Isothermal Martensitic Transformation Research Articles

Effect of isothermal martensite and bainite transformation on microstructure and properties in a medium-carbon high-strength Q&P steel

Effect of isothermal martensite and bainite transformation on microstructure and properties in a medium-carbon high-strength Q&P steel

Recoverable strain induced by martensitic transformation during holding of NiTi-based shape memory alloys under constant stress and temperature

The aim of the present paper was to study the strain variation in the NiTi-based shape memory alloys during isothermal holding under stress. The recoverable strain of Ni51Ti49 and Ti40.7Hf9.5Ni44.8Cu5 alloys was studied on holding under stress after cooling (Regime 1), after active loading (Regime 2), or during holding without stress in the sample that demonstrates the two-way shape memory effect (Regime 3). The strain variation was found during holding under a stress in all regimes. This strain was recovered on subsequent heating or unloading, thus, the strain variation on holding was caused by the isothermal martensitic transformation. This isothermal strain depended on the chemical composition of the alloy, the regime for the isothermal holding, the stress, and the holding temperature. The maximum isothermal strain was 3.4% in Ti40.7Hf9.5Ni44.8Cu5 (Regime 1) and 6.1% in Ni51Ti49 (Regime 2). Holding in Regime 3 was accompanied by a small strain (less than 0.3%). The influence of the stored elastic energy on isothermal martensitic transformation on holding was discussed, and it was shown that a large isothermal strain was found if the transformation was accompanied by a small stored elastic energy.

Isothermal martensitic transformation in Ti-Ni-Cu-Co shape memory alloy: Insight from a thermally activated kinetic model

Recently the isothermal martensitic transformation in shape memory alloys (SMAs) has been reported in many literatures, and several models have been proposed to interpret the isothermal and athermal kinetics. However, the underlying mechanisms remain inadequately understood. In this work, the isothermal transformation from B2 to B19 is confirmed in Ti-Ni-Cu-Co melt-spun ribbons at the temperature range between Ms and Mf. It reaches a saturation point at every isothermal temperature Tiso, and the saturation points correspond to the f−T curve at the cooling rate of 0.5 K/min. The experimental results indicate that the isothermal accumulation of martensite is a relaxation process from the transient state to the thermoelastic balance one. A thermally activated kinetic model is developed in this study to characterize the isothermal and athermal kinetics. The model is able to estimate the evolution of martensite volume fraction under any temperature path T(t) and it agrees with the experimental results well. According to the model, the effects of elastic energy, nucleation density, and activation energy on the kinetics are investigated. Among those, a small nucleation density ni as well as a large activation energy Q will result in a significant isothermal transition. In this work, the slighter isothermal effects originate from the higher value of ni. As for the non-stoichiometric SMAs, the higher value of Q is responsible for the accumulation of martensite at the isothermal process. Accordingly, the present work provides a novel view from a kinetic model to understand the isothermal martensitic transformation in SMAs.

The effects of post-weld aging and cryogenic treatment on self-fusion welded austenitic stainless steel

The effects of post-weld aging and cryogenic treatment on self-fusion welded austenitic stainless steel thick plates were investigated in the present work. The results showed that fusion zone microstructure consisted of austenite matrix and vermicular ferrite. Aging treatment promoted the decomposition of δ-ferrite into σ-ferrites as well as the precipitation of carbides. Isothermal martensitic transformation in the Tungsten Inert Gas Welding (TIG) specimen was induced by cryogenic treatment, and stacking faults were increased. The fusion zone microstructure of Electron Beam Welding (EBW) and Laser Welding (LW) was finer than that of TIG, with acicular ferrite distributed in the austenite matrix. A large number of twins were generated in the austenite matrix after LW. Cryogenic treatment produced a large number of sub-grains in LW specimens, which was due to the entanglement and accumulation of dislocations in the vicinity of ferrite. Post-weld aging and cryogenic treatment have no influence on the strength of weldments with different welding methods while cryogenic treatment could improve the impact toughness of EBW and LW weldments by the extent of 7.4% and 8.8%, respectively. The aging treatment reduced the impact toughness by 49%, 33% and 15.5%, as well as the uniform elongation by 44%, 39% and 17% for TIG, EBW and LW, respectively. Aging treatment reduced the surface residual stress of TIG weldment by 58.8% in Y direction and 61.2% in X direction. Cryogenic treatment at could also release the surface residual stress of TIG weldment by 36.8% in X direction and 16.3% in Y direction.

Application of the Nelder-Mead method to optimize the selecting of the Likhachev-Volkov model constants

A new method was proposed for selecting constants of the modified Likhachev - Volkov microstructural model to describe the reversible strain variation during isothermal martensitic transformation in Ti49Ni51 alloy. The experimental data were approximated by solving the optimization problem using the Nelder - Mead method. A set of model parameters was found that provided the best approximation. Using the selected constants set, model qualitatively and quantitatively describe the strain variation during cooling and heating of the model sample, as well as during isothermal holding under constant stress. The proposed method of selecting constants allowed performing the smallest number of numerical experiments without requiring additional real experiments.

The strain variation during the isothermal B2→B19’ martensitic transformation in the quenched or annealed Ni51Ti49 alloy

The strain variation during the isothermal holding under constant stress was studied in the quenched or annealed Ni51Ti49 alloy samples. The isothermal strain variation was found in both samples and this strain was completely recovered on subsequent unloading and heating. This allowed to conclude that the strain variation on holding was caused by the isothermal martensitic transformation. It was found that the maximum value of isothermal strain depended on the alloy heat treatment. This value was equal to 0.5 % in annealed sample and it was equal to 6 % in quenched sample. It was assumed that the formation of the Ni4Ti3 phase during annealing led to a decrease in concentration of substitutional Ni atoms in NiTi phase that were responsible for the isothermal transformation. As a result, the less volume fraction of the martensite formed during holding that supresses the strain variation in annealed samples.

Isothermal Martensitic Transformation in Ti-Ni-Cu-Co Shape Memory Alloy: Insight from a Thermally Activated Kinetic Model

Isothermal Martensitic Transformation in Ti-Ni-Cu-Co Shape Memory Alloy: Insight from a Thermally Activated Kinetic Model

Strain variation in the Ti40,7Hf9,5Ni41,8Cu8 alloy during isothermal martensitic transformation under a constant stress

Strain variation on holding under a constant stress was studied in the Ti40,7Hf9,5Ni41,8Cu8 alloy. It was found, that on holding under stress, the isothermal strain rose up to saturation, which value depended on holding temperature and stress. It was found that the dependencies of the isothermal strain on the holding temperature and stress were non-monotonic. This allowed to find the optimal value of stress and time at which the isothermal strain attained the maximum value of 3.2 %. It was found that the maximum isothermal strain in the Ti40,7Hf9,5Ni41,8Cu8 alloy was less than in the Ti40,7Hf9,5Ni44,8Cu5 alloy.

Thermal Activation Theory of Isothermal Martensitic Transformations: Classical Interpretation of Nonergodicity in Transformation Strain

Thermal Activation Theory of Isothermal Martensitic Transformations: Classical Interpretation of Nonergodicity in Transformation Strain

Martensitic transformations in Fe-Mn alloy with low stacking fault energy

Object of investigation is highly alloyed binary Fe-Mn alloys which in austenitic state are characterized with different stacking fault energy (SFE). Experimental results using the internal friction and X-Ray methods are obtained for the martensitic non isothermal transformation upon cooling and heating. It was determined that mainly the structural characteristics of the alloys define the type of the possible transformation mechanisms and the products of the martensitic reaction. The role of the SFE for the mechanisms of formation and growth of the martensite phase and the relation between the micro-structural parameters of the solid solution and the mechanism of non isothermal martensitic transformations is shown. On the basis of the existing models of dynamic relaxation associated with phase transformations, here are pivotal conclusions about the characteristics of the non-diffusion mechanism of transformation into a low SFE.

Microstructure evolution and mechanical properties improvement of selective laser melted Co-Cr biomedical alloys during subsequent heat treatments

Abstract Selective laser melting (SLM) technology has garnered considerable attention as an advanced manufacturing method that produces biomedical Co-Cr devices specifically developed for dental restoration applications. However, SLM technology inevitably causes unstable microstructures and excessive residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. Also, information on the phase transformation and precipitation behavior during subsequent heat treatments is limited. In this work, SLM-manufactured Co-Cr specimens were initially subjected to stress-relieving annealing (SRA) at various temperatures (850, 950, 1050, and 1150 °C) and then porcelain firing (PF) treatments, and the microstructure evolution and mechanical properties improvement during subsequent heat treatments was investigated in detail. Scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometer, transmission electron microscopy, and Image Pro Plus 6.0 (IPP) software were used to analyze alloy microstructures, whereas tensile tests were performed to evaluate corresponding performance. Results showed that after SRA and/or PF treatments, a large number of nanoscale precipitates were observed, and acicular or lamellar e phase appeared. For SRA-treated specimens, as annealing temperatures increased, the volume fraction of γ phase decreased, especially after annealing at 950 °C, the γ phase was completely transformed into the e phase. In contrast, for (SRA + PF)-treated specimens, the volume fraction of γ phase first decreased and then increased with the increase of annealing temperatures. IPP analysis found that the amount, size, and morphology of the precipitates varied with the change in annealing temperature. During annealing, isothermal martensite transformation (γ →e) dominated due to the long duration between 600 and 900 °C, while the amount of athermal martensite transformation (γ →e) gradually increased when porcelain firing. Changes in the above-mentioned microstructures affected corresponding mechanical properties. From 850 °C to 1050 °C, tensile strength and elongation continue to improve. At 1150 °C, both tensile strength and 0.2% yield strength decreased, but the elongation was further improved. The variation in hardness (HV10) with temperature was the same as that of the volume fraction of e phase, that is, the higher the volume fraction of e phase, the higher the hardness. SEM/EDX analysis of tensile fracture surfaces revealed a quasi-cleavage fracture. In conclusion, 1150°C-SRA and PF treatments are an effective method for homogenizing microstructure and improving mechanical properties.

Extremely rapid age hardening behavior in solution treated Ti-5Al-2Fe-3Mo

To grasp age hardening and phase transformation behaviors in a β rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, during aging at 300-500°C after the solution treatment at high α+β temperature. Vickers hardness and microstructure changes during aging were closely investigated using XDR and TEM/EDS. Vickers hardness rapidly increased with increasing holding time and reached about 440HV by aging at 450°C for only 5 min. It further increased to 510HV in 8h of aging time. The initial stage of age hardening is extremely fast compared to that in other conventional α+β and β type titanium alloys. After aging for only 5min, extremely fine acicular products of about 2 to 10 nm in width were formed in the transformed β phase. TEM/EDS analysis revealed that all substitutional alloying elements, Al, Fe and Mo, homogeneously distributed after the aging, indicating that the transformation is diffusionless as far as substitutional elements are concerned just like martensite transformation although it has time dependency. To explain the mechanism of this unique phase transformation having features of isothermal martensite transformation, we propose bainitic transformation where interstitials such as O diffuse without conspicuous diffusion of substitutional elements.

Isothermal martensitic transformation behavior of Ti–Nb–O alloy

Abstract The isothermal martensitic transformation (IMT) behavior of Ti–20Nb–0.7O (mol %) alloy was investigated via X-ray diffractometry. Isothermal formation of the α″ martensite phase was observed, and the activation energy was determined using the time-temperature-transformation diagram coupled with differential scanning calorimetry. IMT in Ti–20Nb–0.7O alloy was explained by nanometer-sized local lattice modulations (nanodomains) and diffusion of oxygen atoms in the β phase. The latter was found to be the primary rate-determining process.

Analysis of interface migration and isothermal martensite formation for quenching and partitioning process in a low-carbon steel by phase field modeling

A modified two-dimensional phase field microelasticity model coupling the Cahn–Hillard equation is applied to investigate the microstructural evolution of interface migration and isothermal martensitic transformation accompanying carbon diffusion and redistribution during quenching and partitioning (Q&P) process in a low-carbon steel. In the modeling, the Kim–Kim–Suzuki model is adopted to depict interface conditions for carbon diffusion, while the transformation behavior during partitioning is assumed as a displacive manner. Simulations were done for one-step Q&P processes with various partitioning temperatures. Simulated results reveal that carbon enrichment within untransformed austenite varies significantly with the local microstructural features formed during Q&P process. With the proposed phase field model, the isothermal martensite formation and migration of existing austenite–martensite interfaces during partitioning are realized by simulation. It is also revealed that the interface migration behavior during partitioning is strongly correlated with the spatial distribution of the local elastic strain energy inherited from quenching process. Furthermore, temperature dependence of isothermal martensitic transformation kinetics is also studied, with the competition between martensitic transformation and reverse transformation during partitioning, a favorable partitioning temperature is found to have the maximum amount of isothermal martensite formed during partitioning.

Strain variation during the isothermal martensitic transformation in the Ni51Ti49 shape memory alloy

The strain variation under constant tensile stress of 50 MPa during isothermal holding of the quenched Ni51Ti49 shape memory alloy is reported for the first time. A maximum strain of 0.4% is observed during holding at a temperature of −37 °C, which is 6 °C less than the start temperature of the martensitic transformation (Msσ). A shift in the holding temperature above or below the Msσ temperature decreases the isothermal strain. It is shown that the strain that appears during isothermal holding completely recovers on subsequent heating. This allows one to conclude that the strain variation on holding is caused by the martensitic transformation that occurs under isothermal conditions.

Strain variation during the isothermal martensitic transformation in Ti40.7Hf9.5Ni44.8Cu5 alloy

The aim of the paper was to study the strain variation during the isothermal B2 → B19′ transformation in NiTi-based alloy. It was found that isothermal holding of the Ti40.7Hf9.5Ni44.8Cu5 alloy under a stress of 235 MPa at constant temperatures, which were close to the start temperature of the B2 → B19′ transition, was accompanied by an additional strain variation. This strain was completely reversible on further heating and depended on the position of the holding temperature compared to the start temperature of the forward transformation. The maximum of isothermal strain of 2.9% was observed during holding at a temperature of 50 °C.

Thermal and Mechanical Stability of Austenite in Metastable Austenitic Stainless Steel

The roles of grain size, texture, strain, and strain rate on the thermal and mechanical stability of austenite in AISI 321 metastable austenitic stainless steel were studied. Ultrafine grain (UFG), fine grain (FG), and coarse grain (CG) specimens with average grain sizes of 0.24, 3, and 37 µm sizes, respectively, were investigated. To determine the thermal stability of austenite (TSA), samples were soaked in liquid nitrogen (− 196 °C) for varying times between 0.5 and 24 hours. On the other hand, the mechanical stability of austenite (MSA) was studied by subjecting cylindrical specimens to both quasi-static (4.4 × 10−3 s−1) and dynamic loading conditions (between 1300 and 8800 s−1). Thermally-induced α′-martensite was only observed at an incumbent time in AISI 321 to suggests an isothermal martensitic transformation occurred. Both Kurdjumov-Sachs ($$ \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} $$ and $$ \langle \bar{1}01\rangle_{\gamma } ||\langle 1\bar{1}1\rangle_{{\alpha^{\prime } }} $$) and Nishiyama–Wasserman ($$ \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} $$ and $$ \langle 112\rangle_{\gamma } ||\langle 011\rangle_{{\alpha^{\prime } }} $$) orientation relationships existed between the untransformed γ and thin-plate α′-martensite. The thermally-induced phase transformation was highly suppressed in UFG specimens. While TSA decreased with an increase in grain size, MSA decreased with a decrease in grain size. While thin-plate α′ predominantly formed in the thermally-treated AISI 321 steel (FG and CG specimens only), lath and irregularly-shaped α′ formed in the specimens deformed under quasi-static and dynamic loading conditions, respectively. Irrespective of strain rate, deformation-induced α′ in UFG specimens inherited the morphology of the deformed austenite grain that is equiaxed. Irrespective of grain size, MSA also decreased with increase in strain (up to a critical strain for specimens deformed under dynamic loading condition) and decrease in strain rate. In the event of adiabatic shear band (ASB) formation in a specimen deformed at high strain rate, MSA increased as the ASB was approached due to the temperature rise in the ASB region. Electron backscattered diffractometry examination revealed that the evolution of both thermally- and deformation-induced martensite is orientation-dependent in FG and CG specimens. The instability (thermal and mechanical) of the austenite phase is highest in the RD/CD||[100]-oriented grains (RD and CD are rolling and compression directions, respectively), followed by grains oriented near RD/CD||[110] and RD/CD||[111], in that order. These findings could open a new window of engineering the initial texture of metastable austenitic stainless steel to either aid thermally and/or mechanically-stable austenite phase or promote both isothermal and deformation-induced martensitic phase transformation.

Effect of Minimum Temperature on the Mechanical Properties and Reversed Austenite Content of 9%Ni Steel Subjected to Cryogenic Treatment

The effect of minimum temperature on the mechanical properties and reversed austenite content of 9%Ni steel subjected to cryogenic treatment was experimentally investigated. Cryogenic treatment with different temperatures and soaking time were conducted by combining with the newly developed quenching, lamellarizing and tempering (QLT) heat treatment of 9% Ni steel. The results show that the cryogenic treatment at −80 and −110 °C has no obvious influence on the reversed austenite content and mechanical properties of 9%Ni steel. However, the room temperature impact toughness is improved by cryogenic treatment at −140 °C for 24 h, which is attributed to the modification in the reversed austenite morphology from bulks into strips. The volume fraction of the reversed austenite decreases slightly due to the isothermal martensitic transformation at −140 °C. Cryogenic treatment at −196 °C for 24 h increases the volume fraction of reversed austenite and refines the secondary martensite lath, thereby improving both the room temperature impact toughness and cryogenic ductility. The observed results are mainly due to the precipitation of ultra-fine carbides and the increase of internal stress during cryogenic treatment, which provides more nucleation sites for the reversed austenite in the process of tempering.

Structural Aspect of Isothermal Bainitic Transformation in High-Carbon Manganese–Silicon Steel

Transmission and scanning electron microscopy and X-ray diffraction analysis were used to compare the structural states of high-carbon manganese–silicon steel which underwent low-temperature isothermal bainitic and martensitic transformations. It was shown that bainite consisting of thin α-phase plates and residual austenite interlayers forms during isothermal holding at 300°С. As the isothermal holding time increases, fine carbide particles precipitate within the bainite α-phase.

Metallurgical principles of microstructure formation in sub-zero treated cold-work tool steels – a review

The beneficial influence of a sub-zero treatment on wear resistant tools and components has been known for over 100 years. On the other hand, the basic metallurgical principles being responsible for enhanced hardness and wear performance, changes in the tempering response and toughness and improved dimensional stability have become known only over the past decade. The sub-zero treatment has, thus, been changed from an art to accepted science. The topic of the current conference paper is the latest theory explaining the metallurgical background for this kind of treatment. This theory states that it is the low-temperature isothermal martensitic transformation that induces secondary microstructural effects, such as an accelerated precipitation rate for transient carbides, formation of very small globular carbides and overall refinement of the microstructure. Consequently, secondary microstructural effects have a clear impact on the most relevant properties. The extent of the improvement or deterioration of these properties may be a result of competitive microstructural effects.