Start Temperature Research Articles

Role of Magnetism in Lattice Instability and Martensitic Transformation of Heusler Alloys

Heusler alloys are subject of considerable interest because they exhibit a martensitic transformation (MT), a shape-memory effect and a giant magnetocaloric effect. As it is commonly believed, the pronounced magnetoelastic coupling plays a crucial role; however, the effect of alloy composition on MT is still under discussion. To describe the features of MT in Ni0.75−xMnxGa0.25 Heusler alloys, the phenomenological model that consistently considers the magnetic and lattice degrees of freedom and their mutual interplay has been developed. The magnetic entropy contribution was estimated within the framework of the microscopic approach. The proposed model allows us to describe the dependence of the martensitic transformation start temperature Ms(x) on the Mn concentration x in reasonable agreement with the experiment.

Microstructural evolution and ultra-high impact toughness of austempered lamellar bainitic steel far below Ms temperature

The present study investigated the effects of the austempering process far below Ms (martensite start temperature) on the microstructural evolution and impact toughness of bainitic steel with ultrahigh impact toughness and compared the results with the tempered martensitic steel. The results revealed that the lamellar bainitic steel composed of bainitic ferrite laths and retained austenite films is obtained by austempering far below Ms temperature. The obtained steel exhibited the Charpy V-notch impact absorbed energy of up to 131 ± 6 J, which was 2.7 times higher than that of the tempered martensitic steel (49 ± 1 J). The ultra-high impact toughness of bainitic steel was attributed to the deflection of the crack propagation direction by the refined bainitic ferrite laths and the passivation of the crack tip by the retained austenite films. Furthermore, the proportion of V2, V3, and V5 variants increased and formed HAGBs (high-angle grain boundaries) with the V1 variant in the austempered bainitic steel B-32. These HAGBs inhibited crack propagation effectively, thereby significantly improving the impact toughness of the austempered bainitic steel.

Zero thermal expansion and high Young's modulus in Ti-Nb achieved by concurrent α″iso and ω precipitation

This work develops a novel and efficient processing strategy for creating multi-phase Ti-alloys with zero thermal expansion and enhanced rigidity. By heating to between the start and finish temperatures for martensite reversion, the cold-rolled, α″ martensitic microstructure in Ti22Nb mostly transforms to austenite β, which in turn facilitates the simultaneous precipitation of nano-scale α″isoand ω phases during aging. The diffusional partitioning of Nb leads to the formation of β + α″iso+ ω multi-phase microstructures. The crystal-level thermal expansion of α″isois analogous to athermal α″ martensite, featuring negative thermal expansion along [010]α″. Macroscopic expansion behavior and Young's modulus are related to the microstructural evolution during aging. Holding at 250 °C increases the initially negative sample-level coefficient of thermal expansion and makes it become zero after aging for 1000 h. A considerable improvement in Young's modulus of 44% could be verified.

Study on the characterization of surface ferrite transformation in carbon steel using in-situ thermovoltage measurement

In this paper, a self-designed in-situ thermovoltage measuring system was built to monitor thermovoltage of 0.45% carbon steel during heat treatment with precisely designed cooling rate. The results show that an abrupt change in thermovoltage curve was observed, and the transition temperature of the surface ferrite transformation was precisely obtained, which are 765 °C and 723 °C, respectively. Based on the measured austenite to ferrite transition temperature range, designed cooling rate during heat treatment of 0.45% carbon steel was performed, which realizes precise control of surface ferrite transformation. In addition, quenching at the phase transition start temperature (765 °C) determines the formation mechanism of the surface ferrite layer. The carbon distribution at the interrupted ferrite grain development was measured by electron probe microanalyzer (EPMA), and which corresponds to the formation mechanism of surface ferrite layer presented in the author's previous study. The results in current study confirm the practical application of the self-designed in-situ thermovoltage measuring system which provides a new method of monitoring and controlling material phase transformation during heat treatment.

Effect of Microstructure on Mechanical Properties of Quenching & Partitioning Steel

The microstructure and mechanical properties of a low-carbon steel produced via the quenching & partitioning (Q&P) heat treatment was investigated, with particular focus on the hole expansion ratio, which is an index of the stretch-flange-formability. 0.19mass%C-1.5mass%Si-2.9mass%Mn steel was annealed at 850 ˚C, then cooled to 150~400 ˚C (QT: quench temperature), followed by holding at 400˚C for 1100 s. Yield strength and hole expansion ratio drastically increased when the QT was below the Ms (martensite start) temperature. The steel with QT of 300 ˚C exhibited not only a higher elongation, which has been well documented, but also a higher hole expansion ratio, when compared to the conventional TRIP steel with QT of 400 ˚C having equal tensile strength around 1200 MPa. The micro-void formation during deformation was suppressed in the steel with QT of 300 ˚C due to the smaller volume fraction of large blocky martensite compared to the TRIP steel. These excellent mechanical properties are attributed to its unique microstructure consisting of a certain amount of tempered martensite, lath-shaped retained austenite and bainitic ferrite, which was generated via the Q&P heat treatment.

Effects of Cu and Nb on martensite hardening and transformation-induced plastic deformation behavior of maraging TRIP-aided steel

Martensite hardening and transformation-induced plastic deformation (TRIP) are crucial to the mechanical properties of steels. In this study, maraging TRIP-aided steel was developed. The addition of Cu and Nb can effectively promote transformation-induced plastic deformation, this is due to the characteristic of the alternating combination of soft and hard phases. In contrast, the addition of Nb has a more significant effect on martensite hardening, which can be attributed to the smaller effect of Nb on the martensite transformation start temperature (Ms). The multi-phase structure and precipitation strengthening, lead to the ultimate tensile strength and elongation of the maraging TRIP-aided steel is ∼1300 MPa and ∼24%, respectively. It was found that the prior austenite formed a continuous network surrounding bainite and martensite. The high-density dislocations in bainite and martensite increased the hardness. In addition, the metastable austenite phase was transformed into the martensite phase, and a heterostructure with the alternating distribution of bainite/austenite was formed during deformation. The refinement of grains effectively reduced the concentration of stress. Massive nanometer-scaled precipitates were formed in Nb-containing steel after peak aging, and the co-precipitation of Nb-rich and Mo-rich phases occurred in the steel. The large-sized Laves-Fe2(Nb, Mo) effectively enhanced the strength of steel, and the fine and dense Ni3Ti was beneficial to the simultaneous improvement of strength and plasticity. Therefore, the jointed mechanisms of martensite hardening, and the TRIP effect improved the strength while retaining good plasticity.

Controlling the Thermal Stability of a Bainitic Structure by Alloy Design and Isothermal Heat Treatment.

The aim of this work was to develop a novel bainitic steel that will be specifically dedicated to achieving a high degree of refinement (nano- or submicron scale) along with increased thermal stability of the structure at elevated temperatures. The material was characterized by improved in-use properties, expressed as the thermal stability of the structure, compared to nanocrystalline bainitic steels with a limited fraction of carbide precipitations. Assumed criteria for the expected low martensite start temperature, bainitic hardenability level, and thermal stability are specified. The steel design process and complete characteristics of the novel steel including continuous cooling transformation and time-temperature-transformation diagrams based on dilatometry are presented. Moreover, the influence of bainite transformation temperature on the degree of structure refinement and dimensions of austenite blocks was also determined. It was assessed whether, in medium-carbon steels, it is possible to achieve a nanoscale bainitic structure. Finally, the effectiveness of the applied strategy for enhancing thermal stability at elevated temperatures was analyzed.

Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry

Continuous cooling transformation (CCT) diagrams represent roadmaps for producing all heat-treatable steels. CCT curves provide valuable information on the solid-state phase transformation sequence, depending on the defined cooling strategies, the alloying concept of the steel and previous processing steps. The experimental characterization of CCT diagrams is usually done on a laboratory scale applying thermal analysis of dilatometry. In current research studies, however, also other in-situ methods such as high-temperature laser scanning confocal microscopy (HT-LSCM) or differential scanning calorimetry (DSC) are frequently used to investigate phase transformations during thermal cycling. In the present study, HT-LSCM observations and DSC analysis are critically compared with dilatometry results by investigating the CCT diagram of a 0.4%C-1.8%Si-2.8%Mn-0.5%Al (in mass pct.) advanced steel grade. Furthermore, classical examinations by optical microscopy and hardness measurements were performed to support the analysis. In general, very good consistencies between all experimental techniques were identified in determining the transformation start temperature for pearlite, bainite and martensite. The optical microscopy confirmed the observed phase transformations and the results correlated with the measured hardness response. Based on the results, coupling of HT-LSCM and DSC is considered as a valuable novel approach to plot CCT diagrams in future research.

The role of austenite grain size on martensite start temperature and phase transformation path in SUS321 stainless steel

The change in Ms and transformation path of γ→α′ transformation with austenite grain size (AGS) has been investigated by a thermodynamic model of martensitic transformation and the stacking fault energy (SFE) in SUS321 stainless steel. With increasing AGS, the Ms increases and the SFE decreases. A good linear relationship is established between the Ms and the SFE. This result suggests that the martensitic transformation path changes as the AGS decreases, and the γ→ε→α′ sequence is gradually suppressed when the AGS is smaller than 3.5 μm.

Atypical phase transformation behavior of Fe-33%Ni alloys induced by shot peening

The phase transformation behaviors of two Fe-33 mass%Ni alloy specimens subjected to shot peening (SP) were investigated. The first sample consisted solely of an austenite (γ) phase while the second also contained a high proportion of a martensite (α′) phase. When the specimen containing only the γ phase was subjected to SP, a stress-induced martensitic transformation occurred. In contrast, the second sample underwent the reverse transformation from the α′ to γ phase. Moreover, the volume fraction of the γ phase in the vicinity of the peened surface of each specimen was found to plateau as the SP treatment was prolonged. The final volume fraction of the γ phase was the same in both specimens regardless of their initial microstructure prior to the SP process. In addition, the volume fraction of the γ phase was found to increase with increasing peening pressure, meaning that the reverse transformation was promoted by SP at higher pressures. Calculations of the deformation heat generated during SP indicated that this heat was not primarily responsible for the reverse transformation induced by SP. Estimation of the change in the phase transformation temperature resulting from the residual compressive stress imparted by SP showed that this treatment changed the value of T0 (the arithmetic mean of the martensitic transformation start temperature and the reverse transformation start temperature) for the Fe-33 mass%Ni alloy from 328.5 K to 287.8 K. Because T0 after SP was lower than the temperature to which the specimens were raised during SP, this process could have caused stress-induced reverse transformation. Therefore, the decrease in the phase transformation temperature stemming from residual compressive stress is believed to explain the reverse phase transformation observed during SP of this alloy.

Microstructural characterization and thermal stability of He charged amorphous silicon films prepared by magnetron sputtering in helium

Sputtering of silicon in a Helium magnetron discharge has been reported as a bottom-up procedure to obtain amorphous Si films containing high amounts of gas-filled nanopores. Here we compare the microstructure and composition of Si–He nanocomposite films deposited by magnetron sputtering (MS) with 4He in DC or RF and 3He in RF operation modes. Electron microscopy (SEM and TEM), X-ray diffraction (XRD) and ion beam analysis (IBA) have been used to analyze the films and to investigate the in-situ and ex-situ thermal evolution. Depending on deposition conditions different in depth compositions, nanopore size and shape distributions, porosity and He content could be obtained. The presence of impurities (i.e. oxygen) has shown to promote He diffusivity reducing He accumulation. The start temperature of He-release varied in the range 473–723 K without films crystallization. Films grown in RF mode reached contents of 32 and 29 at% of 4He and 3He and were respectively stable up to 573 and 723 K both in vacuum and under inert gas flow. In-situ p-EBS (proton Elastic Back Scattering) allowed monitoring the He release accompanied by blistering/delamination effects visualized by SEM. These results show the potentiality of annealing to hold nano-porous structures after liberation of trapped gas.

Molecular dynamics simulation of phase transformation and wear behavior of Ni35Al30Co35 high temperature shape memory alloy

Ni–Al–Co alloys as one of the ferromagnetic high-temperature shape memory alloys (HTSMA) are attractive in aerospace and defense applications, due to their high phase transition temperature range (400–700K), high recoverable strain and good machinability. However, the underlying physics for the phase transformation behavior of β single-phase and β+γ two-phase Ni–Al–Co ternary alloys remain unclear. In this work, molecular dynamics simulation of phase transition, pseudoelasticity (PE) and wear behavior of Ni35Al30Co35 alloy were analyzed using the EAM potential. It shows that Ni35Al30Co35 alloy has a high martensite start temperature of 825K and an austenite start temperature of 631K, respectively. After five thermal cycles, martensite and austenite start and finish temperatures are stable around 500K and 700K. The initial structure of Ni35Al30Co35 alloy at 1000K is identified as BCC (β phase) and FCC (γ phase) structure, transforming to L10 structure through martensitic transformation. Furthermore, PE of Ni35Al30Co35 alloy exhibits about a 9.1% recoverable strain under <001> oriented uniaxial tension tests and about a 6.6% recoverable strain under <001> oriented uniaxial compression tests at a temperature of 1200K, showing the potential used in extreme environment. Moreover, spherical indenters with a radius of 15 Å were used to investigate the wear behavior of Ni35Al30Co35 alloy under different temperature. It was found that the PE effect affects the frictional behavior during nano-scratching. Combining the effect of high temperature softening and phase transition PE, the wear rate increases nearly by 5.3% when the temperature rises from 300K to 1000K. Furthermore, the friction force curve obtained is consistent with the friction force curve obtained by simulation at 300K.

Design and fabrication of NiTi shape memory alloy/aramid composite fabric for thermal protective clothing

Developing fabrics with enhanced thermal protection is a cutting-edge topic in area of thermal protective clothing (TPC) for workers under heat hazards. Here, temperature-responsive NiTi shape memory alloy (SMA) filaments, which transform into a sinusoid form under a stimulus of temperature exceeding their austenite start temperature of 45.1 °C, were prepared. Nine composite fabrics (CFs) using SMA filaments and aramid yarns, different in filament interval, air gap orientation, aramid yarn type and fabric density, were fabricated and incorporated into traditional three-layer TPC fabric system, resulting in a four-layer smart fabric system (SFS) with adaptive structure. Thermal protective performance (TPP) tests demonstrated the addition of CF could significantly slow down the rise of temperature at skin surface and prolong the time to the first-degree burn. With the decrease of filament interval or increase of fabric density, TPP of SFS was improved due to more heat accumulation within and less heat conduction through the fabric. This study would inspire engineering CFs with enhanced TPP, thus pushing forward the development of smart technology in textile engineering.

Impacts of Nb on grain refinement in a simulated coarse-grained-heat-affected-zone of ultra-high-strength steels

ABSTRACT The impact of Nb contents on grain refinement in a simulated coarse-grained heated-affected zone (CGHAZ) in steels was investigated. The austenite grain grew in multiple directions and then impinged against each other during the α→γ transformation. Subsequently, austenite grains coarsened via the coalescence of small grains and boundary migration. With the increased Nb content, the austenite in CGHAZ became coarser due to the decreased quantity and increased size of Nb precipitates. Furthermore, the austenite decomposed into bainite during the cooling process. Compared with the low-Nb specimen, the crystallographic grain size in CGHAZ of high-Nb specimen is slightly coarser, relating to the coarser prior austenite grain. However, higher Nb content delays the bainite transformation start temperature, leading to multiple variants selection of bainite, increasing the refinement ratio.

Lattice parameters of austenite and martensite during transformation for Fe–18Ni alloy investigated through in-situ neutron diffraction

Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, in-situ neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe–18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Furthermore, the lattice parameter of quenched full martensite and the specimen diameter were both decreased by tempering, demonstrating that there is a direct correlation among lattice parameter, specimen volume and dislocation density. As a result, various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.

Homogeneous cooling control technology for high-strength low-alloy hot-rolled strip

ABSTRACT Heterogeneities of transverse temperature during laminar cooling of high-strength low-alloy hot-rolled strip can cause heterogeneous distribution of microstructure, mechanical property and internal stress. Edge masking is a control technology that can be used to improve laminar cooling heterogeneities. In this paper, different edge masking schemes were set up based on the transverse transformation characteristics of the strip. The results show that a slow cooling of 7.1 s during transformation could reduce the heterogeneity of grain size, internal stress and microhardness at the edge and middle of the strip. In addition, the relationship model between grain size and six variables such as transformation start temperature was established by PCA. It provided a new idea for the construction of a microstructure prediction model under nonlinear cooling.

Reassessment of low-temperature Gibbs energies of BCC and FCC in steel for T0-temperature evaluation

In this work, we present an improved thermodynamic modeling of the mc_fe multicomponent database on the Fe-rich side at low temperatures. This is of special interest for the prediction of bainite and martensite start temperatures. The Fe–Ni, Fe–Mn, and Fe–Cr subsystems of the Fe–Al–C–Cr–Mn–Ni–Si steel system are reassessed with particular focus on the magnetic contributions to Gibbs energies. Moreover, the predicted C dependence of the T0 line in various extensions of the Fe–C system needed improvement, which is realized by the revision of the Fe–C base in terms of the solubility limit of C in ferrite (BCC). Ternary system descriptions are adapted to the changes made in the binaries.

Smart Stressing of Concrete Beams with Nitinol Shape Memory Alloy Cables

In this study, a novel method is presented to smartly prestress concrete beams using Nitinol Shape Memory Alloy (SMA) cables. Concrete beams having different compressive strengths and steel fiber contents prestressed with SMA cables are studied in comparison with concrete beams prestressed with steel and carbon fiber reinforced polymer (CFRP) cables. SMA cables with Nickel: Titanium ratio of 0.558:0.442 are embedded in concrete beams and prestrained at temperatures below their martensite start temperature, <img src=image/14829457_01.gif>=20℃±1℃. Smart prestressing is accomplished by heating the Nitinol SMA cables to temperatures above their austenite finish temperatures, <img src=image/14829457_02.gif>=28℃±1℃, and thus subjecting the concrete to compressive forces as the SMA cables attempt to regain their parent form. The study shows that SMA prestressed concrete beams have improved cracking behavior with fewer cracks, larger crack spacing, partial crack closure upon load removal, and an increase in cracking loads. The 35MPa beams prestressed with SMA had 24.2% and 6% higher cracking loads than beams prestressed with steel and CFRP cables, respectively. Meanwhile, SMA prestressed concrete beams had reduced failure loads varying from 4% to 34% in comparison with beams prestressed with steel and CFRP cables. The addition of steel fiber increased the magnitudes of the failure loads in SMA loaded beams. Further, SMA loaded beams possessed large load deflections accompanied with a unique shape retaining capability. This study adverts that SMA cables are attractive alternatives to steel and CFRP cables in post-tensioned beams and slabs, where the use of a complicated jacking system could be substituted by a self-prestressing cable.

Modelling of strain-induced martensite formation in advanced medium-Mn automotive sheet steel

The modelling of strain-induced martensite formation is simulated in advanced medium-Mn steel. The fraction of retained austenite (8%) embedded in the bainitic matrix is transforming into the strain-induced martensite during progressive static tensile tests. The originally elaborated technique and algorithms (using C++ language) are presented. The finite element method and LS-DYNA (LSTC Company, USA) have been deployed. The calculations of the stress-induced martensite start temperature were performed to characterize the austenite stability. The structural investigations using the SEM and EBSD have been conducted. The comparison of the experimental and numerical results has been made in terms of mechanical austenite stability.

A study of grain boundary effects on the stress-induced martensitic transformation and superelasticity in NiTi alloy via atomistic simulation

The stress-induced martensitic transformations and superelasticity behavior in the NiTi alloy with a single crystal model and a twist grain boundary bicrystal model at different temperatures are studied using molecular dynamics simulations. An atomic tracing method is proposed to identify specific numbers of B19′ martensite variants. Under uniaxial compressive loading, the stress-induced martensitic transformation takes place accompanied by the formation of &amp;lt;011&amp;gt;M type II twins, and the deformation process can be divided into three distinct stages based on microstructure evolution and average atomic total energy. It is found that the twist grain boundary induces an increase in the martensite start temperature, which is consistent with the experimental results. There is no residual B19′ martensite at the end of the unloading process, and the irrecoverable strain mainly results from plastic deformation at the grain boundary through the analysis of atomic local shear strains and has hardly changed with increasing deformation temperature. Remarkably, the grain boundary brings about the acceleration of martensite nucleation and an earlier occurrence of stress plateau. Further simulation results manifest that the presence of the twist grain boundary leads to weakened temperature dependence of martensitic transformation stress and a reduction in the hysteresis loop area.