanocrystalline materials.

Phenomenological uncoupled ductile fracture model considering different void deformation modes for sheet metal forming

A mathematical model of ductile fracture behavior is devised by considering fracture incipience as a void deformation limit phase. To this end, two major void deformation modes, viz., void dilation and void elongation/rotation are carefully considered in the model. Void dilation is governed by the normalized first principal stress, whereas void elongation/rotation is characterized by the normalized maximum shear stress. To model the ductile damage accumulation process caused by plastic deformation, the above two void deformation modes are carefully combined with a strain-based nonlinear void nucleation function in an integral framework, with model parameters with clear physical meanings being introduced. Through hypothesis of a proportional loading, a 3D fracture surface function is obtained. The 3D fracture surface is calibrated using a robust experimental-numerical approach for a dual-phase steel (DP780) sheet. In addition, the fracture data of two aluminum alloys are used for construction of their 3D fracture surfaces. The acceptable deviation of the predictions from the experimental results confirms the good prediction performance of the proposed model for various metals under the distinctly different stress states that often manifest in sheet metal forming processes. Moreover, scanning electron microscopy analysis of the DP780 specimens further demonstrates that the proposed model expresses a reasonable correlation between the stress state, void evolution, and material ductility.

Experimental study on the deformation and damage of cylindrical shell-water-cylindrical shell structures subjected to underwater explosion

Experimental investigations were carried out on Q235 steel cylindrical shell-water-cylindrical shell (CWC) structures subjected to near field or contact underwater explosion loading. All the tests were conducted in an artificial water pool detonating 50 g charge weight of trinitroluene (TNT) explosive. Corresponding plastic deformation of the CWC structures were measured for different structural parameters and standoff distances. Three major deformation modes including seven typical subdivided deformation characteristics were classified and analyzed considering the effect of standoff distance, shell thickness and water interlayer thickness. Microscopic analysis was performed using the SEM method to further compare two typical subdivided fracture modes with the combination of the macroscopic deformation features.

Tension–compression asymmetry in yield strength and hardening behaviour of as-extruded AZ31 alloy

The effects of texture and twinning on the yield asymmetry and differential work-hardening behaviour of an as-extruded AZ31 alloy under uniaxial compression and tension are investigated using a viscoplastic self-consistent model. The results show that the yield asymmetry and differential work-hardening behaviour can be attributed to the combined effects of strong fibre texture and the polar nature of twinning. The variation in the relative activity of each deformation mode under compressive deformation is much more complex than that under tension. Under tensile deformation, the major deformation modes are prismatic and pyramidal slips, while twinning and basal slip are the major deformation modes under compressive deformation. The presence of extension twins and contraction twins is confirmed by microstructural observations.

Deformation mechanisms in nanoporous metals: Effect of ligament shape and disorder

The current work presents a numerical modelling approach for investigating the effect of ligament shape and disorder on the macroscopic mechanical response of nanoporous gold (NPG). The approach starts from a ‘single ligament’ analysis with respect to three fundamental deformation modes, bending, torsion, and compression, that depend on the ligament shape. It can be shown that the predictive capability of the highly computationally efficient beam model is sufficient for a large variation in ligament shapes. Using a representative volume element (RVE) composed of such ligaments, different degrees of disorder are included. For both the single ligament and RVE models, the cylindrical beam serves as a common reference to compare the results when varying the ligament shape. From the comparison of the RVE elastic response with the single ligament results and the further analysis of statistical information from the elements in the RVE, it is found that bending is the major deformation mode for perfectly ordered RVEs, whereas torsion gains importance for increasing RVE disorder. The effect of compression of the ligaments can be neglected in general. It is concluded that the transition to torsion deformation due to disorder is the cause of the strongly reduced lateral expansion during compression deformation of NPG.

Competitive effect of stacking fault energy and short-range clustering on the plastic deformation behavior of Cu-Ni alloys

Uniaxial tensile tests were conducted to investigate the plastic deformation behavior and deformation microstructures of coarse-grained Cu-Ni alloys containing a wide range of Ni contents (5–20at%), which possess higher stacking fault energies (SFE) than pure Cu. The mechanical testing results show that, with increasing Ni content, i.e., jointly increasing SFE and degree of short range clustering (SRC), the ultimate tensile strength increases, but the ductility keeps almost unchanged; meanwhile, there exists an obvious increasing stage (or “bump”) in the strain-hardening rate curves at around 3% engineering strain. Microstructural examinations demonstrate that dislocations are apt to slip on primary slip planes at the initial stage of deformation (e.g., 3% engineering strain) to form planar slip bands, indicating that the existence of SRCs in Cu-Ni alloys is beneficial to the promotion of planar slip, leading to the occurrence of a “bump” phenomenon in the strain-hardening rate curves. With increasing deformation amount to a certain degree, wavy slip becomes the major deformation mode under the joint influence of high SFE and diminution of SRCs, and the final deformation microstructures transform from dislocation cells and cell blocks into extended dislocation walls with increasing Ni content. Both the cell block structures and extended dislocation walls subdivide the coarse grains uniformly to disperse local strain concentration, thus enabling the Cu-Ni alloys to maintain a high ductility with an increased tensile strength with increasing Ni content. In a word, the plastic deformation behavior of Cu-Ni alloys is actually governed by the competitive influence of SRC and SFE.

Thermally induced failure mechanism transition and its correlation with short-range order evolution in metallic glasses

The effect of temperature on the short-range order (SRO) structures, deformation mechanisms and failure modes of metallic glasses (MGs) is of fundamental importance for their practical applications. However, due to lack of direct structural information at the atomistic level from experiments and the absence of previous molecular dynamics (MD) simulations to reproduce experimental observations over a wide range of temperature, this issue has not been well understood. Here, by carefully constructing the atomistic models of Cu64Zr36 and Fe80W20 MGs, we are able to reproduce the major deformation modes observed experimentally, i.e. single shear banding (SB) at low temperatures, multiple shear-bandings at intermediate temperatures and homogeneous plastic flow at elevated temperatures. By examining the evolution of SRO, we find that the different failure modes exhibit distinctively different full-icosahedron (FI) evolution pathways at different temperatures. Specifically, at low temperatures, the FI concentration first deceases to a minimum, then recovers slightly, and finally comes to a plateau; at intermediate temperatures, it first decreases and then reaches a plateau; while at elevated temperatures, it decreases simply monotonically. These different pathways arise from the dynamic competition between the destruction and recovering of FI clusters. We further show that the local softening caused by the destructions of FI clusters is crucial for the formation of localized shear planes and further shear bands. Since our simulations exhibit the same trend for both MG systems, it is expected that these findings may be generic for a wide range of MGs.

Effect of initial orientation on the microstructure and mechanical properties of textured AZ31 Mg alloy during torsion and annealing

Abstract We studied the effect of crystallographic orientation and temperature on the microstructure and mechanical properties of extruded AZ31 magnesium alloy bar by torsion and subsequent annealing. The results show that the orientation between torsion axis (TA) and extrusion direction (ED) has a significant impact on the microstructure evolution. With TA parallel to ED, profuse extension twins appeared. After annealing, zones close to the surface were completely recrystallized and refined, while the center still had some extension twins left. With TA perpendicular to ED, extension twins were inhibited. It is speculated that, except extension twins, contraction twins is also acting as a major deformation mode. Upon annealing, this specimen was completely recrystallized, even at the center, which is considered as a result of more preferred nucleation sites for static recrystallization from contraction twins. As demonstrated, the temperature has little impact on the microstructure development when twisted at room temperature or liquid nitrogen temperature. Moreover, the compression tests show that the compression ductility and yield strength were improved simultaneously for both samples when compressed on the direction either along ED or perpendicular to ED, due to the combined effects of grain refinement and texture weakening.

Effect of aluminum content on the texture and mechanical behavior of Mg–1 wt% Mn wrought magnesium alloys

The mechanical response of as-extruded magnesium–aluminum–manganese alloy with varying (1–4wt%) aluminum content has been examined, while the concentration of manganese is kept constant at 1wt%. Results indicated that microstructure exhibits more uniform and equiaxed grain structure, and gradual decrease of texture intensity with increasing aluminum content. Mechanical testing results revealed tensile and compressive yield strengths decreased, while yield anisotropy, tensile uniform elongation and strain hardening exponent increased with increasing aluminum content. Weaker texture was considered to be responsible for lower yield strength, and in particular for significant decrease in tensile yield strength. In addition, simulation results revealed that prismatic <a> and basal <a> slips played a very important role during tensile deformation, and the contribution of prismatic <a> slip decreased with increase in aluminum content. On the other hand, {101¯2} twin was the major deformation mode at the initial stage of the compressive deformation in addition to the basal <a> slip, and the relative activity of the {101¯2} twin decreased with increasing aluminum content.

An experimental investigation of energy absorption in TRIP steel under impact three-point bending deformation

TRIP (Transformation-induced Plasticity) steel is nowadays in widespread use in the automobile industry because of their favorable mechanical properties such as high strength, excellent formability and toughness because of strain-induced martensitic transformation. Moreover, when TRIP steel is applied to the components of the vehicles, it is expected that huge amount of kinetic energy will be absorbed into both plastic deformation and martensitic transformation during the collision. Basically, bending deformation due to buckling is one of the major crash deformation modes of automobile structures. Thus, an investigation of energy absorption during bending deformation at high impact velocity for TRIP steel is indispensable. Although TRIP steel have particularly attracted the recent interest of the scientific community, just few studies can be found on the energy absorption characteristic of TRIP steel, especially at impact loading condition. In present study, experimental investigations of bending deformation behaviors of TRIP steel are conducted in the three-point bending tests for both smooth and pre-cracked specimen. Then, energy absorption characteristic during plastic deformation and fracture process at high impact velocity in TRIP steel will be discussed.

Mechanical and Morphological Properties and Deformation Mechanisms of Acrylonitrile-Butadiene-Styrene/Poly(ϵ-Caprolactone) Blends with Varied Matrix Composition

Acrylonitrile-butadiene-styrene and poly(ϵ-caprolactone) blends (ABS/PCL) were prepared by mixing styrene-co-acrylonitrile (SAN), polybutadiene-g-SAN (PB-g-SAN), and PCL with varied SAN and PCL composition. PCL is miscible with SAN and can improve the matrix toughness. The impact strength and elongation at break of the ABS/PCL blends increased with the PCL content. When the PCL content was lower than 20 wt%, the improvement of impact strength for the blends was not obvious. A significant increase of impact strength took place when the PCL content was between 20 and 25 wt%. When PCL content was more than 20 wt%, the impact strength was higher than 800 J/m which shows the super toughness. The addition of PCL improved the dispersed phase morphology of PB-g-SAN in the matrix and the interfacial adhesion increased. Deformation observations showed that, when the PCL content was lower than 20 wt%, crazing was the major deformation mode. When the PCL content was 20 wt%, crazing and slight shear yielding could be found. When the PCL content was more than 20 wt%, cavitation of rubber particles and shear yielding of the matrix were the major deformation modes. The cause of the change of the deformation mode lies in the varied matrix composition which modifies the crazing and yielding stresses of the matrix and the final fracture mode and impact toughness.

Influence of yttrium content on phase formation and strain hardening behavior of Mg–Zn–Mn magnesium alloy

The aim of this study was to identify the effect of yttrium (Y) addition on the phase development and strain hardening behavior of an extruded Mg–Zn–Mn (ZM31) magnesium alloy. The addition of a small amount (0.3wt.%) of Y in the alloy led to the formation of icosahedral quasicrystalline I (Mg3YZn6) phase. Both I-phase and W-phase (Mg3Y2Zn3) were present in the extruded ZM31+3.2Y alloy, while long period stacking ordered (LPSO) X-phase (Mg12YZn) and Mg24Y5 were observed in the extruded ZM31+6Y alloy. The Y addition significantly refined grains in the extruded state. The presence of I-phase in the extruded ZM31+0.3Y alloy increased hardness, compressive yield strength, and Stage B strain hardening rate. The extruded ZM31+3.2Y alloy exhibited a lower hardness and Stage B hardening rate due to the formation of W-phase. Both extruded ZM31+0.3Y and ZM31+3.2Y alloys showed a yield point phenomenon with an initial negative strain hardening rate. The extruded ZM31+6Y alloy had a high hardness and compressive yield strength without Stage B hardening, suggesting a change of major deformation mode from twinning to slip mainly due to the role of LPSO X-phase. After solution treatment and aging, the hardness and compressive yield strength gradually increased with increasing Y content, while the strain hardening exponent and the extent of Stage B strain hardening decreased due to the dissolution of I- and W-phases and the presence of LPSO X-phase.

The Effect of Nd on the Tension and Compression Deformation Behavior of Extruded Mg-1Mn (wt pct) at Temperatures Between 298 K and 523 K (25 °C and 250 °C)

The tension and compression deformation behavior of extruded magnesium-1 wt pct manganese alloys with nominally 0.3 wt pct (MN10) and 1 wt pct neodymium (MN11) was studied over the temperature range of 298 K to 523 K (25 °C to 250 °C). Nd additions to Mg alloys tend to reduce the strong basal texture exhibited by conventional wrought Mg alloys and this work was intended to study the effect of Nd on the deformation behavior of Mg alloys. Insitu tensile and compressive experiments were performed using a scanning electron microscopy, and electron backscatter diffraction was performed both before and after the deformation. A slip trace analysis technique was used to identify the distribution of the deformation systems as a function of strain, and based on this analysis and the texture of the undeformed samples, the critical resolved shear stress ratios between the deformation systems were estimated. In the case of MN11, the deformation behavior under tension at all temperatures was dominated by slip, while in compression, extension twinning was the major deformation mode. In tension at 323 K (50 °C), extension twinning, basal, prismatic 〈a〉, and pyramidal 〈c + a〉 slip were active in MN11. Much less extension twinning was observed at 423 K (150 °C), while basal slip and prismatic 〈a〉 slip were dominant and presented similar relative activities. At 523 K (250 °C), twinning was not observed, and basal slip controlled the deformation. With the reduction of Nd content, less slip deformation and more twinning were observed during the tensile deformation. However, like for MN11, the extent of twinning in MN10 decreased with increasing temperature and basal slip was the primary deformation mode at elevated temperatures. Extension twinning was the major deformation mode under compression for all test temperatures in MN10 and MN11. The tensile strength decreased with increasing temperature for both alloys, where MN10 was slightly stronger than MN11 at 323 K (50 °C), which was expected to be a result of the stronger basal texture exhibited by MN10 due to its lower Nd content. However, MN11 maintained its strength more at elevated temperatures compared with MN10, and this was explained to be a result of the greater Nd content.

Nanoindentation on the doubler plane of KDP single crystal

The nanohardness is from 1.44 to 2.61 GPa, the Vickers hardness is from 127 to 252 Vickers, and elastic modulus is from 52 to 123 GPa by the nanoindentation experiments on the doubler plane of KDP crystal. An indentation size effect is observed on the doubler plane in the test as the nanohardness and elastic modulus decreases with the increase of the maximum load. Slippage is identified as the major mode of plastic deformation, and pop-in events are attributed to the initiation of slippage. And the variation of unloading curve end is the result of stick effects between the indenter and the contact surface. The depth of the elastic deformation, which is between 40 and 75 nm, is responsible for the elastic deformation. The doubler plane of KDP crystal has anisotropy, and the relative anisotropy of nanohardness is 8.2% and the relative anisotropy of elastic modulus is 8.0%.

Evolution of microstructure and texture during planar simple shear of magnesium alloy

A rolled magnesium alloy was deformed using a planar simple shear apparatus, in which prismatic slip is expected to be a major deformation mode. Examination of the microstructure and texture indicated considerable tension twinning activity. Simulation of flow stress and texture using the viscoplastic self-consistent model was able to reproduce this twinning activity. Although the deformation twinning had a large influence on the microstructure and texture, the flow stress was mostly controlled by prismatic slip.

Energetics of twinning in martensitic NiTi

This work elucidates the details of twinning in martensite as the major deformation mode in NiTi undergoing thermoelastic phase transformation. We report on generalized planar fault energy and generalized stacking fault energy barriers in NiTi shape memory alloys in the monoclinic martensite state (B19′). The energy barrier for (0 0 1) twin migration is 7.6 mJ m −2, which is smaller than that for twinning in soft face-centered cubic metals. The conjugate (1 0 0) twinning corresponds to a higher energy barrier of 41 mJ m −2. Both modes of twinning have been observed experimentally, the (0 0 1) at lower stresses, and the (1 0 0) mode at higher stresses. The unstable stacking fault energy for slip is also calculated, and the results show that twin migration is favored over slip in the B19′ state. We suggest that the results provide a quantitative methodology for the development of new shape memory alloys in which twinning can occur at stress levels far below that corresponding to plastic deformation.

Simulations of stress-induced twinning and de-twinning: A phase field model

Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction. The twin microstructures at grain boundaries are in agreement with experimental observation. It was found that de-twinning occurs during unloading in the simulations, however, a strong dependence of twin structure evolution on loading history was observed.