Plastic Instability Research Articles

Tensile properties of modified 316 stainless steel (PNC316) after neutron irradiation over 100 dpa

ABSTRACT The effects of fast neutron irradiation on tensile properties of 20% cold-worked modified 316 austenitic stainless steel (PNC316), which was improved swelling resistance and creep strength, claddings and wrappers for fast reactors were investigated. PNC316 claddings and wrappers were irradiated in the experimental fast reactor Joyo at irradiation temperatures between 400°C and 735°C to fast neutron doses ranging from 21 to 125 dpa. The post-irradiation tensile tests were carried out at room and irradiation temperatures. Elongations of PNC316 measured by the tensile tests were maintained at an engineering level, although the material incurred significant irradiation hardening and softening depending on irradiation temperature. The maximum swelling of PNC316 wrappers was about 2.5 vol.% at irradiation temperature between 400°C and 500°C up to 110 dpa. Japanese 20% cold-worked austenitic steels, PNC316 and 15Cr-20Ni, had sufficient ductility and work-hardenability even after above 10 vol.% swelling, while they had very weak plastic instabilities.

A novel mechanical joining method for sheet-tube structures by annular ring compression

Joining by forming has lately received great attention for connection between sheets and tubes due to its high joining efficiency, simple operation, and excellent joining strength. However, protrusion or indentation often occurs adjacent to the joints and, in some cases, the sheet warps, seriously affecting the performance of the connector. To overcome this problem, we propose a novel joining by forming process by compressing an accessory ring to produce the annular squeezing of the sheet, thus forming high-quality joints with smooth surfaces. A numerical-experimental analysis was adopted to identify the influence of the ring width and height on the joint cross-section, effective strain, tube thickness, and flow velocity. To assess the performance of the enhanced joint, its mechanical properties, microstructure, and failure modes were also analyzed. The results showed that the ring width and height have a significant effect on joint strength. Compared with the original joining process without the ring, the joint utilizing a wider ring can obtain higher pull-out loads (58.9% higher) to detach the sheet from the tube, while no sheet warpage happens for excessive local deformation. The utilization of larger and higher rings also motivates the increase of the pull-out strength. In some cases, tube plastic instability can be triggered during the pull-out tests, thus avoiding the full detachment of the components and contributing to the safety of the mechanical joint. Compared with the non-deformation zone, the grains in the deformation zone of the sheet are compressed and become elongated, and there are inclined separation bands between the two regions. This paper provides a new and efficient method for manufacturing sheet-tube structures with high joining strength.

In-situ EBSD observation and simulation of free surface roughening and ductile failure in the ultra-thin ferritic stainless steel sheet

Surface roughening is one of the most fundamental issues which greatly affects the plastic deformation and ductile fracture behavior of ultra-thin sheet materials. The present study investigated the influence of free surface roughening on the micromechanical failure in the ultra-thin ferritic stainless steel (FSS) sheet for bipolar plate in proton exchange membrane (PEM) fuel cell under in-situ tensile deformation. The microstructural evolution during uniaxial tension was monitored using in-situ electron backscatter diffraction (EBSD) technique. Then the initial microstructure was directly mapped onto the finite element mesh of representative volume element (RVE) in the crystal plasticity finite element (CPFE) simulation. The CPFE model was incorporated with the microscopic ductile fracture criterion in terms of plastic energy dissipation in order to simulate the initiation of ductile fracture caused by the free surface roughening. The in-situ EBSD observations reveal that the microstructure of the ultra-thin FSS sheet exhibits heterogeneous plastic deformation and strain localization on the surface at the large deformation owing to the considerable free surface roughening. The CPFE simulations well reproduce the stress/strain heterogeneity and the local hotspots of the ductile fracture initiation. It reveals that the increase of surface roughness with the deformation results in the increased inhomogeneity of thickness which accelerates the strain localization, consequently the premature necking and failure in the ultra-thin steel sheet. Due to the free surface roughening which can facilitate strain localization, the initiation of ductile fracture is obviously observed in both the ridges and valleys in the free surface and its propagation along the pronounced localization band through the thickness, leading to material failure. The good correlation of free surface roughening to the onset of plastic instability and ductile fracture further advances the insight into the micromechanical failure during plastic deformation of the ultra-thin FSS sheet for bipolar plate in PEM fuel cell.

Improved Mn–segregation bands and its influence on deformation behavior in a severely warm–rolled 10Mn lightweight steel

The hot–rolled microstructures of medium Mn lightweight steels led to the formation of Mn–segregation bands due to a relatively high Mn content of 9–12 wt%, ultimately deteriorating their overall mechanical properties. In this study, a multi–pass warm rolling of 95% reduction was proposed to minimize the Mn–segregation bands in the 10 wt% Mn lightweight steel. This was followed by the detailed microstructural examinations and ex–situ tensile testing to evaluate the mechanical behavior response and deformation mechanism operating upon straining. Severe warm rolling accelerated the dynamic ferrite transformation and recrystallization in the Mn–rich austenitic bands, thus improving the microstructural homogeneity. The austenite stability was further improved so as to enable continuous strain hardening by transformation/twinning–induced plasticity (TRIP/TWIP) effects. This, thereby, led to obtain high tensile strength of 985–1201 ± 15 MPa together with large elongation of 34.3–76.0 ± 1.1%. In addition, the strain compatibility between ferrite and austenite also proves to give rise to an alleviation of strain localization at the ferrite/austenite interface, which also postponed the plastic instability and contributed to the large ductility by the coordinated TRIP/TWIP effects.

Analysis of localized necking in anisotropic sheet metals

The ductility of sheet metals during forming operations is limited by a plastic instability leading to necking in the thickness direction along a narrow band. The critical condition for the onset of localized necking depends on constitutive effects such as plastic anisotropy, strain hardening and softening due to damage growth. Using the classical bifurcation analysis of Rice and co-workers, we derive a general analytical solution for localized necking in elastic–plastic thin sheets obeying arbitrary yield criteria and the normality flow rule. The criterion is illustrated for the case of plastically orthotropic sheets obeying Hill-type yield criteria, for which the model predictions for the localization strains are compared with explicit finite element simulations of sheet necking. Existing results in the literature are recovered for the special case of materials that exhibit in-plane isotropy and power law strain hardening.

Thermo-kinetic characteristics on stabilizing hetero-phase interface of metal matrix composites by crystal plasticity finite element method

Using dislocation-based constitutive modeling in three-dimension crystal plasticity finite element (3D CPFE) simulations, co-deformation and instability of hetero-phase interface in different material systems were herein studied for polycrystalline metal matrix composites (MMCs). Local stress and strain fields in two types of 3layer MMCs such as fcc/fcc Cu–Ag and fcc/bcc Cu–Nb have been predicted under simple compressive deformations. Accordingly, more severe strain-induced interface instability can be observed in the fcc/bcc systems than in the fcc/fcc systems upon refining to metallic nanolayered composites (MNCs). By detailed analysis of stress and strain localization, it has been demonstrated that the interface instability is always accompanied by high-stress concentration, i.e., thermodynamic characteristics, or high strain prevention i.e., kinetic characteristics, at the hetero-phase interface. It then follows that the thermodynamic driving force ΔG and the kinetic energy barrier Q during dislocation and shear banding can be adopted to classify the deformation modes, following the so-called thermo-kinetic correlation. Then by inserting a high density of high-energy interfaces into the Cu-Nb composites, such thermo-kinetic integration at the hetero-phase interface allows a successful establishment of MMCs with the high ΔG-high Q deformation mode, which ensures high hardening and uniform strain distribution, thus efficiently suppressing the shear band, stabilizing the hetero-phase interface, and obtaining an exceptional combination in strength and ductility. Such hetero-phase interface chosen by a couple of thermodynamics and kinetics can be defined as breaking the thermo-kinetic correlation and has been proposed for artificially designing MNCs.

Evaluation of high-temperature formability for Ti-xMo-4Fe metastable beta alloys based on Mo contents

High-temperature formation technology is widely used to fabricate components with metastable β titanium alloys, a material characterized by balanced mechanical properties, including superb formability, corrosion resistance, and high strength. In high-temperature formation, it is crucial to control the formation conditions by linking the changes in those conditions caused by processing variables with microstructures and identifying the optimal conditions for high-temperature formation. As such, in this study, a Ti–Mo–Fe alloy was designed that is different from conventional metastable β titanium by adding molybdenum (Mo) and iron (Fe), which are relatively economical β stabilization elements. The mechanical characteristics of the alloy at room temperature were also examined by varying the Mo content, and a high-temperature compression test was conducted to evaluate its high-temperature characteristics and formability. The high-temperature compression test was performed at five different temperatures (700 °C, 750 °C, 820 °C, 900 °C, and 950 °C) to avoid β-transus and with four different strain rates 1 × 10−3/sec, 1 × 10−2/sec, 1 × 10−1/sec, and 1 × 100/sec. Then, the test results were utilized to create a processing map, which evaluated formability by examining plastic instability and energy distribution efficiency at different temperatures, strain rates, and reduction rates, the processing variables for high-temperature formation. Afterward, their connections with the microstructures were analyzed to determine the optimal conditions for the temperature formation of the newly designed Ti–Mo–Fe metastable β titanium alloy.

The effect of high solid solution Mg contents (7–13 wt%) on the dynamic strain aging behavior of Al–Mg alloys

Plastic instability caused by Portevin–Le Chatelier (PLC) due to the interaction of dislocations with solute atoms, known as dynamic strain aging (DSA), remains a serious hindrance for sheet metal forming, especially for 5000 series Al alloys. Here, we investigated the DSA behavior of hot extruded Al–Mg alloys with high solid solution Mg contents varying from 7 to 13 wt% at different tensile strain rates. With increasing the Mg content and decreasing the strain rate, the critical strain for the onset of serrated stress flow increases gradually, indicating the greatly suppressed plastic instability. Moreover, serrations changes from type B to type C with increased concentrations of Mg at 5 × 10−4 s−1 and 1 × 10−3 s−1. In the Al–7Mg alloy, the critical strain exhibits an inverse behavior at lower strain rates (< 1 × 10−3 s−1) but a normal behavior at higher strain rates (> 1 × 10−3 s−1). Whereas, the inverse behavior predominates the entire range of strain rates in the Al–10Mg and Al–13Mg alloys, where the waiting time of dislocations at temporary barriers is longer than the diffusion time of Mg solutes at the critical strain. Dislocations are pinned prior to the critical strain and the first plastic instability depends on the first unpinning event.

Low-temperature tensile properties, deformation and fracture behaviors in the ferrite and austenite duplex stainless steel with various grain sizes

Ferrite and austenite duplex stainless steels with different grain sizes ranging from 2 to 11 μm were tensile deformed at room temperature and 77 K, and the effects of grain size on the tensile properties were discussed. The mechanism of improvement of the tensile properties by grain refinement was also discussed based on the observation results of the deformation and fracture behaviors. Specimens having identical phase ratios and chemical compositions in two phases but three different grain sizes ranging from 2 to 11 μm were prepared. At room temperature (RT), the strength increased, but the elongation decreased with decreasing grain size. At 77 K, both the strength and elongation were improved by grain refinement. In the specimen with the smallest grain size, a characteristic increase in the work-hardening rate after satisfying the plastic instability condition occurred during the tensile test at 77 K. Deformation-induced α′ martensite was formed during the tensile test at 77 K, and the volume fraction of α′ martensite increased as the grain size decreased from 11 to 2 μm. This resulted in the highest total strain at 77 K in the fine-grained specimen. The number density and size of voids were decreased by refining the grain size. The distance between two adjacent voids, calculated from the number density and size of voids, assuming a random distribution of voids, increased with grain refinement. It suppressed void coalescence and provided the highest true fracture stress in the fine-grained specimen. The Hall-Petch coefficient of the present duplex stainless steel became larger at 77 K than that at RT, resulting in a higher strength increase by grain refinement at 77 K. It can be concluded that the grain refinement in duplex stainless steel remarkably improves both strength and ductility at low temperatures owing to the high Hall-Petch coefficient, an increase in the volume fraction of deformation-induced α′ martensite, and a decrease in the number density of voids.

Hot deformation behavior, dynamic recrystallization mechanism and processing maps of Ti–V microalloyed high strength steel

The hot deformation behavior, dynamic recrystallization mechanism and processing maps of Ti–V complex microalloyed high strength steel under temperatures ranging from 840 to 1040 °C and strain rates varying from 0.01 to 10 s−1 were studied by using thermal-mechanical simulation (Gleeble-3800), optical microscopy (OM), and electron backscatter diffraction (EBSD). The true stress–strain curves of the investigated steel were obtained under the different deformation conditions, the thermal deformation activation energy was calculated, the thermal constitutive equations based on the true stress–strain curves were established, and the 3D power dissipation diagrams, the 3D plastic instability diagrams, and the thermal processing maps under different true strains were drawn. The results show that the flow stress curves are of dynamic recrystallization type at 960–1040 °C and strain rate of 0.01 s−1 and at 1040 °C and strain rate of 0.1 s−1, and the peak stress decreases gradually with the increase of the deformation temperature and the decrease of the strain rate. The hot deformation activation energy of Ti–V microalloyed steel was calculated to be 349.681 kJ/mol. The average grain size of austenite first decreases and then increases with increasing the strain rate under the deformation temperatures of 1000 °C and 1040 °C, and has the smallest value of 15.0 μm and 16.1 μm, respectively, after deformation at the strain rate of 1 s−1. With the true strain increasing from 0.2 to 0.6, the area occupied by the destabilized region increases and the machinable region area decreases. It reveals that the most available hot processing parameter ranges of Ti–V complex microalloyed steel are at the temperature range of 1000∼1040 °C with the strain rate of 0.01–1 s−1.

Radial–axial feeding process design for large-scale flat ring by quantitative estimation of ring roundness error

Due to the large size and thin wall characteristics, the large-scale ring is prone to plastic instability, resulting in loss of ring roundness. An analytical calculation model of roundness error is proposed to design the feeding process. This model is established by analytically deriving the relationships between feeding process, ring geometry, contact arc length, rolling force, ring vibration angle, ring bending angle, contact arc length increment and ring roundness error at different rolling stages. Compared with numerical calculation and production trial, the analytical model can predict the roundness error with reasonable accuracy and high efficiency and guide feeding process design.

Development of ultrahigh-strength low-alloy Mg-0.7Al-0.3Ca-0.4Mn (wt.%) alloy with excellent ductility via controlling extrusion temperature

Mg-0.7Al-0.3Ca-0.4Mn (AXM070304, wt.%) alloy was subjected to extrusion with a ram speed of 1 mm/s and an extrusion ratio of 25:1. The extrusion temperature varies from 170 °C to 400 °C. The influence of extrusion temperature on microstructure and mechanical properties of the dilute AXM070304 alloy was systematically explored. With raising the extrusion temperature, the average size of the dynamic recrystallized (DRXed) grains was increased, while the intensity of the basal fiber texture was decreased. When the extrusion temperature was escalated from 170 °C to 400 °C, the tensile yield strength (YS) of the low-alloy AXM070304 extrusion alloy was decreased from 437 to 246 MPa, while the elongation to failure (EL) was increased from 2.9 to 16.9%. The alloy extruded at 200 °C obtains an YS of 412 MPa, an ultimate tensile strength (UTS) of 418 MPa and an EL of 14.8%, exhibiting excellent ductility and ultrahigh strength. The ultrahigh YS was mainly due to the strengthening from ultra-fine recrystallized grains (0.61 μm) with grain boundary segregation of solute Al and Ca atoms. The growth of DRXed grains was inhibited mainly by the co-segregation of Al atoms and Ca atoms, nanosized Al2Ca and β-Mn particle phases at the grain boundaries. Plastic instability occurred under tensile loading in the ultrafine-grained AXM070304 alloy, which may be due to the fact that the grain boundary segregation of Al atoms and Ca atoms requires a high energy barrier for dislocation emission. Once the tensile stress reaches the peak value, the mobile dislocation density increases suddenly.

The effect of phase distribution of constituent-fiber structure on the deformation heterogeneity of TRIP-assisted lean duplex stainless steel

The cold-rolled TRIP-assisted lean duplex stainless steel (TLDX) showed the directional distribution of constituent-fiber structure. The effect of phase distribution of constituent-fiber structure on the deformation heterogeneity during tensile tests and deep drawing of cylindrical cup was investigated through crystal plasticity finite element model (CPFEM) involved in the martensitic transformation. The results showed that the strain for the plastic instability was lower in the transverse direction (TD)-loaded case (true strain εT ∼0.408) compared with the rolling direction (RD)-loaded case (εT ∼0.485). Also, the strain localization bands formed at a lower strain (engineering strain εE ∼40%) in the TD-loaded case than that in the RD-loaded case (εE ∼55%). At the same strain, the volume fraction of transformed martensite of the TD-loaded case was relatively less than that of the RD-loaded case. For the transformed martensite, it only accommodated a low proportion (<5%) of global strain in the two cases. The proportion of partitioned strain into austenite (Pεγ) in the RD-loaded case was always higher than that in the TD-loaded case, while the partitioned strain into ferrite (Pεα) showed a converse law, which indicated that the deformation heterogeneity in the TD-loaded case is more pronounced. Similarly, the high risk of cracking of deep-drawn cylindrical cups with different sizes also always formed at the specific zones corresponding to the TD-loaded case.

Burst of Internally Pressurized Steel Torispheres

Abstract The paper begins with derivation of true stress–true strain data, including postnecking section. Available results of past uni-axial tests on round 10 mm diameter and 200 mm long mild steel samples are the basis of the conversion. The steel in question was used to manufacture ten torispherical domes that were in the past tested for burst. Hence, the relevance of matching material model is necessary for the finite elements (FE), analyses. In the past, plastic instability and constraints on the magnitude of plastic strains were postulated as criteria for the burst of internally pressurized torispheres. These criteria for burst pressure are being examined and benchmarked against the tests. The current paper, using the FE analyses, shows that modification of constraints on plastic strains has only marginal effect on the burst which still remains on the unsafe side of test data by a sizeable margin. The same is found to be true for plastic instability criterion. Subsequent computations moved back to the use of engineering stress–strain. Then, two types of computing are carried out here, based on multisegment and bilinear modeling of material. Computed results of burst pressure follow the test data to within (−6%, +10%). These results are far better than all the previous.

Dynamic Response Law and Failure Mechanism of Slope with Weak Interlayer under Combined Action of Reservoir Water and Seismic Force

The southwestern region of China is close to the Eurasian earthquake zone. Many engineering areas in southwestern China are affected by earthquakes and are close to the epicenter of earthquakes that occur in this region. During earthquakes, slopes with weak interlayers are more likely to cause large-scale landslides. In response to the low stability of slopes with weak interlayers in reservoir dam areas, the dynamic response law and failure mechanism of weak interlayered slopes under the combined action of reservoir water and seismic forces were studied through shaking table model tests and finite element numerical simulation software. The height of the water level and the size of the seismic waves were changed during these tests. The research results indicate that seismic waves are influenced by weak interlayers and are repeatedly superimposed between the weak interlayers and the slope surface, resulting in an acceleration amplification effect that increases by approximately 1.8 times compared to homogeneous slopes. Vertical earthquakes have a significant impact on the dynamic response of slopes, and their peak acceleration amplification coefficient can reach 0.83 times the horizontal peak acceleration. The stability of weak interlayers during earthquakes is the worst within the range of the direct action of reservoir water. The failure mode of a slope is as follows: earthquake action causes cracking in the upper part of the slope, and as the earthquake increases in intensity, and the infiltration of reservoir water intensifies, the cracks expand. The soft and muddy interlayer in the front section of the slope forms a sliding surface, and ultimately, the sliding failure forms an accumulation body at the foot of the slope. In reservoir dam areas, the stability of a slope is closely related to the engineering safety of the reservoir dam. Therefore, when a strong earthquake and the water level in a reservoir jointly affect a weak-interlayer slope, the slope is in the stage of plastic deformation and instability. The stability of the slope may be overestimated, and the slope is likely vulnerable to sliding instability, which needs to be monitored and treated.

On the influence of tool contact stress on tensile instabilities in plane strain stretching and bending of sheet metals

It is well known that the widely used forming limit curve (FLC) based upon in-plane stretching fails to account for the delay in plastic instability in the presence of appreciable bending and tool contact pressure. The recently proposed General Incremental Stability Criterion (GISC) – a generalization of the Modified Maximum Force Criterion (MMFC) to triaxial loading – revealed the dependence of the boundary conditions on the formation of tensile instabilities. Nevertheless, it remains unclear how the combined effect of radial bending stresses and tool contact pressure affects the formation of an acute neck. It is shown in this study that a compressive normal stress causes a shift of the strain state from initial plane strain tension to positive minor strains for material layers within the cross-section. The convex (outer) layer remains in a state of plane strain-plane stress. Accounting for the shift in the strain state is key to capture the dependence of the contact pressure on necking. A simplified modelling approach, which enforces plane strain tension of the cross-section, was able to capture the formability gain due to bending but failed to account for the delay in plastic instability due to the contact pressure. An incremental multi-layer model was required to account for non-monotonic loading due to bending over the punch and the biaxial shift in the strain path caused by the contact pressure. The models were then applied to the experimental forming limits of a 3rd Gen 1180 advanced high strength steel to demonstrate the formability gains under triaxial loading due to the contact pressure.

Inhibiting plastic tensile instability of non-symmetric thin-walled shell component via increasing regional metal inflow based on heterogeneous pressure-carrying medium

Heterogeneous pressure-carrying medium was employed to establish a differentiated pressure field on sheet metal in flexible die forming process in this work, which aimed at matching the non-symmetric shape of target component and improving metal inflow to avoid local tensile instability. Specifically, metal inflow corresponding to the differentiated pressure field was analytically evaluated. Forming of a typical non-symmetric shell component was experimentally and numerically studied based on the proposed method. Compared with forming processes based on the uniform pressure, difference of metal inflow in two sides of the non-symmetric component increased from 2.16 mm to 3.36 mm and metal inflow in critical region increased by 11.9% when differentiated pressure field (taking heterogeneous elastomer #4–3 for example) was employed. The resultant maximum thinning ratio decreased by 4.2% and the uniformity of shell thickness increased by 16.9%. With the decrease of Shore hardness of elastomer in the formed region, stress path in the ready-to-form region transferred towards the bi-axial tension stress state, i.e., stress ratio (α) increased. And, stress triaxiality (η) in characteristic regions were regulated appropriately, which decreased the risk of tensile instability. It was attributed to the decreased normal pressure and frictional resistance at sheet/elastomer interface in the formed region.

Ultrastrong steel strengthened by multiple shearable nanostructures

Precipitation of multiple strong nanoprecipitates is crucial for the development of ultrahigh-strength structural materials with a strength of 2.5 GPa or above. Nevertheless, the ductility usually loses rapidly with strength due to limited dislocation mobility and high cracking tendency if coarse non-deformable precipitates are employed. Herein, we report a 2.5 GPa maraging steel strengthened by an ultrahigh density of intermeshed shearable nanostructures consisting of Ni(Al, Fe) nanoprecipitates and Mo-rich (∼30 at.%) disordered clusters, both of which assume coherent interfaces. The fully coherent B2-Ni(Al, Fe) particles precipitate in an extremely fast fashion, effectively accelerating local aggregation of low-diffusivity Mo atoms and promoting the formation of Mo-rich clusters surrounding them. This elemental partition was found to be further enhanced by Co addition via depleting both residual Al and Mo within the matrix, leading to the formation of copious yet fine intermeshed nanostructures. During plastic deformation, the interlocked nanostructures not only enhance local cutting stress by combining long-range elastic and short-range chemically ordering effects but also improve dislocation activity and resist shear-induced plastic instability. The multiple shearable nanostructures endow decent ductility (> 6%) of the 2.5 GPa steel, suggesting a new paradigm for designing ultrastrong steels.

Impact of dispersoids’ distribution on portevin-le-chatelier effect and surface quality in Al–Mg-Sc-Zr alloys

The 5xxx-series Al–Mg alloys are susceptible to plastic instabilities visible as stress serrations during uniaxial tensile testing and known as the Portevin Le Chatelier (PLC) effect. This effect also results in stretcher strain markings on the surface of the deformed sheet which limits the application of 5xxx-series Al–Mg alloys in the automotive industry. Sc and Zr have recently been shown to efficiently suppress the plastic instabilities in Al–Mg alloys due to the formation of Al3(Sc,Zr) precipitates that affect the dynamics of dislocations pinning and unpinning and reduces the severity of PLC. The impact of the size distribution of Al3(Sc,Zr) on PLC and related surface quality is currently unknown. In this work, we investigate the PLC and surface quality of an Al–Mg-Sc-Zr alloy and an Al–Mg alloy free of Sc and Zr. The size and distribution of the Al3(Sc,Zr) dispersoids are varied by subjecting the alloys to different homogenization treatments. The PLC behaviour is quantitatively examined with tensile tests and surface roughness of deformed samples is assessed with optical profilometry. In general, the presence of dispersoids resulted in a decreased magnitude and number of stress drops compared to the Sc- and Zr-free Al–Mg alloys. The lowest PLC severity and optimal surface quality was obtained for dispersoids with an average size of 6–7 nm. This optimal size range is rationalized by discussing the combined impact of dispersoids and Mg solute on the waiting time between dislocation pinning and unpinning.

Additive manufacturing of titanium alloys with enhanced strength and uniform ductility via multi-element alloying

The application of commercially pure titanium (CP–Ti) has long been limited by its insufficient strength and plastic instability. However, strength enhancement typically comes at the expense of ductility since the two important mechanical properties are often mutually exclusive. In the present work, multi-element Fe–Cr–Ni–Mo in the form of 316L steel was added into CP-Ti by additive manufacturing (AM), so as to realize the simultaneous enhancement of strength and uniform ductility. The localized homogenization of CP-Ti and 316L alloy melts during laser melting deposition produced the microstructure comprised of basket-weave ɑ laths, discontinuous ɑGB, and retained β phases, which endowed Ti-1.35Fe-0.36Cr-0.24Ni-0.05Mo alloy with related higher ultimate tensile strength (635 MPa) and larger uniform ductility (12%). This approach could provide guidance for designing high-performance metal materials starting with elements powder through additive manufacturing.