Role of mechanical activation in the dynamic transformation of austenite

When austenite is deformed above the equilibrium transformation temperature Ae₃, it is dynamically transformed into Widmanstätten ferrite by a displacive mechanism. On removal of the load it is slowly retransformed into austenite by diffusional processes. The forward transformation has recently been explained in terms of a thermodynamic model in which the lower free energy of austenite is raised above that of normally unstable ferrite as a result of the additional stored energy associated with the dislocations introduced by straining. This model is here shown to be unable to account for the initiation of transformation at critical strains of about 0.1, at which only low densities of dislocations are present. Of particular importance is the observation that dynamic transformation can be initiated at temperatures 100 °C and more above the Ae₃ and that the critical strain actually decreases with increasing temperature and increasing chemical free energy barrier. This discrepancy is removed by allowing for mechanical (stress-based) activation of the transformation. The latter provides the energy required to accommodate the shear of the parent austenite into Widmanstätten plates, as well as the volume change or dilatation accompanying ferrite formation. The work of dilatation and the shear accommodation work, omitted from the previous analysis, are introduced here as barriers to the transformation that are overcome by the applied stress. This modified approach is able to account for the very rapid forward (mechanically activated) transformation compared with the much slower reverse transformation that takes place in the absence of stress.     

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was 44.3° 〈1 1 6〉. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.     

Experimental evidence and thermodynamics analysis of high magnetic field effects on the austenite to ferrite transformation temperature in Fe–C–Mn alloys

The non-isothermal decomposition of austenite into ferrite and pearlite in Fe–xC–1.5 wt.% Mn steels with x = 0.1, 0.2 and 0.3 wt.% C is investigated by in situ dilatometry and microstructure characterization in magnetic fields up to 16 T. The global shift towards higher temperatures of the respective austenite, ferrite + austenite and ferrite + pearlite stability regions is experimentally quantified. A systematic increase in the ferrite area fraction and proportional reduction of the Vickers hardness values with the magnetic field intensity are also reported. Moreover, the steels’ magnetizations, measured up to 3.5 T and 1100 K, are used to calculate the magnetic contribution to the free energy of the transformation and to account thermodynamically for the field dependence of the transformation temperature. The impact of magnetic field is found to be greater with increasing carbon content in the steels.     

Microstructure,creep, and tensile behavior of a Ti–21Al–29Nb(at.%) orthorhombic+B2 alloy

The creep and tensile deformation behavior of a Ti–21Al–29Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques. Heat treatments at all temperatures above 1050 °C, followed by water quenching, resulted in fully-B2 microstructures. Below 1050 °C, either equiaxed or Widmanstätten O-phase precipitated within the B2 grains. RT elongation-to-failure values of less than 2% were recorded for aged microstructures containing 72–78 volume percent O phase. Tensile-creep experiments were conducted in the temperature range 650–710 °C and stress range 48–250 MPa. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress and microstructure. Microstructural effects on the tensile properties and creep behavior are discussed and the data was compared to that for other Ti₂AlNb-based alloys.     

A novel thermomechanical approach to produce a fine ferrite and low-temperature bainitic composite microstructure

A novel thermomechanical processing was developed in the present study to produce a unique microstructure consisting of fine ferrite grains (i.e. ～4 μm on average) and low-temperature bainite in a relatively low-carbon steel with a modest hardenability. The thermomechanical route consisted of warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to the bainitic transformation regime (i.e. 400–200 °C). The low-temperature bainite consisted of high dislocation density bainitic laths and very fine retained austenite films. This microstructure offered a high work hardening rate leading to a unique combination of ultimate tensile strength and elongation. This was due to the presence of ductile fine ferrite grains and hard low-temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy, electron backscatter diffraction and atom probe tomography techniques.     

Time-dependent synchrotron X-ray diffraction on the austenite decomposition kinetics in SAE 52100 bearing steel at elevated temperatures under tensile stress

We have studied the decomposition kinetics of the metastable austenite phase present in quenched-and-tempered SAE 52100 steel by in situ high-energy synchrotron X-ray diffraction experiments at elevated temperatures of 200–235 °C under a constant tensile stress. We have observed a continuous decomposition of austenite into ferrite and cementite. The decomposition kinetics is controlled by the long-range diffusion of carbon atoms into the austenite ahead of the moving austenite/ferrite interface. The presence of a tensile stress of 295 MPa favours the carbon diffusion in the remaining austenite, so that the activation energy for the overall process decreases from 138–148 to 82–104 kJ mol^?1. Before the austenite starts to decompose, a significant amount of carbon atoms partition from the surrounding martensite phase into the metastable austenite grains. This carbon partitioning takes place simultaneously with the carbide precipitation due to the over-tempering of the martensite phase. As the austenite decomposition proceeds gradually at a constant temperature and stress, the elastic strain in the remaining austenite grains continuously decreases. Consequently, the remaining austenite grains act as a reinforcement of the ferritic matrix at longer isothermal holding times. The texture evolution in the constituent phases reflects both significant grain rotations and crystal orientation relationships between the parent austenite phase and the newly formed ferritic grains.     

Influence of transformation temperature on microtexture formation associated with α precipitation at β grain boundaries in a β metastable titanium alloy

The influence of transformation temperature on microtexture development associated with α precipitation at β/β grain boundaries (GB) in the near-β Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior β grain because of a larger number of α_WGB colonies (standing for α Widmanstätten GB) formed per β grain, each colony increasing by one the number of α orientations inside each prior β grain. This larger number of α_WGB colonies was a consequence of faster formation along β/β GB of their precursors, the allotriomorphic α_GB grains (standing for α-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of α_GB formation. α_GB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)β//(0 0 0 1)α_GB//(0 1 1)β. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final α microtexture, considered at the scale of one prior β grain. α_GB grains that followed the VS criterion emitted two α_WGB colonies on either side of the β/β GB more frequently than those with no particular orientation.     

Corrosion resistance of duplex stainless steel subjected to long-term annealing in the spinodal decomposition temperature range

The effect of thermal annealing up to 15,000 h between 300 °C and 500 °C on the corrosion resistance of the duplex stainless steel (DSS) 7MoPLUS has been investigated by using the DLEPR test. Spinodal decomposition in 7MoPLUS is unabated even after annealing for 15,000 h and no healing has been observed. The possible healing mechanisms in this temperature range (back diffusion of Cr atoms from the Cr-rich ferrite (α_Cr) and diffusion of Cr atoms from the austenite) and its absence in the present steel have been discussed.     

Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels

Complex multiphase microstructures were obtained in transformation induced plasticity C–Mn–Si–(Nb–Al–Mo) steels by simulated controlled thermomechanical processing. These microstructures were characterized using transmission electron microscopy, X-ray diffraction and three-dimensional atom probe tomography (APT), which was used to determine the partitioning of elements between different phases and microconstituents. The measured carbon concentration (∼0.25 at%) in the ferrite of carbide-free bainite was higher than expected from para-equilibrium between the austenite and ferrite, while the concentrations of substitutional elements were the same as in the parent austenite suggesting that incomplete bainite transformation occurred. In contrast, the distribution of substitutional elements between the ferrite lath and austenite in carbide-containing bainite indicated a complete bainite reaction. The average carbon content in the retained austenite (3.2 ± 1.6 at%) was somewhat higher than the T₀ limit. On the basis of the APT measured composition, the calculated M_s temperatures for retained austenite were above room temperature, indicating its low chemical stability.     

Effect of annealing temperature on microstructural modification and tensile properties in 0.35 C–3.5 Mn–5.8 Al lightweight steel

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α + γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795 °C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980 °C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830 °C or 880 °C annealed steels.     

Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

Crystallography of Widmanstätten austenite in duplex stainless steel weld metal

Abstract

Two kinds of austenite grow from δ-ferrite during the cooling of the duplex stainless steel weld deposits studied here, Widmanstätten plates and allotriomorphs which precipitate at δ–δ grain boundaries. It is found using microtexture measurements that the preferred crystallographic orientation of the Widmanstätten austenite can be estimated using established theory if it is assumed that there is an interaction between external stress and the growing plates. It is also demonstrated that the Widmanstätten and allotriomorphic forms of austenite may not be identically oriented even when the former appears to originate from the latter; this may be a consequence of differences in the transformation mechanisms of these two forms of austenite.     

Memory effects of transformation textures in steel and its prediction by the double Kurdjumov–Sachs relation

The phenomenon that the transformation texture near the initial texture reproduces after the phase transformation cycle such as ferrite (α, body-centered cubic) → austenite (γ, face-centered cubic) → α is called a texture memory. In this study, the texture change in a 0.1% C–1% Mn hot-rolled steel sheet during the α → γ → α transformation cycle was studied via neutron diffraction and the transformation texture prediction based on a variant selection rule that we call the double Kurdjumov–Sachs (K–S) relation. The texture change observed by neutron diffraction, which clearly showed the texture memory, could be quantitatively reproduced by the proposed variant selection rule adopted into the calculation method based on the spherical harmonics expansion of orientation distribution functions. Therefore, it is most likely that the texture memory in steel is caused by the preferential selection of those K–S variants that reduce the interfacial energy between a precipitate and two adjoining parent phase grains at the same time, which we call the double K–S relation.     

Influence of long-term low-temperature aging on the microhardness and corrosion properties of duplex stainless steel

An investigation was made of the influence of long-term aging (up to 7000 h) at low temperatures (300 and 400 °C) on the corrosion and mechanical properties of a 2205 duplex stainless steel. The selective corrosion behavior of austenite and ferrite phases was found to reverse in response to aging treatment at 400 °C. The degree of Cr depletion (I_r/I_a) increased considerably with the increase in aging time at 400 °C and no self-healing process was observed even after aging for 7000 h. A good correlation was observed between the electrochemical results and the microhardness of ferrite phase.     

Kinetics of austenitization under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Differential dilatometry has been employed to study the kinetics of the massive ferrite (α) → austenite (γ) transformation upon isochronal heating (i.e. austenitization) of the substitutional Fe–2.96 at.% Ni alloy subjected to a range of applied constant uniaxial compressive stresses. A phase-transformation model, involving site saturation, interface-controlled (continuous) growth and incorporating an impingement correction for an intermediate of the cases of ideally periodically and of ideally randomly dispersed growing particles, has been employed to extract the interface-migration velocity of the α/γ interface and the transformation-induced deformation energy taken up by the specimen. The value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the α/γ volume misfit depends on the austenite fraction and increases distinctly with an increase in the applied uniaxial compressive stress, which is compensated by, in particular, an increase in the chemical driving force corresponding to an increase in the onset temperature. The opposite effects of an applied uniaxial compressive stress on the α → γ transformation and on the γ → α transformation can be discussed as the outcome of constrained plastic deformation due to transformation-induced strain.     

Carbon supersaturation of ferrite in a nanocrystalline bainitic steel

The extremely slow transformation kinetics of a nanocrystalline bainitic steel allows the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography as the bainite transformation progresses at 200 °C. A high level of carbon, well above that expected from para-equilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.     

The effect of boron and alpha grain size on the massive transformation in Ti–46Al–8Nb–xB alloys

Samples of Ti–46Al–8Nb containing up to 1 at.% B have been examined using optical microscopy after cooling over a wide range of cooling rates from the α phase field in order to understand the influence of boron and grain size on the massive transformation. The grain size of the samples was controlled either by varying the boron level or by appropriate processing of B-free and B-containing alloys. The results show that the addition of boron suppresses the feathery and the Widmanstätten transformation. The massive transformation and the lamellar transformation are strongly influenced by prior α grain size independent of whether the grain size was achieved by heat treatment or by addition of boron. In fine-grained samples the range of cooling rates over which the massive transformation occurs is restricted by formation of the lamellar microstructure at high cooling rates. These observations are discussed in terms of the factors controlling the nucleation and the progression of these transformations.     

Phase compositions,nanoscale microstructures and thermoelectric properties in Ag2−ySbyTe1+y alloys with precipitated Sb2Te3 plates

Nonstoichiometric ternary thermoelectric materials Ag_2?ySb_yTe_1+y (y = 1.26, 1.29, 1.32, 1.35, and 1.38) were prepared by a direct melt-quench and hot press process. In situ composites of AgSbTe₂ and Sb₂Te₃ were obtained over the entire composition range with a typical Widmanstätten pattern. Thermoelectric properties were measured from 300 K to 673 K, which changed systematically with Sb₂Te₃ ratio. The phase transition occurring at about 633 K, forming the single phased AgSbTe₂, can significantly influence the electrical transport properties. Various crystallographic defects in different scales, such as atomic ordering, nanodomains, dislocations and stacking faults, have been observed by high-resolution transmission electron microscopy and their influences on lattice thermal conductivity have been discussed. Due to the extremely low thermal conductivity (about 0.6 W m^?1 K^?1) and large positive Seebeck coefficient of ～250 μV K^?1 detected in Ag_0.71Sb_1.29Te_2.29, the maximum dimensionless figure of merit ZT of 1.37 was obtained at 600 K.     

Orientation relationships of intragranular austenite in duplex stainless steel weld metals

Formation and characteristics of fine intragranular austenite were studied for low energy input duplex stainless steel welds. Microstructures were largely ferritic with some allotriomorphic grain boundary austenite, Widmanstätten type austenite, fine intragranular austenite and nitrides. Electron backscattered diffraction analysis revealed that grain boundary austenite had a random orientation relationship (OR) with one of the adjacent ferrite grains and was close to Kurdjumov–Sachs (KS) with the other, whereas Widmanstätten austenite always showed an OR near KS. The finest intragranular austenite was mainly randomly oriented, whereas coarser austenite more often was close to KS. It is argued that the OR of intragranular austenite with the ferritic matrix is governed by a combination of composition, determining driving force for nucleation at temperature, cooling rate and the availability of nitrides acting as nucleation sites. A random OR is most likely for higher cooling rates and compositions promoting nucleation at lower temperatures.