TRIP Effect Research Articles

Insight into austenite reversion and mechanical behavior of quenching and partitioning steel inherited hierarchical structure of martensite

In this paper, the start microstructure of full martensite and dual phases comprising ferrite and martensite were introduced into quenching and partitioning (Q&P) steel to tailor the microstructural characteristics and mechanical behaviors, which was achieved by means of a pre-quenching treatment with various temperatures. It is revealed that the conventional Q&P sample possessed bimodal microstructure with predominantly blocky feature, while gradually changed from combination of blocky and lath structures into individually lath feature in pre-quenched Q&P samples. The reversed austenite presented different variants, which predominantly reproduced the orientation of parent austenite and occasionally possessed twin relationship, low angle misorientation or extraneous with parent austenite. The twin-related austenite included annealing twin and reversed twin, whilst the latter showed twin relationship with both neighbored annealing twin and parent austenite. The variant type of reversed austenite significantly influenced the characteristics of its orientation relationship with neighbored pre-quenched martensite. The pre-quenching treatment was favorable to improve the carbon content of meta-stable austenite before Q&P treatment, while the final carbon content in retained austenite was mainly controlled by the synergistic carbon partitioning of martensite and bainite on basis of thermodynamic calculation, accompanied with the partitioning of Mn and Si elements. Moreover, the pre-quenched martensite deformed coordinately in a unit of martensite packet and was beneficial to retard necking to improve ductility. The optimal mechanical properties were obtained with pre-quenching temperature near Ac3, possessing yielding strength of 623 MPa, ultimate tensile strength of 861 MPa and product of strength and elongation exceeding 28 GPa%, attributing to the positive TRIP effect originating from lath-like retained austenite with high carbon content and multi-level hierarchical microstructure inherited from pre-quenched martensite.

Effect of microstructures tuning on mechanical properties of an ultra-high strength marine steel via the TRIP effect

Effect of microstructures tuning on mechanical properties of an ultra-high strength marine steel via the TRIP effect

High strength-high ductility combination in a low-density medium Mn steel prepared by a highly efficient route

Medium Mn steels are the primary candidates in the fields of automobile, construction machinery et al. due to the advantages of high strength-ductility combination, and low costs. By now, the balance of good performance and high preparation efficiency is still a critical issue to be unsolved, which restricts their further applications. This work designs a novel Al-contained medium Mn low-density steel and explores the effect of a short intercritical-annealing process. The microstructure evolution of multi-scales, especially the austenite characteristics and Mn partitioning behaviors is analyzed, and the relationship between austenite and tensile properties is discussed. The results indicate Al can broaden the annealing processing window and improve the annealing temperature. When annealed at 780 °C, the austenite reverted transformation happens quickly and after annealing for 3 min, the austenite volume fraction maintains stable in the range of 52.4–56.7 vol%, but the morphology and Mn concentration are very different. A large amount of blocky austenite with a coarse size of more than 1 μm and a low Mn concentration of 8–9 wt% forms at the longer annealing time of 20 min, while the austenite almost presents a lath-like shape with a high Mn concentration exceeding 10 wt% at annealing time of 4 min. The tensile properties are closely related to the volume fraction and mechanical stability of austenite, and the sample annealed for 4 min exhibited the best performance with tensile strength of 1192 MPa and total elongation of 33 %, due to its slow TRIP effect.

Дюрометричний аналіз зміцнення приповерхневого шару ADI при терті під впливом TRIP-ефекту

Features of strengthening of the near-surface layer of ADI during friction due to strain-induced martensitic transformation were analyzed by duromeric methods. Indentation under continuous loading (Meyer hardness, HM) and Vickers microhardness Hμ were used. Pop–ins are observed on the ADI continuous load curves, which indicate martensitic transformations during indentation. The effect usually exists at a load of ~0,1 H and an depth of ~1,5 μk. The average microhardness of the initial sample is Hμ ≈ 4,89 GPa. After wear, the average value increases to Hμ ≈ 6,92 GPa. Statistical analysis of the microhardness distribution of the sample after wear revealed that a third of the indentations have abnormally high hardness, which is characteristic of deformation-induced martensite. Probably, these indents are obtained from regions of the structure where deformation-induced martensitic transformation took place. Increasing the indentation load practically does not affect the determination of the microhardness of the initial sample, but reduces the hardness of the sample after wear. This indicates the gradient nature of deformation and phase-structural rearrangements in the near-surface layer during wear. As the friction temperature increases, there is a decrease in microhardness in the wear zone. This is explained by the departure from the temperature range of the martensitic transformation, due to which the TRIP effect is weakened. The maximum degradation of microhardness is observed between room temperature and 50 oC. Keywords: ADI materials, durometric studies, TRIP-effect, wear.

Microstructural evolution and deformation behavior of an interstitial TRIP high-entropy alloy under dynamic loading

The mechanical properties and microstructural evolution under dynamic loading conditions are of paramount importance in the development and utilization of interstitial high-entropy alloys (iHEAs). This study focuses on the investigation of a currently trending material, C-doped TRIP-assisted HEA Fe49.5Mn30Co10Cr10C0.5 (at.%). By subjecting the alloy to uniaxial quasi-static and dynamic tensile loading, the study systematically elucidates mechanical property testing under different strain states, characterizes the microstructural evolution during deformation, analyzes deformation mechanisms, and delves into the essence of mechanical responses. Particularly, by employing digital image correlation (DIC) techniques, local severe deformation regions under high-speed deformation conditions are sampled to investigate microstructural changes under extreme deformation conditions. The results indicated that, in comparison to quasi-static tensile deformation, the dynamic tensile deformation of the HEA demonstrated a slight increase in yield strength, a decrease in ultimate tensile strength, and a trend toward an increase in total elongation. Specifically, at a strain rate of 103 s−1, the tensile strength was ∼825.5 MPa, and at a strain rate of 10 s−1, the elongation was ∼57.7 %. In quasi-static deformation, the TRIP and TWIP effects served as the primary deformation mechanisms. In dynamic deformation, the proportion of ε-martensite phase transformation was significantly reduced, with deformation twinning (DT) and dislocations becoming the dominant mechanisms, originating from the impact of adiabatic heating on the stacking fault energy (SFE). Furthermore, in the locally severe deformation region, multiple twinning systems were activated, coupled with a substantial accumulation of dislocations. A minor presence of dual-oriented martensite also contributed to this complex mechanism. Additionally, the carbides formed through C-doping exerted a significant hindrance on dislocation slip during high-speed deformation and under high-strain conditions. Simultaneously, they promoted the activation of multiple twinning systems, making an indispensable contribution to maintaining the strain hardening rate (SHR) of the HEA.

Evading the strength-ductility compromise in medium manganese steel by a novel low temperature warm rolling treatment

Strength-ductility trade-off is an enduring problem in a wide range of metals and alloys including the advanced high-strength steels. In this work, we propose a novel non-isothermal low-temperature warm rolling (WR) treatment which was applied after the conventional intercritical annealing (IA) in order to overcome the strength-ductility compromise in a medium manganese steel, Fe-5.1Mn-0.22C-0.85Si-0.42Al (composition in wt%). IA + WR treatment yielded a simultaneous improvement in tensile properties, wherein yield strength (YS), ultimate tensile strength (UTS), and ductility improved by 32%, 12%, and 6% respectively, compared to the conventional IA treatment. Despite an increase in the yield point elongation (2%) and Lüders strain (3%) in the IA + WR specimen, the ductility of the same improved as compared to the IA specimen. Microstructure of the IA + WR specimen was tailored by introducing a significantly higher dislocation density in the constituent ferrite and retained austenite (RA) phases along with a relatively higher density of planar defects (stacking faults and nano-twins) in the RA phase. X-ray line profile analysis quantified the increased dislocation density (order ∼1015 vs 1014 m−2) and twin fault probability (∼1.35% vs ∼0.12%) in the RA phase of the IA + WR compared to the IA specimen. The improvement in YS was primarily due to the increase in the dislocation density of the constituent phases in the IA + WR specimen. Introduction of a high density of lattice defects in the RA phase during the WR treatment above the Md30 temperature enhanced the mechanical stability of the RA phase, leading to a decreased rate of TRIP effect and formation of hierarchical twins during tensile deformation. The near-ubiquitous strength-ductility trade-off was overcome via a tailored microstructure consisting of regulated austenite stability leading to discontinuous TRIP effect, hierarchical twinning in RA phase, and favorable texture in the constituent phases. The present study provides a pathway to tailor the microstructure for escaping the strength-ductility paradox in different metastable FCC alloys such as stainless steel, HEAs, TWIP steels etc.

Studying the Formation of the Microstructure of Powdered Steel with Trip Effect by Direct Metal Deposition

Studying the Formation of the Microstructure of Powdered Steel with Trip Effect by Direct Metal Deposition

Effects of Pre-Stretching on the Mechanical Behavior of Cold-Rolled 5%Mn Medium Manganese Steel.

Studies about the pre-stretching effect on the mechanical behavior of cold-rolled 5%Mn medium manganese steel have adopted optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction techniques. Results showed that pre-stretching would change the ferrite morphology from massive and lath-like to strip-like. With the pre-stretching increasing from 0% to 14%, the dislocation density and yield strength both grew gradually, which corresponded to growth from 6.49 × 1014 m-2 to 7.98 × 1014 m-2 and growth from 765 MPa to 1109 MPa, respectively. Meanwhile, the austenite volume fraction, elongation and product of strength and elongation were all reduced with the pre-stretch increase. The stabilized retained austenite with pre-stretch delayed the occurrence of the TRIP effect and improved the work hardening rate. As a result, the Lüders band disappeared at 2% pre-stretch and the PLC band vanished from the stress-strain curve at 14% pre-stretch.

Effect of a novel controlled thermomechanical treatment on the microstructure and mechanical properties of a high-carbon nanobainitic steel

The effect of the novel controlled thermomechanical treatment, including torsion components in the elastic strain range during the isothermal holding on the microstructure and mechanical properties of the high-carbon nanobainitic steel, was investigated. TEM observations of the thermo-mechanically treated steel revealed bainitic ferrite laths with an average size of 68 ± 40 nm and films of retained austenite with an average size of 34 ± 17 nm, along with the blocky morphology of retained austenite in sub-micron scale. The XRD synchrotron diffraction allows estimating the amount of retained austenite at 43.1 ± 1.2% volume fraction with a carbon concentration of 1.17 ± 0.09 wt.%. Furthermore, the deconvolution of (200) Fe-γ reflections corresponding to two different low-carbon and high-carbon retained austenite peaks and, simultaneously, the blocky and film-like retained austenite was performed. In addition, the Nishiyama–Wassermann (N–W) crystallographic orientation relationship between bainitic ferrite and retained austenite was described as dominant using the misorientation distribution function (MDF). The crystallographic texture results indicated that the main growth of bainitic ferrite plates occurred after removing external stress during isothermal holding. The tensile tests and hardness measurements showed a high tensile strength achieved mainly by nano-metric bainitic ferrite plates and a high dislocation density. The high level of elongation is most likely attained due to a high amount of retained austenite in steel and both TRIP and TWIP effects during tensile deformation.

Strength-toughness synergic Ti-Mo double harmonic alloys prepared via powder metallurgy

A series of double harmonic Ti-Mo alloys are prepared using a powder metallurgy method. These alloys display chemical and structural double harmonic (CSDH) heterogeneity. The Ti-10Mo CSDH alloy is composed of α″, β(s), and residual β-Mo phases. As Mo increases, β(s) and β-Mo increase but α″ decreases. EDS and SEM results prove the chemical and structural harmonic distributions. All Ti-Mo CSDH alloys exhibit excellent mechanical properties. Especially, the Ti-10Mo CSDH alloy exhibits superior strength-toughness synergy (938.7 ± 49.5 MPa for Rp0.2, 1075.5 ± 54.7 MPa for Rm, ∼10.38 % for A and 111.6 ± 6.4 MPa for UT). The double harmonic heterogeneity producing the TRIP effect and back stress during plastic deformation is the major reason for strength-toughness synergy.

Achieving High Plasticity and High Toughness of Low-Carbon Low-Alloy Steel through Intercritical Heat Treatment

Medium manganese steel has excellent comprehensive properties due to the TRIP effect of retained austenite, but its welding performance is unsatisfactory for its high alloy content. This study obtained retained austenite in low-carbon low-alloy steel with low contents of silicon and manganese elements through intercritical heat treatment. The influence of intercritical quenching temperature on the content and characteristics of the retained austenite, as well as the functional mechanism of the retained austenite during low-temperature impact, was studied. The results showed that the content of the retained austenite increased from 12% to 17%, and its distribution extended from grain boundaries to martensite lath boundaries, with increasing intercritical quenching temperature. The retained austenite on the grain boundaries was in blocks, and that on the martensitic lath boundaries formed slender domains. The stability of the retained austenite was achieved through the enrichment of C and Mn during intercritical heat treatment. The contribution of retained austenite to low-temperature mechanical properties was closely related to its stability. The retained austenite with poor stability underwent martensite transformation at low temperatures, and the high-carbon martensite was a brittle phase that became the nucleation site of cracks or the path of crack growth during impact. Stable retained austenite passivated crack tips and hindered crack propagation during impacts, which improved the impact performance of the steel.

Effect of microstructure heterogeneity on the cryogenic fracture toughness of the dissimilar joint between SA645 and 304L made by laser welding

The cryogenic fracture toughness of the SA645/304L dissimilar weld, produced using continuous wave IPG fiber laser welding with a 0.3 mm beam offset towards the 304L side, and the microstructural effects on the fracture behavior were systematically investigated. The weld metal consists of ∼89% columnar dendritic martensite and ∼11% retained austenite (RA), where the ununiform distribution of constituent phases as well as the distinctions between martensite and RA in terms of morphology and mechanical properties lead to the microstructure heterogeneity of weld metal. Pop-in phenomenon appears on the Force-Displacement (F-V) curve during the crack tip opening displacement (CTOD) test for the weld. The CTOD value for the weld is ∼0.057 mm, about 8 times less than that when pop-in effect is ignored (∼0.563 mm). Rapid propagation of the pre-fatigue crack tip along the center of the weld leads to the pop-in phenomenon. Fracture surface in pre-fatigue crack tip (PCT) region shows quasi-cleavage features, while fracture surface in stable crack propagation (SCP) region presents a ductile-fractured surface with dimples. The crack propagation path in SCP region is frequently deflected. Inhibition effect of grain boundaries on crack propagation and TRIP effect of retained austenite (RA) result in the improvement in fracture toughness of the SCP region.

Pre-quenching induced lath structures and enhanced TRIP effect to optimize the strength-ductility of ultrahigh-strength hot-galvanized steel

Compared with the conventional hot-dip galvanized steel, an optimized strength-ductility combination was obtained by the combination of continuous galvanizing line (CGL) compatible quenching and austempering (QAT) treatment and pre-quenching, i.e., “pre-quenching + CGL-QAT”. The effects of the pre-quenching on the microstructure and mechanical performance were analyzed by using dilatometry, SEM, AES-EBSD, TEM and XRD technique. Compared with QAT samples, pre-quenching could eliminate the blocky M-A islands and promote the formation of lath reversed austenite and bainite ferrite in “pre-quenching + CGL-QAT” samples. Meanwhile, a large fraction (22.7–25.8%) of lath RA were obtained, such that brittle intergranular fracture can be prevented. The lath bainite/ferrite structures and sufficient TRIP effect enhanced the strength-ductility combination with an UTS reaching 1000 MPa and TEL around 30%. The fracture mainly presents ductile fracture and results in a ductile fracture surface with large dimples. The excellent strength-ductility combination with a UTS above 980 MPa, a YS of 625 MPa, a TEL of 29.1% and a higher PSE around 30 GPa·% can be obtained, which exceeds the current commercial hot-dip galvanized steel grades.

INVESTIGATION OF THE DEGREE DEFORMATION INFLUENCE DURING TENSILE TESTS ON THE TEXTURE, PHASE COMPOSITION AND RESIDUAL STRESSES IN THE α- AND ɣ-PHASES OF STEEL VNS9-SH

X-ray diffraction methods were used to study the influence of the degree of deformation during tensile testing on the phase composition, texture and stress state of the α- and ɣ-phases of the VNS9-SH alloy. It is shown that during testing to failure, the amount of the α-phase increases on the surface from 75 to 91% and from 45-50 to ~70% in the subsurface layers. To assess the susceptibility of two-phase steels to the trip effect, a parameter of austenite metastability is proposed in the form of the relative fraction of decomposed austenite at individual stages of tensile deformation. It has been established that in the initial steel strip 0.3 mm thick, as a result of the positive volumetric effect of the transformation ɣ = α, compressive stresses are formed in austenite, reaching a value of –1000 MPa on the surface, in contrast to tensile stresses in martensite. Their presence is associated with heating of the metal, the cooling of which leads to tensile stresses in martensite due to its significantly lower temperature coefficient of linear expansion value compared to austenite.

Modeling of Mechanical Properties of Metastable Austenitic Cr–Mn–Ni Steels with TRIP Effect

The mechanical properties of two metastable austenitic cast steels are examined by performing tensile tests in a temperature range of −70 to 300 °C. The 16‐6‐6 steel has an Ms temperature of −30 °C and an Md temperature of 100 °C. Athermal martensite formation down to −196 °C can be excluded for X3CrNiCuN17‐6‐4 steel. The Md temperature of X3CrNiCuN17‐6‐4 steel is 130 °C. The results obtained by a semiempirical thermodynamic‐mechanical calculation model are linked to experimental results from the tensile tests. The presented model combines chemical energy amounts from thermodynamic databases with mechanical energy amounts obtained by tensile tests with the aid of the conversion rule proposed by Patel and Cohen. Thermodynamic calculations are performed using the Thermo‐Calc software with the database TCFE10. As a result, the stress–temperature–transformation diagram showing the temperature dependence of proof stress, tensile strength, triggering stress for martensite formation, and the associated critical transformation temperatures (Ms, Md) is obtained. With the aid of the semiempirical thermodynamic‐mechanical model, it can be shown that the mechanical properties and critical temperatures of a metastable austenitic steel can be determined at very low experimental effort.

Second-order homogenisation of crystal plasticity and martensitic transformation

Second-order computational homogenisation is combined with constitutive modelling of crystal plasticity and martensitic transformation to analyse the influence of strain gradients on the multi-scale response of polycrystalline materials and the kind of size effects that can be captured. The impact of the length of the Representative Volume Element (RVE) model and the size of the grains is analysed independently, resorting to parametric studies with several random realisations. It is observed that the captured size effects are due to the RVE length, while the size of the grains, which is related to the number of grains in the RVE, is mainly associated with the representativeness of the homogenised response. Macroscopic deformation states are extracted from a plate subjected to bending to guarantee the physical admissibility of the strain gradients and deformation gradients applied to the RVE. It is also found that the influence of the RVE length on the multi-scale response of polycrystals is also dependent on the relationship between the magnitude of the first and second-order deformations. The TRIP effect is also studied by performing these analyses with and without martensitic transformation.To quantify the differences between the results obtained with first- and second-order homogenisation, analytical expressions that take into consideration the macroscopic deformation measures and the RVE length are identified to approximate numerical observations. Finally, an adaptive framework for multi-scale simulation accounting for strain gradients is proposed, where these expressions can be employed in a criterion for the transition from first- to second-order homogenisation.

Effect of austempering time on multiphase microstructure evolution and properties of carburized Cr-Mn-Si alloyed steel subjected to bainitization quenching & partitioning heat treatment

An innovative multistage heat treatment, that combines Bainitization and Quenching & Partitioning processes (BQ&P) can be applied to obtain multiphase microstructure in the carburized surface layer comprising nanobainite, ultra-fine martensite and retained austenite. In this study, a carburized medium-carbon Cr-Si-Mn alloyed steel was treated by BQ&P treatments with 20%, 40% and 100% of advancement of bainitic transformation in the top surface to investigate the effect of isothermal holding time on microstructure evolution and properties.Extending the bainitization time leads to an increase in the bainitic ferrite content in the microstructure and more significant fragmentation of the residual austenite blocks. It was shown that multiphase microstructure consisting of nanobainite, ultra-fine martensite and retained austenite results in the higher hardness and wear resistance of the case as compared to the fully bainitic microstructure. The enhanced properties results from the strengthening effect of additional martensite fraction and higher amount of stabilized austenite prone to transformation under friction (transformation induced plasticity, TRIP effect). Furthermore, adjusted parameters of the process lead to favorable mechanical properties in a medium-carbon core (YS ∼ 1430 MPa, UTS ∼ 1820 MPa, KV ∼ 20 Jcm−2). The ability to tailor mechanical and service properties of steel by controlling the fraction of particular phases along with a significant reduction of heat treatment time might designate the BQ&P process to industrial application, e.g. carburized gear wheels.

Microstructure evolution and mechanical properties of metastable austenitic medium Mn steel fabricated by warm caliber rolling

Warm caliber rolling at 550, 600, and 650 °C were carried out to refine the austenite grains of medium Mn steel by dynamic recrystallization (DRX). The effect of rolling temperature on the microstructure characteristics and mechanical properties of metastable austenitic steel was investigated in detail. The results show that the hierarchical structure of coarse fiber grains (CFGs) and ultrafine grains (UFGs) is obtained by warm caliber rolling method. With the increasing rolling temperature, more obvious DRX behavior occurs, resulting in a high percentage of UFGs. The preactive TRIP effect shortens the yield stress plateau in tensile stress-strain curves caused by CFGs with low stability. Furthermore, the unique hierarchical structure induces crack deflection and delamination, and significantly improves the impact toughness of medium Mn steel at room temperature. However, at the low temperature of −80 °C, CFGs are easy to induce martensite transformation, which seriously deteriorates the impact toughness.

A β-type titanium alloy with high strength and low elastic modulus achieved by spinodal decomposition

In the present work, a Ti-40Zr-12Ta (in atom percent, at%) alloy with the spinodal decomposition was designed based on the isothermal section of the Ti-Zr-Ta ternary phase diagram. The alloy was first solid solution treated at 1000 °C and then cold rolled with a thickness reduction of 70% at room temperature. After that, the as-rolled samples were annealed at the temperatures ranging from 500 °C to 900 °C, and the effects of annealed temperature on the microstructure and mechanical properties were investigated. It is found that the spinodal decomposition occurred in the alloy at the temperature ranging from 500 °C to 700 °C, leading to the formation of the periodically distributed Ta-rich (β1) and Ta-lean phases (β2). The as-rolled Ti-40Zr-12Ta alloy exhibited a relative high strength due to the work hardening and solid solution strengthening. Annealing treatment at 500 °C promoted the strength of the alloy but significantly decreased its plasticity due to the formation of nano-scale isothermal ω phases. The sample annealed at 700 °C exhibited good tensile mechanical properties, and the modulated structure introduced by spinodal decomposition and TRIP effect contribute to the improvement of its strength and plasticity.

Comparative study on effects of annealing temperature on microstructures and mechanical properties of high-Al low-Si steel

We elucidate here profound effects of quenching and partitioning process after partial (IQP) or full austenitization (FQP) on the microstructure and mechanical properties of high strength hot-dip galvanized steel with high Al and low Si content. The results showed that the FQP steel contains 3.38% vol.% retained austenite with mainly film-like morphology, while the IQP steel mainly consists of 6.44–8.13 vol.% retained austenite with block shape. Additionally, retained austenite in the IQP sample transforms into martensite more easily duo to higher nucleation rate, which means lower austenite stability. A combination of various austenite grain size and morphology make the TRIP effect throughout the whole deformation process and increased the elongation for IQP sample. When the steel was treated by annealing temperature of 967 °C, partitioning temperature and time of 460 °C and 100 s respectively, the steel exhibited excellent mechanical properties with tensile strength of 754 MPa, total elongation of 27% and the product of strength and ductility exceeds 20 GPa%. The proposed content in the present study demonstrates compatibility with the continuous hot-dip galvanizing process.