Forming response of retained austenite in a C‐Si‐Mn high strength TRIP sheet steel

TRIP sheet steels typically consist of ferrite, bainite, retained austenite, and martensite. The retained austenite is of particular importance because its deformation‐induced transformation to martensite contributes to excellent combinations of strength and ductility. While information is available regarding austenite response in uniaxial tension, less information is available for TRIP steels with respect to the forming response of retained austenite in complex strain states. Therefore, the purpose of this work was to study the austenite transformation behaviour in different strain paths by determining the amount of retained austenite before and after forming. Forming experiments were performed on a high strength 0.19C‐1.63Si‐1.59Mn TRIP sheet steel 1.2 mm in thickness in two different strain conditions, uniaxial tension (ε₁ = ‐2ε₂) and balanced biaxial stretching (ε₁ = ε₂). Specimens were formed to strains ranging from zero to approximately 0.2 effective (von Mises) strain. Specimens were tested both longitudinally and transverse to the rolling direction in uniaxial tension, and subtle mechanical property differences were found. The volume fraction of austenite, determined with X‐ray diffraction subsequent to forming, was found to decrease with increasing strain for both forming modes. Some modification in the crystallographic texture of the ferrite was observed with increasing strain, in specimens tested in the balanced biaxial stretch condition. This trend was not evident in the uniaxial tensile test results. Slight differences were found in the transformation behaviour of the austenite when formed in different strain conditions. More austenite transformed in specimens tested parallel to the rolling direction than transverse to the rolling direction in uniaxial tension. The amount of austenite transformed during biaxial stretching was determined to be greater than the amount transformed in uniaxial tension for specimens tested transverse to the rolling direction at an equivalent von Mises strain. The amount of austenite that transformed in biaxial tension, however, was comparable to the amount of austenite that transformed in specimens tested longitudinal to the rolling direction in uniaxial tension.     

Deformation microstructures in titanium sheet metal

A metallographic study has been made of the microstructures produced by room temperature deformation of 0.6mm thick commercially pure titanium sheet metal in uniaxial, plane strain and biaxial tension. Deformation twinning becomes increasingly important as the deformation mode changes from uniaxial through plane strain to equibiaxial tension, and is more significant for strain transverse to the rolling direction than for strain in the longitudinal direction. In uniaxial tension, 1122 twins are dominant in longitudinal straining, while 1012 twins dominate in transverse straining. In plane strain and equibiaxial straining, 1012 twinning is suppressed and largely replaced by 1122 twinning. The observed changes in twin occurrence and type are attributed to the interaction of the imposed stress system and the crystallographic texture of the rolled sheet, which alters the distribution of the grain basal-plane poles with respect to the operative stress axes. In uniaxial tension parallel to the longitudinal direction, twins favored by ‘c’ axis compression are produced, while in the transverse direction twins favored by ‘c’ axis tension appear. In plane strain and biaxial tension the dominant stress is through-thickness compression, which produces twins favored by ‘c’ axis compression in nearly all cases. The alterations in twin orientation and numbers are associated with changes in stress-strain behavior. As twin volume fraction increases and twins are aligned more closely to the principal stress axis, the instantaneous work-hardening rate tends to stabilize at a nearly constant value over a large strain range. Formerly Chief Metallurgist, The APV Company.     

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10^-3 per second) and high (10³ per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.     

拉伸应变下中碳贝氏体钢中残留奥氏体转变及其稳定性

摘要：采用分阶段拉伸应变试验,研究中碳贝氏体钢在拉伸过程中TRIP效应的影响,探究残留奥氏体转变量与应变量之间的关系。试验结果表明,拉伸变形初期残留奥氏体转变较快（0%到3.0%应变量(体积分数)对应残留奥氏体转变量为13.72%）,主要为分布在贝氏体束与束之间的块状残留奥氏体（尺寸为微米级）发生相变;拉伸变形后期残留奥氏体转变较慢（10.0%到18.5%应变量对应残留奥氏体转变量为7.78%）,主要为分布在贝氏体铁素体板条与板条之间的薄膜状残留奥氏体（尺寸为纳米级或者亚微米级）发生相变。归因为块状残留奥氏体稳定性低,在外部应力作用下容易转变为硬而脆的马氏体组织,而薄膜状/条状残留奥氏体稳定性高,在外部应力的作用下不易发生相变,TRIP效应的发生使得钢种强度和塑性同时得到提高。通过3种模型（Hollomon模型,C-J模型,修正C-J模型）描述试验钢的加工硬化行为,将整个拉伸过程可分为3个阶段,并结合微观组织变化,详细解析了每个阶段所对应的主要加工硬化机制。     

不同应变方式下TRIP钢中残余奥氏体的体积分数随应变量的变化

对TRIP钢板在单向拉伸、双向拉伸和平面应变3种应变方式下残余奥氏体的体积分数随应变量变化的规律进行了实验研究。结果表明：在变形过程中，残余奥氏体的体积分数随应变量的增加而减小；该变化量不仅与应变量有关，还与应变方式有关；平面应变时变化最大，双向拉伸次之，单向托伸下变化最小。还对同一零件不同区域残余奥氏体的含量对零件形状精度的影响进行了讨论。     

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Forming limit diagram (FLD) of cold- rolled TRIP steel was established by experiments. The microstructures of samples before and after deformation were examined by metalloscopy and scanning electron microscopy and at the same time the contents of retained austenite after different strain ratios were measured by X- ray diffraction. The results show that the ultimate strain under plane strain state(FLD0) is 0. 397. With the strain ratio increasing, strain path changes from uniaxial stretching to plane strain and then biaxial stretching and the transformation amount of residual austenite increases gradually. Compared with dual- phase steel, the higher FLD0of TRIP steel is ascribed to TRIP effect and necking area is wider during deformation.     

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part II. Microstructural Study

The γ→α transformation in 304 stainless steel was induced by plastic deformation under various conditions of strain, strain state, and strain rate, and the transformation microstructures were examined by transmission electron microscopy (TEM). The nucleation of α martensite embryos was always confined to microscopic shear band (faults, twins, and ε-martensite) intersections. In cases where shear bands consisted of bundles of intermixed faults, twins, and ε-martensite, α nucleated preferentially only within specific portions of the intersection volume. At sufficiently large strains α appeared to grow into polyhedral shapes. We postulate that growth occurs by repeated nucleation of new α embryos and coalescence of such embryos into polyhedral shapes. These shapes can grow either within an active slip plane or out of it, depending on how many shear band intersections are produced during deformation. Actual measurements of the number of intersections indicated that more intersections are formed in biaxial tension per unit effective strain than in uniaxial tension. This accounts for the more irregular, blocky α morphology observed in biaxial tension. At high strain rates we also found an increase in the number of intersections. However, adiabatic heating at large strains and high rates restricts repeated nucleation and coalescence and limits the amount of α transformation product.     

On the influence of interactions between phases on the mechanical stability of retained austenite in transformation-induced plasticity multiphase steels

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M _s temperature induced by carbon enrichment.     

Reverse transformation of ferrite and pearlite to austenite in an ultrafine-grained low-carbon steel fabricated by severe plastic deformation

Reverse transformation characteristics of a low-carbon steel consisting of ultrafine-grained (UFG) ferrite and severely deformed pearlite by severe plastic deformation were investigated and compared to those of the steel having coarse-grained (CG) ferrite and undeformed pearlite by austenitization and subsequent air cooling. Coarse-grained steel exhibited two serial transformation stages, i.e., pear-lite → austenite followed by ferrite → austenite. Contrarily, UFG steel transformed with the three serial stages, i.e., probably carbon-supersaturated ferrite → austenite, not-fully-dissolved pearlite → austenite, and ferrite → austenite transformations.     

Resolving the Martensitic Transformation in Q&P Steels In-Situ at Dynamic Strain Rates Using Synchrotron X-ray Diffraction

The dynamic deformation response of two quenching and partitioning (Q&P) steels was investigated using a high strain rate tension pressure bar and in-situ synchrotron radiography and diffraction. This allowed for concurrent measurements of the martensitic transformation, the elastic strains/stresses on the martensite and ferrite, and the bulk mechanical behavior. The steel with the greater fraction of ferrite exhibited greater ductility and lower strength, suggesting that dislocation slip in ferrite enhanced the deformability. Meanwhile, the kinetics of the martensitic transformation appeared similar for both steels, although the steel with a greater ferrite fraction retained more austenite in the neck after fracture.

     

Transformation superplasticity of iron and Fe/TiC metal matrix composites

Unreinforced iron was thermally cycled around the α/γ phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the α/γ phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.     

A kinetic model of the γ → α + Gr eutectoid transformation in spheroidal graphite cast irons

The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper, the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite, has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing number density of graphite spheroids. Predictions of the model are compared with data available in literature.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900°C and 1060°C and strain rates between 10⁻⁴ and 10⁻² s⁻². Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperatur and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy), were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of Mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallo graphic texture components determined for the B2 phase ((001) [100] and (−112) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

Biaxial deformation of 70-30 brass: Flow behaviors,texture, microstructures

70-30 brass tubes have been tested in combinations of tension/internal pressure and in pure torsion. The flow properties, from these loading conditions, were measured from yield until local necking. We found that the pure hoop tension, plane strain, and torsion flow curves were 15 pct lower than those in uniaxial tension and balanced biaxial tension when compared on the basis of a von Mises effective stress-strain criterion. Microstructures, at a von Mises strain of 0.4, were examined; no differences were observed between plane strain, torsion, and uniaxial tension deformation modes. Based on the microstructural measurements, we estimate that at most 13 pct of the deformation at an effective strain of 0.40 was by twinning. The initial texture and final textures, after balanced biaxial and uniaxial tension, were measured by pole figure analysis. The initial texture was qualitatively consistent with measured flow stress levels of the axisymmetric deformation modes (uniaxial tension, balanced biaxial tension, and hoop tension). A crystallographic effective stress-strain criterion was also applied to the torsion data. This method of analysis gave results which were better than the von Mises criterion.     

Coherency stresses in lamellar Ti-Al

General formulas are given for the coherency strains and stresses in a multilayer far from the free surfaces. The multilayer is assumed to be a periodic stack of different elastically isotropic materials, but there may be any number of layers in the stack and they may each have any thickness and any elastic constants. The results are applied to lamellar Ti-Al alloys, in which there are shear misfits between different γ layers and both shear and biaxial misfits between the γ and α ₂ layers. In a fully coherent multilayer, the stresses would be large, in the GPa range, and in high strength, thin lamella alloys, the coherency stresses are a substantial fraction of a GPa. The shear stresses act principally on hard mode deformation systems, and the biaxial stresses place every α ₂ lamella in biaxial compression. This biaxial compression, which, for dislocation glide, is equivalent to a uniaxial tension normal to the lamella, is particularly large when the α ₂ volume fraction is small.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900 °C and 1060 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and ( 12) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.