Induced Plasticity Effect Research Articles

Tailored deformation behavior of 304L stainless steel through control of the crystallographic texture with laser-powder bed fusion

Laser-powder bed fusion (L-PBF) has gained significant research interest, not only for its profound advantage of producing near-net shape complex geometries of metallic parts, but also for the possibility of producing tailored microstructures. Here we exploit the capability of manipulating the crystallographic texture by L-PBF to tailor the deformation behavior of austenitic stainless steels. In specific, by adjusting the laser power and the laser scanning speed, tailored crystallographic textures can be obtained, along the uniaxial loading direction in 304L stainless steel samples produced by L-PBF. In situ neutron diffraction and uniaxial tension and compression tests are undertaken to investigate the extent of the transformation induced plasticity effect and to correlate it with the tailored macrostructures. The influence of the initial and the evolving crystallographic texture on the deformation behavior is demonstrated and elaborated accordingly. The observed asymmetry in the deformation behavior between tension and compression is also discussed in detail.

Microstructural and mechanical optimization of selective laser melted Ti6Al4V lattices: Effect of hot isostatic pressing

Ti6Al4V triply periodic minimal surface (TPMS) and Trabecular (Trab.) lattices were manufactured by selective laser melting (SLM). The defect distribution, phase composition, microstructural evolution and mechanical properties after hot isostatic pressing (HIPing) treatment were investigated. For both lattices, only micro-sized pores could be observed without any other defects. HIPing treatment had limited influence on pore elimination when the pore diameters were larger than 30 μm. The as-fabricated acicular martensites were in-situ decomposed into the homogeneous basket-weave α + β structure during HIPing process, and the microstructural anisotropy was effectively eliminated after HIPing. The two lattices had similar α/β phase fraction, but the average grain size of the TPMS lattice (2.206 μm) was much smaller than that of the Trab. One, which was attributed to the original microstructural difference in their as-fabricated counterparts. The compressive strength of the two lattices had no significant change after HIPing treatment, but the energy absorption capacity significantly improved due to the transformation induced plasticity effect of α + β laths formation. The fracture mechanism was altered from the brittle-dominated quasi-cleavage mode to ductility-dominated mode. The findings in this work provide an approach for microstructure and compression ductility improvement of SLM Ti6Al4V porous structure by specific heating treatments.

Decreasing Zr content to improve tensile properties of non-equiatomic TiZrHfNb medium entropy alloys with transformation-induced plasticity

Compositional tuning of phase stability of the body-centered cubic (BCC) matrix is an effective way to improve mechanical properties of refractory medium/high entropy alloys, which are generally brittle or lack of strain hardening capability. Here, reducing Zr content in Ti(80-x)ZrxHf10Nb10 (at.%) alloys was demonstrated to destabilize the BCC phase and trigger the BCC to hexagonal close-packed (HCP) phase transformation. Two metastable Ti50Zr30Hf10Nb10 and Ti60Zr20Hf10Nb10 medium entropy alloys were designed, which exhibit superior strength-ductility combination and evident strain hardening due to the transformation induced plasticity effect.

Segregation engineering of grain boundaries of a metastable Fe-Mn-Co-Cr-Si high entropy alloy with laser-powder bed fusion additive manufacturing

Laser-powder bed fusion (L-PBF) additive manufacturing offers unprecedented microstructural fine-tuning capabilities. Naturally, benefitting from such capability requires alloys that are amenable to microstructural heterogeneity and hierarchy (MHH) and that exhibit a low hot-cracking susceptibility (HCS). However, columnar growth, which is characterized by capillary effects and poor strain accommodation capabilities, is prevalent in L-PBF and increases the HCS of the processed alloys. Further, while solute segregation is prominent in cellular and dendritic growth modes during L-PBF, the effects of solute segregation on the alloy HCS and L-PBF processing window remain widely unexplored. Here, we demonstrate that solute segregation affects columnar growth, grain coalescence behavior during solidification, MHH and mechanical properties of a metastable Fe40Mn20Co20Cr15Si5 (at.%) high entropy alloy (CS-HEA) doped with 0.5 wt.% B4C (termed CS-BC). A theoretical framework is proposed, which reveals that a boundary-strengthening segregant may reduce the alloy HCS during L-PBF. In as-built CS-BC, boron, a boundary strengthener, segregated to the solidification cell boundaries, whereas carbon remained in the solid solution. The as-built CS-BC exhibited suppressed columnar growth, more random texture, smaller cell size and higher strength as compared to the as-built CS-HEA. Further, a wide crack-free L-PBF processing window of CS-BC allowed fine-tuning of its MHH and thus the mechanical properties. Upon annealing, as carbon-containing precipitates formed, CS-BC exhibited a metastable microstructure and transformation induced plasticity effect, which led to high synergistic strength-ductility. These findings will foster design of alloys that facilitate application-specific manufacture with L-PBF and thus, an extended outreach of L-PBF for structural applications.

Tribological investigation of abrasion resistant steels with martensitic and retained austenitic microstructure in single- and multi–asperity contact

Steel making companies are developing new steels with complex microstructure to satisfy abrasion resistance issues. Three newly developed steels (fresh martensitic, tempered martensitic and a TRIP steel) were investigated under single and multi-asperity abrasion test conditions. Abrasive performance was linked to material properties. In the multi-asperity test, increasing the normal load and abrasive particle size resulted in increasing mass loss and in transition in micro-mechanisms from ploughing to micro-cutting. Martensitic materials showed better wear performance due to their higher hardness. As a result of the abrasive action a newly formed tribo-layer was observed in the contact zone of all steel specimen. In the single asperity testing a diamond spherical indenter was used with different normal loads. Wear micro-mechanisms were analysed during the scratching using optical microscopy and 3D-profilometry. The fresh and tempered martensitic steels showed a transition in micro-mechanism from micro-ploughing to micro-cutting at higher loads and smaller tip radius. For the multiphase steel with retained austenite, despite its TRansformation Induced Plasticity effect, micro-cutting was reached already at lower loads. The tempered martensitic material outperformed the fresh martensitic material despite the similar hardness and microstructure. For all materials single-asperity test data showed similar ranking in abrasion wear resistance as in the multi-asperity test.

Unusual relationship between impact toughness and grain size in a high-manganese steel

The high-manganese steels are important structural materials, owing to their excellent toughness at low temperatures. However, the microstructural causes for their unusual properties have not adequately been understood thus far. Here, we report a reversal relationship between impact toughness and grain size in a high-manganese steel and its unrevealed microscopic mechanisms, which result in an excellent low-temperature toughness of the steel. Our investigations show that with increasing grain size the impact toughness of the steel can be improved drastically, especially at low-temperatures. Advanced electron microscopy characterization reveals that the enhanced impact toughness of the coarse-grained steel is attributed to the twinning induced plasticity and transformation induced plasticity effects, which produce large quantities of deformation twins, εhcp-martensite and α′bcc-martensite. Inversely, in the fine-grained steels, the formation of deformation twins and martensite is significantly inhibited, leading to the decrease of impact toughness. Microstructural characterizations also indicate that εhcp-martensite becomes more stable than α′bcc-martensite with decreasing temperature, resulting in characteristic microstructures in the coarse-grained samples after impact deformation at liquid nitrogen temperature. In the coarse-grained samples under impact deformation at -80 °C, εhcp-martensite transformation, α′bcc-martensite transformation and deformation twinning all occur simultaneously, which greatly improves the toughness of the steel.

Future Trends on Displacive Stress and Strain Induced Transformations in Steels

Displacive stress and strain induced transformations are those transformations that occur when the formation of martensite or bainitic ferrite is promoted by the application of stress or strain. These transformations have been shown to be one of the mechanisms by which the mechanical properties of a microstructure can be improved, as they lead to a better ductility and strength by the transformation induced plasticity effect. This review aims to summarize the fundamental knowledge about them, both in fully austenitic or in multiphase structures, pointing out the issues that—according to the authors’ opinion—need further research. Knowing the mechanisms that govern the stress and strain induced transformation could enable to optimize the thermomechanical treatments and improve the final microstructure properties.

TWIP – TRIP effect in single crystalline VFeCoCrNi multi-principle element alloy

Various multi-principle element alloys exhibit superior mechanical properties across a wide range of temperatures including cryogenic temperatures. In most systems, the remarkable properties are attributed to deformation twinning (twinning induced plasticity). The recent VFeCoCrNi multi-principle element alloy displays a combined twinning induced plasticity effect, at room-temperature, and phase transformation at low deformation temperatures (transformation induced plasticity). This work considers single crystalline specimens of VFeCoCrNi loaded along the [111] direction. At room-temperature, slip dominates at low strains, below 22% plastic strain, and a delayed activation of twinning is observed. At 77 K, early activation of transformation, face-center cubic to body-center cubic having {111}FCC//{110}BCC and 〈110〉FCC//〈111〉BCC parallel relations, was revealed.

Surface Tension and Density of Cr–Mn–Ni Steels with Transformation Induced Plasticity Effect

Thermophysical properties of liquid metal alloys play a crucial role in the variety of processes. Present work reports experimental investigations of two new transformation induced plasticity (TRIP) steels. A classical maximum bubble pressure technique is successfully used to measure surface tension and density of the liquid TRIP‐steel samples in the temperatures range of 1500–1800 °C. Experiments are performed in two different measurement cells from TU Freiberg and Commissariat a l'Energie Atomique (CEA), Cadarache. Capillaries are made of zirconia partially stabilized with yttrium oxide, and the samples are molten in Al2O3 crucibles. Measurements are conducted under the argon atmosphere. Surface tension of the investigated samples rises up to 1700 °C and further remains the same. Density of the new steels obtained in this study is in good agreement with the previously reported values for Fe–Cr alloys with similar Cr content. Density of the alloy is estimated as 875 488–11 885 × T (±218.85 kg m−3) for a 15%Cr alloy and 796 042–06 897 × T (±199 kg m−3) for a 19%Cr alloy.

Microstructures and tensile properties of friction stir welded 0.2%C–Si–Mn steel

The 0.2 wt%C steels with the addition of 2 wt%Si and 0–4 wt%Mn were prepared by hot rolling followed by subsequent annealing for normalization. The materials were subjected to friction stir welding (FSW) conducted above A3 temperature. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials with the addition of over 2 wt% Mn showed a relatively good balance of strength and ductility when Si content was 2 wt% due to the transformation induced plasticity effect of retained austenite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zones of the 0.2C–2Si–3Mn (wt%) and 0.2C–2Si–4Mn joints exhibited the high tensile strength of approximately 1700 MPa and the high elongation of about 30%, in particular excellent local elongation of 25%. This is due to the refinement of packet and block sizes in the lath martensite induced by the grain refinement of the prior austenite grains by FSW.

Exceptionally high strain-hardening and ductility due to transformation induced plasticity effect in Ti-rich high-entropy alloys

Ti-rich body-centered cubic (BCC, β) high-entropy alloys having compositions Ti35Zr27.5Hf27.5Nb5Ta5, Ti38Zr25Hf25Ta10Sn2, and Ti38Zr25Hf25Ta7Sn5 (in at%) were designed using bond order (Bo)-mean d-orbital energy level (Md) approach. Deformation mechanisms of these alloys were studied using tensile deformation. The alloys showed exceptionally high strain-hardening and ductility. For instance, the alloys showed at least twofold increment of tensile strength compared to the yield strength, due to strain-hardening. Post-deformation microstructural observations confirmed the transformation of β to hexagonal close packed (HCP, α′) martensite. Based on microstructural investigation, stress–strain behaviors were explained using transformation induced plasticity effect. Crystallographic analysis indicated transformation of β to α′ showed strong variant selection (1 1 0)β//(0 0 0 1)α′, and [1 − 1 1]β//[1 1 − 2 0]α′.

Bainite transformation behavior, microstructural feature and mechanical properties of nanostructured bainitic steel subjected to ausforming with different strain

The interplay between ausforming strain level applied prior to austempering step and bainite transformation kinetic as well as plastic deformation mechanism of nanostructured bainitic steel was essentially investigated. The results indicated that the bainite transformation proceeded rapidly with increasing ausforming strain owing to the added heterogeneous nucleation sites and overall storage energy. Nevertheless, when the ausforming strain further increased to 40%, the acceleration effect became less pronounced attributing to the counteraction mechanism by inhibition effect of retarded bainite growth. Increasing ausforming strain could effectively refine bainite sheaves and minimize the dimension of blocky retained austenite, yet strengthen undercooled austenite, leading to a decreased volume fraction of bainitic ferrite at the termination of transformation. Ausforming strain had a positive effect on mechanical properties, particularly, the sample ausformed at 300 °C for 40% strain achieved the largest tensile strength of 1900 MPa and the best total elongation of 24%. This enhanced strength–ductility synergy mainly ascribed to the benefit of the refined bainitic ferrite lath and the increased dislocation density together with transformation induced plasticity effect. Unfortunately, the apparent variant selection brought about by ausforming could offset the increased impact toughness resulting from microstructure refinement when ausforming strain was higher than 10%, which leaded to an intensive deterioration of impact property.

A novel 13Cr austenitic stainless steel with excellent mechanical properties and high hydrogen embrittlement resistance via heterostructure and TRIP effects

A 13Cr austenitic stainless steel with a good combination of high yield strength, ductility and hydrogen embrittlement resistance was designed by combining hetero-deformation induced strengthening and martensitic transformation induced plasticity effects. Room temperature metastable austenite is promoted by adding 8 wt% Mn to a 13Cr–5Ni–2Mo supermartensitic stainless steel. A heterostructure consisting of remaining martensite grains located between lath austenite grains and micrometer-sized austenite grains embedded inside ultrafine grains, was fabricated by cold rolling (44% reduction) and annealing at 973 K through a shear reversion process. The novel 13Cr austenitic stainless steel exhibits a yield strength of 923 MPa, tensile strength of 1085 MPa and total elongation of 33.2%. The hydrogen embrittlement resistance of the novel steel was determined by incremental step loading technique testing and was increased 40% compared with that of the 13Cr–5Ni–2Mo supermartensitic stainless steel. This study provides an approach for the design and industrial fabrication of high strength (125 ksi grade level), high ductility and hydrogen embrittlement resistant stainless steels that can be used in harsh service environments containing hydrogen.

Ultrafine bainitic steel produced through ausforming-quenching process

The coupling effects of ausforming and quenching process on isothermal bainite transformation behaviors, microstructures and mechanical properties of ultrafine bainitic steel have been analyzed. The synergism of ausforming and prior martensite transformation not only significantly accelerates subsequent bainite transformation, but also effectively refines bainite laths and greatly reduces the size of harmful blocky retained austenite, providing an improved transformation induced plasticity effect. Ultimate tensile strength of 2000 MPa and total elongation of 24% can be achieved in ultrafine bainitic steel by ausforming-quenching process and austempering at 300 °C for 1 h.

Novel deformation twinning system in a cold rolled high-strength metastable-β Ti-5Al-5V-5Mo-3Cr-0.5Fe alloy

A high-strength metastable β-titanium alloy, Ti-5Al-5Mo-5V-3Cr-0.5Fe (wt.%) was cold rolled at room temperature and characterized using scanning and transmission electron microscopy. A novel {10 9 3}<331>β type deformation twining mode (Σ19a) is reported for the first time, and crystallographic analysis was carried-out to elucidate the origin of such high-index deformation twins. The initially β-solutionized alloy contains nano-scale ω phase, which is also subsequently distributed within the twins. Additionally, a thin nano-scale ω layer forms along the twin boundaries, and two variants of α″ martensite phase form inside the deformation twins. The current work opens up new prospects for improving the ductility of high-strength Ti alloys by inducing the twinning and transformation induced plasticity effects.

Fine-tuning of stress-induced martensite in TRIP/TWIP Ti alloys

Fine-tuning of stress-induced martensitic (SIM) transformation was studied in Ti-Mo based β metastable alloys, showing combined Transformation Induced Plasticity (TRIP) and Twinning Induced Plasticity effects (TWIP) effects. The work aimed to clarify the transition and interaction between the two deformation mechanisms and their influences on the mechanical properties of Ti-Mo based alloys. Electron parameter design methods (Bo-Md and e/a ratio) were cross-used to increase the β phase stability from near-TRIP to near-TWIP by adding third alloying elements. SIM α″ transformation and mechanical twinning were traced by in-situ EBSD (Electron backscatter diffraction) mapping under cyclic tension. The deformation modes of β phase exhibited significant changes when shifting its metastability via chemical composition modifications. In near-TRIP conditions (dominated by the growth of SIM α″ in plastic regime), SIM α″ transformation and internal twinning of martensite were the main mechanisms to accommodate the local stress-strain conditions. In near-TWIP ones (dominated by the growth of {332}&lt;113&gt;β twinning in plastic regime), SIM α″ was observed only at twinning interface during strain process and disappeared after stress release.

Deformation and fracture behavior of new strain-transformable titanium alloys: a multi-scale investigation

Titanium alloys possessing Twinning and Transformation Induced Plasticity effects show promising mechanical properties, particularly high ductility, hardenability, impact and fracture toughness. This work focuses on a strain-transformable, coarse-grained β-Ti-Cr-Sn alloy displaying TWIP effect. To account for the enhanced properties of this alloy, compared to more conventional β-Ti alloys, fracture and deformation features were correlated at different scales. Examinations evidenced a major role of twinning and, more generally, of plasticity-induced phenomena in the ductile fracture process. The resistance of this alloy to plastic deformation (work-hardening), and to crack initiation and propagation is interpreted in view of the progressive, multiscale twinning mechanisms that occur up to the very final stages of fracture.

The use of color etching to study the microstructure of laser welded steel used in the automotive industry

Abstract In the present article the results of a microstructure study of dual phase, multiphase (with transformation induced plasticity effect) and manganese-boron steel have been presented in the as-delivered state and after laser fusion by means of color metallography, which, up to now, has not been applied in those types of joints. In order to reveal the components of the microstructure of the steel types studied, five reagents were applied: nital, pikral, Klemm, LePera and sodium metabisulfite, in different combinations. Various phases were revealed in the material in the as-delivered state and after fusion, and this would not have been possible using standard procedures by means of a light microscope. In the as-delivered state all basic phases were observed. In the state after laser remelting, the observed microstructures were completely different than in the received state giving bainite and martensite an advantage. Measurements of the hardness of the as-received material and the welds of the tested steels were performed, showing a significant increase in weld hardness.

Microstructure and mechanical properties of Cr-rich Co-Cr-Fe-Ni high entropy alloys designed by valence electron concentration

Cr element is considered to have strong solid solution strengthening effect in high entropy alloys (HEAs). However, the formation of brittle σ phase is enhanced with the increase of Cr content, which is harmful to the ductility of alloys. The valence electron concentration (VEC) is highly correlated with phase stability of HEAs, which is used to predict the formation of σ phase. The microstructure and mechanical behavior of Cr-rich non-equal molar Co-Cr-Fe-Ni HEAs were investigated. Results indicate the formation of σ phase could be suppressed with the high VEC, while lattice parameter tends to increase as Cr content increases, which enhances the metastability of Co-Cr-Fe-Ni system to accelerate grain-boundary precipitates of σ phase. Minor precipitates of σ phase strengthen the HEAs and do not cause serious embrittlement. Deformation-induced martensitic transformation (FCC →HCP) occurs during the tensile deformation of alloys with the VEC of 8.0 and 7.9, and the transformation induced plasticity effect is the main reason for good combination of strength and ductility.

Coupling nano-carbide strengthening with transformation induced plasticity effect to achieve over 1.5 GPa strength with 30% ductility in cold-rolled medium-Mn steel

An ultra-high strength (over 1500 MPa) and enhanced ductility (30%) of cold-rolled medium-Mn steel is developed by quenching and tempering heat treatment. The highest product of strength and elongation of 45.7 GPa·% is obtained when tempering at 300 °C. The microstructure consists of martensite matirx, substantial nano-carbide and film-like/blocky retained austenite. The ultra-high strength derives from the precipitate strengthening of nano-carbide as efficient obstacles to dislocation motion. While, transformation induced plasticity effect of retained austenite enhances the plasticity.