Induced Plasticity Effect Research Articles

Improving plasticity by Si-addition-induced coherent nanoprecipitates in Cu–Al–Mn alloy fabricated by laser powder bed fusion

In this paper, the Cu-10.8Al-8.3Mn-0.37Si alloy containing Mn5Si3 phase was obtained by adding Si element to the Cu-10.8Al-8.3Mn alloy. The two alloys were printed by laser powder bed fusion. The changes in the organization and tensile properties of the alloys and the mechanism were investigated. After Si addition, the Mn5Si3 nano phase precipitated in the alloy in addition to the β1 austenite phase. The Mn5Si3 phase was co-lattice with the β1 phase with an average size of 10 nm and was diffusely distributed. Standard tensile experiments showed that the yield strength of the Cu-10.8Al-8.3Mn-0.37Si alloy decreased by 57.7% and the elongation increased by 266% compared to the Cu-10.8Al-8.3Mn alloy, while the tensile strength remained essentially unchanged. The presence of Mn5Si3 phase provided nucleation points for martensite and promoted the generation of stress-induced martensite. Hence the yield strength was reduced. TEM results showed that the β1 phase in the Cu-10.8Al-8.3Mn-0.37Si alloy was fully transformed into stress-induced martensite after deformation, which displayed the transformation induced plasticity effect and hindered the slip of dislocations. Meanwhile, the Mn5Si3 phase was sheared by the stacking faults, which promoted the accumulation of hetero-deformation-induced stress. Therefore, the hardening capacity and plasticity of the Cu-10.8Al-8.3Mn-0.37Si alloy were enhanced. This work provides a new idea for the plasticity enhancement of Cu–Al–Mn-based shape memory alloys achieved by co-lattice nano-precipitated phase strengthening.

Better mechanical properties of SAF2507 duplex stainless steel formed by cold rolling and normalizing

The SAF2507 duplex stainless steel with the heterogeneous structure is produced through cold rolling (ε = 90%) and subsequent 1-min normalizing at 1273 K. Compared with the corresponding initial samples, the grain sizes of ferrite and austenite phases in the 90%-1273 sample are refined by 90%, which led to an increase in the number of grain or phase boundaries between the two phases. The yield strength (YS) of the 90%-1273 sample increased to 715 MPa from 541 MPa, while the uniform elongation of 24.8% is slightly smaller than that of the coarse-grained sample. The higher YS is mainly contributed by the strengthening of grain refinement and dislocation strengthening by incorporating geometrically necessary dislocations (GNDs). During tensile deformation, the accumulation of GNDs in the heterostructured sample continuously generates hetero-deformation-induced (HDI) stress. The HDI stress plays a positive role in the 90%-1273 sample showing an ultimate tensile strength of 939 MPa. Additionally, the amount of twin boundaries in the 90%-1273 sample rose from 4.5% to 35% during tensile deformation, which indicated that the twinning induced plasticity effect was helpful in retaining the ductility of the 90%-1273 sample. This work provides strategies for optimizing the mechanical properties of duplex stainless steels and contributes to understanding the essence of a superior combination of strength and ductility of heterostructured materials.

Synergistic improvement of strength and ductility by high magnetic field assisted intercritical annealing in lightweight medium Mn steel

The microstructures and mechanical properties of lightweight medium Mn steels were studied by high magnetic field (HMF) assisted intercritical annealing process (HMF-IA). The results show that the volume fraction of retained austenite increased and reached the maximum value of 56.2 % when the magnetic flux density (MFD) was 10 T at first and then decreased with the increase of MFD. The application of 10 T HMF promoted the diffusion ability of carbon atoms into austenite by magnetic stress-driven dislocation movement and increased the volume fraction and grain size of austenite. HMF also played a critical role in dislocation movement and element partitioning during the IA process. Due to the wide range of bimodal size distribution of austenite after the HMF-IA process, stronger transformation induced plasticity effect and twin induced plasticity effect happened in the process of plastic deformation. For the sample annealed under 10 T, comparing with the sample annealed without HMF, the product of strength and elongation value increased from 45.7 GPa·% to 55.1 GPa·%. Additionally, it also kept a persistent high strain hardening. The present study provides a new understanding of the role of HMF in adjusting the relationship between microstructure and mechanical properties during the annealing process and opens up a new path for designing industrial medium Mn steel with low cost, high performance and simple process.

Study on a novel α+β dual-phase Ti-9V-1Fe alloy with twinning induced plasticity effect

A novel α+β dual-phase Ti-9 V-1Fe alloy with twinning induced plasticity effect is developed in this work. {332}<113> twinning plays a momentous role in the plastic deformation, and effectively enhances the uniform elongation. The alloy displays a considerable mechanical performance including yield strength of 890 MPa and uniform elongation strain of 19 %. {332}<113> twin traversing α precipitate leads to a shear in the precipitate, and Burgers orientation relationship is achieved between {332}<113> twins and some specific α variants. Moreover, the shear releases the stress concentration induced by twinning shear stress field acting on α precipitate, which makes {332}<113> twinning be a compatible deformation between α precipitate and β matrix. The strategy of introducing TWIP effect into α+β dual-phase Ti alloys demonstrated in this work may serve as a new method for the development of high performance Ti alloys.

Thermodynamic Prediction and Experimental Verification of Transformation‐Induced Plasticity/Twinning‐Induced Plasticity Effects in Advanced Low‐C High‐Mn Steel at Different Deformation Temperatures

The effectiveness of transformation‐induced plasticity (TRIP)/twinning‐induced plasticity (TWIP) effects in Fe–0.06C–26Mn–3Si–3Al advanced ferrous alloy is analyzed over a wide temperature range of −40 to 200 °C. The specific deformation mechanisms in the steel are experimentally investigated and predicted using thermodynamic calculations of stacking fault energy (SFE). These predictions were correlated with the microstructure of the steel and its mechanical behavior. The study finds that applied thermodynamic models effectively predict the occurrence of TRIP/TWIP effects at different deformation temperatures, aligning well with experimental observations in terms of ε/α′ martensite formation, indicative of the TRIP mechanism. Some discrepancies in SFE results are observed at Mn contents significantly higher or lower than the nominal composition of steel.

Development of TRIP phenomenon using power refining of retained austenite by niobium

AbstractThe illustrations above demonstrate that as the percentage of niobium increases, the grain size decreases, leading to an increase in the stability of transformation induced plasticity steel. This, in turn, results in improved mechanical strength. The optimal volume fraction of retained austenite for a significant transformation induced plasticity effect to occur is reported to be in the range of 10 vol. %–20 vol. %. The volume fraction of retained austenite directly determines the carbon content and grain size of the retained austenite, its two main stabilization factors. The stability of the retained austenite dictates when the strain‐induced martensite transformation (SIMT) occurs during straining of transformation induced plasticity high strength steel. Unstable retained austenite transforms almost immediately upon deformation, increasing work hardening rate and formability during the stamping process. At the appropriate stability of the retained austenite, the Strain‐Induced Martensite Transformation begins only at strain levels beyond those produced during stamping and forming, and the retained austenite is still present in the final part; it can transform into martensite in the event of a crash, providing greater crash energy absorption. The tensile strength level of micro‐alloyed transformation induced plasticity steels may exceed 1 GPa. Grain refinement mechanism is considered as the most applied mechanism used in increasing of steel strength without deterioration of ductility and toughness. Recently, it was proposed that the grain refinement could act positively on promoting the transformation induced plasticity effect of advanced high strength steel through improving the stability of retained austenite. In this research, quenching and partitioning heat treatment technique was applied to four grades of low carbon steel with different percentage of niobium. Then, the effect of niobium on grain refinement has been detected by microstructure observations. Mechanical properties and strain hardening properties of investigated steel have been determined. In addition, by using x‐ray diffraction, as well as a new electric resistance‐based sensor, it was possible to characterize the stability of the retained austenite through the plastic deformation of steel. Grain refinement by using of niobium has a great impact on promoting the stability of retained austenite against the plastic stress at the plastic deformation zone.

Segregation-dependent mechanical stability of reverted austenite and resultant tensile properties of Co-free maraging steel fabricated by wire-arc directed energy deposition

The segregation-dependent mechanical stability of reverted austenite (RA) and resultant tensile properties was investigated on Co-free maraging steel fabricated by wire-arc directed energy deposition (DED). The results showed that the segregation in the interdendritic regions enabled the formation of gradient reverted austenite in Co-free maraging steel fabricated by wire-arc DED when subjected to aging heat treatment. The content and mechanical stability of reverted austenite and resultant tensile properties were strongly dependent on the degree and compositional gradient of segregation in the interdendritic regions as well as the post heat treatment process. The reverted austenite with compositional gradient inherited from the segregation, transformed into martensite successively from the edge to the interior within individual reverted austenite grain at different strain levels. Besides, the variations in the degree and compositional gradient of segregation in the interdendritic regions widened the range the mechanical stability of the reverted austenite and thereby activating martensite transformation over a broad strain regime. Annealing process prior to aging could modify the degree and compositional gradient of segregation in the interdendritic regions and accordingly tune the content and the range of mechanical stabilities of reverted austenite grains, thus attaining a gradual and consistent transformation induced plasticity effect over whole deformation regime and thereby increasing ductility without sacrificing strength of Co-free maraging steel fabricated by wire-arc DED.

Correlation of microstructure, mechanical properties, and residual stress of 17-4 PH stainless steel fabricated by laser powder bed fusion

17-4 precipitation hardening (PH) stainless steel is a multi-purpose engineering alloy offering an excellent trade-off between strength, toughness, and corrosion properties. It is commonly employed in additive manufacturing via laser powder bed fusion owing to its good weldability. However, there are remaining gaps in the processing-structure-property relationships for AM 17-4 PH that need to be addressed. For instance, discrepancies in literature regarding the as-built microstructure, subsequent development of the matrix phase upon heat treatment, as well as the as-built residual stress should be addressed to enable reproducible printing of 17-4 builds with superior properties. As such, this work applies a comprehensive characterisation and testing approach to 17-4 PH builds fabricated with different processing parameters, both in the as-built state and after standard heat treatments. Tensile properties in as-built samples both along and normal to the build direction were benchmarked against standard wrought samples in the solution annealed and quenched condition (CA). When testing along the build direction, higher ductility was observed for samples produced with a higher laser power (energy density) due to the promotion of interlayer cohesion and, hence, reduction of interlayer defects. Following the CA heat treatment, the austenite volume fraction increased to ∼35 %, resulting in a lower yield stress and greater work hardening capacity than the as-built specimens due to the transformation induced plasticity effect. Neutron diffraction revealed a slight reduction in the magnitude of residual stress with laser power. A concentric scanning strategy led to a higher magnitude of residual stress than a bidirectional raster pattern.

Laser-directed energy deposition of a high performance additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloy with a novel core-shell structured strengthening phase

Additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloys (MEAs) were fabricated by laser-directed energy deposition. The strengthening induction mechanism in the additively manufactured (CoCrNi)94(TiAl)6 MEAs was investigated using electron backscatter diffraction and transmission electron microscopy. The results showed that the addition of TiAl powder led to increases of 29.6% and 43.5% in the ultimate tensile strength (UTS) and yield strength (YS), respectively, at 298 K, and a decrease of 17.1% in the elongation. For the samples tested at 77 k after the addition of the TiAl powder, the UTS and YS improved by 26.6% and 28.0%, respectively, and the elongation increased by 26.3%. The microstructural observation results indicated that the matrix grains changed from initial columnar grains with a uniform growth direction to fine dendrites. Numerous TiO clad Al2O3 strengthening nano-precipitates with a novel core–shell structure were found in the additively manufactured (CoCrNi)94(TiAl)6 MEAs, and both featured a face-centered cubic structure. No crystallographic orientation was observed between the nano-precipitates and the matrix. At 298 K, the dislocation line bypassed the nano-precipitates during the plastic deformation period and Lomer–Cottrell locks appeared near the nano-precipitates, thereby strengthening the additively manufactured (CoCrNi)94(TiAl)6 MEAs. At 77 K, plastic deformation resulted in the synergistic deformation of the nano-precipitates and matrix, the formation of a large number of dislocations, and a twinning induced plasticity effect, which further increased the strength and plasticity of the additively manufactured (CoCrNi)94(TiAl)6 MEAs.

Tailoring Strength and Ductility of an Fe–Mn–Al–C Low‐Density Duplex Steel by Controlling the Cooling Path after Hot Rolling

The low‐density duplex steels provide a potential for the industrial application owing to their good strength‐ductility match and reduced density. In the present work, the microstructure of an Fe–8.03Mn–6.10Al–0.4C (wt%) low‐density duplex steel was adjusted by controlling the cooling path after hot rolling, which is a conventional process during the production of steel sheets and strips. A microstructure of ferrite and austenite has been achieved by annealing the air‐cooled samples, and the microstructure of furnace‐cooled annealed samples consists of banded ferrite, austenite, and martensite. This is mainly due to the fact that the austenite is more stable by refining the grain size via embedding the needle‐like α‐ferrite. The existing of martensite and the banded microstructure in furnace‐cooled annealed samples induces the occurrence of cracks during deformation, resulting in an abrupt fracture at a relatively lower strain level. However, for air‐cooled annealed samples, a good ductility of duplex microstructure ensures that the deformation can be sustained up to a much higher strain level, and the transformation induced plasticity effect plays a positive role at the later deformation stage; this contributes to the occurrence of an extra work hardening rate recovery during deformation and thus to a good strength‐ductility match.

A novel low-cost Ti–Cr–Fe β-Ti alloy with TWIP effect: Sensitivity of tensile properties to microstructural characteristics

A novel low-cost Ti-4.6Cr–3Fe β-Ti alloy with twining induced plasticity effect was designed, and its tensile properties and deformation mechanisms were sensitive to its microstructural characteristic. The received sample with the microstructure of β matrix embedded isothermal ω (ωiso) precipitations exhibited the largest tensile strength (Rm), but the limited ductility (<3 %). When the solution treatment (ST) temperature increased from below to above α→β transition temperature, the microstructure changed from β matrix with αs precipitations to β matrix with athermal ω (ωath) precipitations, and the deformation mechanisms correspondingly changed from dislocation slip to the combination of dislocation slip and {332} deformation twins. Among different treatment samples, the ST800 sample obtained a relatively good combination of Rm (1146 MPa) and elongation to failure (EL, 12.5 %). It is surprising that, as the ST temperature increased from 800 °C to 900 °C, the Rm increased from 1146 MPa to 1230 MPa, but the fracture mode changes from ductile fracture to brittle fracture due to the formation of Laves phase (TiFe2) at grain boundaries. These findings provide a basis for improving the tensile properties of Ti-4.6Cr–3Fe alloy by adjusting the microstructure, and a reference for optimizing the composition of Ti–Cr–Fe system low cost β-Ti alloys.

Macroscopic and nanoscale investigation of the enhanced wear properties of medium-Mn steel processed via room-temperature quenching and partitioning

High-manganese austenitic and high hardness martensitic steels are widely used in wear-related applications. However, their abrasive wear performance is unsatisfactory because of the insufficient twinning induced plasticity effect of the former and the weak strain hardening and low crack resistance during wear of the latter. Here, novel wear-resistant steels were developed via alloy design and simple room-temperature quenching and partitioning (RT-Q&P) of medium Mn steels (0.3C–10Mn and 0.3C–10Mn-0.25V). Their microstructure and sliding abrasive wear behavior were compared to those of conventional Hardox400 and Hadfield steels. The two medium-Mn steels had better wear properties than conventional wear-resistant steels. Moreover, the V-containing steel with a multiphase microstructure of tempered martensite, retained austenite, and nano-vanadium carbide precipitates, possessed 2.98 and 3.45 times higher abrasive wear resistance than the Hardox400 and Hadfield steels, respectively. Besides the stimulated transformation-induced-plasticity effect in RT-Q&P steels, the enhanced abrasive wear resistance of V-containing steel was attributed to microstructural refinement and improved strain hardening during wear imparted by V element. The main wear mechanism of RT-Q&P wear-resistant steels was cutting, while it was grooves and pits for Hadfield steel, and peeling pits and delamination for Hardox400 steel. This study can guide the design of structural materials with excellent wear resistance through multiphase microstructural and nano-precipitate regulation to overcome the wear challenges of components.

Stability of retained austenite and work hardening behavior in ultra-fine medium carbon bainitic steel

The effect of retained austenite stability on plastic deformation was investigated by stepwise tensile test of medium carbon steel material at two different bainite isothermal temperatures below or above the martensite start (Ms) point. The results show that higher retained austenite transformation leads to higher elongation. Compared with the above-Ms bainitic steel (320 °C-12 h), the below-Ms bainitic steel of retained austenite (240 °C-48 h) has higher mechanical stability. This is because the transformation induced plasticity effect (i.e., the TRIP effect) of retained austenite plays a significant role in the process of tensile strain. The block retained austenite distributed between the bainite sheaf deforms rapidly in the initial stage, and the film retained austenite between the bainitic-ferrite lath transformation is slow in the later stage of tensile deformation. This is attributed to the low stability of block retained austenite, which easily transforms into hard and brittle martensite under external stress, while the film retained austenite has high stability and cannot easily undergo phase transformation under the action of external force. Meanwhile, the work hardening ability of the below-Ms bainitic steel is higher than that of the above-Ms bainitic steel at the initial stage of tension, but the opposite occurs in the later stage of tension.

Microstructures and mechanical properties of cold-rolled 21Cr lean duplex stainless steel with medium to high cold rolling reductions

The effects of medium-high cold rolling reduction (40%~80%) on the microstructure, phase transformation, and mechanical properties of 2101 lean duplex stainless steel (LDSS) are studied. The results show that the alternately arranged band-like microstructure is formed along the cold rolling direction. A large number of dislocations appear in the ferrite and gradually evolve into dislocation plug clusters, dislocation walls, and dislocation cells with the deformation. The nanoscale dislocation substructures can refine the ferrite. Shear bands, mechanical twinning, and α' -martensite are detected in the austenite. The content of α' -martensite increases with the increase in the deformation degree, and a wavy "pancake structure" form under large strains. The strength and hardness of the steel after the cold rolling deformation increase significantly, while the plasticity is drastically reduced. Nevertheless, with the cold rolling reduction increasing from 40% to 80%, the plasticity remains almost unchanged, which may be related to the ultrafine ferrite, fine α' + γ mixed grains, and reduced transformation induced plasticity effect. The high-density dislocations in the ferrite, strain-induced martensite, and retained austenite synergistically transform the fracture mode from a typical ductile fracture to a ductile and quasi-cleavage mixed fracture. • The austenite phase is filled with shear bands, twins, and α' -matensite. • The transformation route follows γ → twins → α' -matensite and γ → α' -matensite. • The ferrite grain size is refined by a large number of dislocation substructures. • The maintained elongation is related to the fine ferrite and α′ + γ mixed structure.

Stabilizing austenite via intercritical Mn partitioning in a medium Mn steel

In this study, a novel processing route of intercritical Mn partitioning is proposed to stabilize the reverted austenite in a medium Mn steel. The Fe-5Mn-0.2C-1.5Al (wt.%) steel is annealed unceasingly at two intercritical temperatures to produce a compositional core-shell structured reverted austenite which consists of a core in low Mn content and a shell in relatively high Mn content. This distinctive microstructure can not only allow much more retained austenite to be obtained compared with the conventional processing route, but also produce active transformation induced plasticity effect over a broad strain range, leading to excellent mechanical properties of high strength and enhanced ductility.

Mechanical property regulation of transformation induced plasticity (TRIP) multi-principal element alloys through multi-phase microstructural design

Transformation induced plasticity in multi-principal element alloys involves several types of phase transformation, among which the alloys exhibiting Face-centered cubic (FCC) to Body-centered cubic (BCC) phase transformation during deformation have a place. However, the current multi-principal element alloys with single FCC phase show relatively low room temperature tensile yield strength, thus usage of these alloys is limited for room temperature applications. We report a revised microstructural design by tuning the fractions of FCC and other harder phases via annealing according to equilibrium phase diagrams, which gives rise to both higher yield strength and tensile strength, and still good ductility at room temperature. The increase of the strength can be attributed to the contribution of the harder BCC and Sigma phase particles formed during annealing and the still operational FCC to BCC transformation induced plasticity effect. Hence, properly selected annealing treatment to produce a multi-phase microstructure is a way to exert both the transformation induced plasticity effect meanwhile the composite effect from both soft and hard phases.

Effect of a New Prestrain, Intercritical Annealing, Quenching, and Partitioning Process on the Microstructure and Mechanical Properties of Medium‐Manganese Steels

A new process combining 4% prestrain, intercritical annealing, quenching, and partitioning (prestrain–IA–Q&amp;P) is used to investigate the effect of tensile number on the microstructure and mechanical properties of medium‐manganese steels. A microstructural model is built based on the specimens’ microstructure and mechanical properties. The amount of tiny brilliant white carbide particles precipitated in the microstructure seems to grow with the number of stretches. The test steel's tensile strength first decreases and then increases. However, the product of tensile strength and elongation exhibits the opposite trend. The new process improves test steel elongation compared with IA–Q&amp;P. This might be because multiple stretches have destroyed the original microstructure of the test steel. Consequently, the test steel generated more deformation bands and dislocation energy. Simultaneously, grain refinement and carbide precipitation in the test steel increase the nucleation point of austenite, resulting in a diffuse distribution of the retained austenite. The tested steel is stretched numerous times, enhancing the capacity of the microstructure's ferrite/martensite/retained austenite to accept dislocations, and there is increased dislocation slip and back stress production between the microstructures. As a result, test steel's ductility is increased by the transformation induced plasticity effect of retained austenite and the synergistic deformation of ferrite.

Enhanced cryogenic tensile properties through cryogenic cyclic plastic strengthening in a metastable austenitic stainless steel

Pre-strain with large deformation is an effective method to improve the yield ratio of metastable austenitic stainless steel, but it will lead to a significant reduction in elongation. Therefore, a cryogenic cyclic plastic strengthening method is proposed in this paper. Compared with the base metal without strengthening, the strengthened material improved the yield strength at 77 K and maintained high elongation. It is shown that the strengthening eliminated the yield plateau and changed the characteristics of first adequately nucleating and then rapidly phase transformation. This was achieved by the special microstructure prepared by cryogenic cyclic plastic loading, which allowed the strengthened material to contain a large number of phase transformation while still retained the ability of stable phase transformation, so that the phase transformation induced plasticity effect played a full role in the subsequent tensile process and allowed the material to show an excellent combination of strength and ductility.

Torsion of a rectangular bar: Complex phase distribution in 304L steel revealed by neutron tomography

Metastable austenitic stainless steel (304L) samples with a rectangular cross-section were plastically deformed in torsion during which they experienced multiaxial stresses that led to a complex martensitic phase distribution owing to the transformation induced plasticity effect. A three-dimensional characterization of the phase distributions in these cm-sized samples was carried out by wavelength-selective neutron tomography. It was found that quantitatively correct results are obtained as long as the samples do not exhibit any considerable preferential grain orientation. Optical microscopy, electron backscatter diffraction, and finite element modeling were used to verify and explain the results obtained by neutron tomography. Altogether, neutron tomography was shown to extend the range of microstructure characterization methods towards the meso- and macroscale.

Improvement in toughness and ductility of friction stir welded medium-Mn steel joint via post-welding annealing

The medium-Mn steels have a great application potential in the engineering fields due to its excellent strength and toughness. However, the presence of coarse martensite and segregation of Mn in the fusion-welded medium-Mn steel joint severely deteriorated toughness and ductility. In this work, 6 mm thick medium-Mn steel plates were joined by friction stir welding, and then were intercritically annealed to enhance the toughness and ductility of the joint. The as-welded nugget zone (NZ) contained lath martensite with low proportion of high angle boundaries (HABs), leading to the joint with lower elongation of 22.5% and the impact energy of 73.6 J/cm 2 . However, the as-annealed NZ consisted of ferrite and austenite because partial martensite changed to reversed austenite, while other martensite became ferrite during annealing. In addition, the proportion of HABs in the as-annealed NZ increased obviously. In comparison with as-welded joint, the impact energy and elongation of the as-annealed joint was increased by 70.7% and 44.4%, respectively. The abovementioned excellent mechanical properties were attributed to the high proportion of HABs and a large amount of ultra-fine reversed austenite with high thermal and mechanical stability, which provided a significant transformation induced plasticity effect, obviously inhibiting crack initiation and propagation. This systematic work provided a reference for welding similar materials containing retained austenite.