Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Constitutive analysis of the hot deformation behavior of Fe–23Mn–2Al–0.2C twinning induced plasticity steel in consideration of strain

In this study, constitutive analysis has been carried out on Fe–23Mn–2Al–0.2C twinning induced plasticity (TWIP) steel. For this purpose, hot compression tests were conducted on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 900–1150 °C and the strain rate range of 0.001–20 s⁻¹. The effects of deformation heating and friction on flow stress were analyzed and corrected. On the basis of Sellars–Tegart–Garofalo equation, the strain-dependent constitutive equations of the steel were derived. The results show that deformation heating has a significant influence on the flow stress at lower temperatures and higher strain rates, while the frictional effect is slight even at the highest strain level investigated. Comparison of the calculated flow stress with the experimental data suggests that the developed constitutive equations can adequately describe the relationships between the flow stress, strain rate, temperature and strain of the steel during hot deformation. This is supported by a high correlation coefficient (R = 0.996) and a low average absolute relative error (AARE = 3.31%) for the entire deformation condition range investigated.     

Hot deformation behavior of spray-deposited Al–Zn–Mg–Cu alloy

The flow behavior of spray-deposited Al–10.21Zn–2.76Mg–1.45Cu–0.16Zr (wt.%) alloy has been systematically investigated by thermal compression tests with temperature and strain rate ranging from 613 K to 733 K and 0.001–1 s⁻¹, respectively. Microstructural observations revealed that the average grain size of spray-deposited alloy was below 25 μm due to the high cooling rate. Both relatively high temperature and low strain rate could promote the formation of dynamic recrystallization (DRX). The stress level of the alloy decreased with increasing deformation temperature and decreasing strain rate, which could be characterized by a Zener–Hollomon parameter in the hyperbolic-sine equation. Furthermore, the strain-dependent constitutive equation could lead to a good agreement between the calculated and measured flow stresses in the elevated temperature range for spray-deposited alloy. The deformation activation energy for spray-deposited alloy was relatively lower than that of the as-cast alloy owing to ultrafine grains and high supersaturated solid solubility.     

Influences of Nb-microalloying on microstructure and mechanical properties of Fe–25Mn–3Si–3Al TWIP steel

The Fe–25Mn–3Si–3Al TWIP steel was microalloyed by niobium in this paper, and the appropriate heat treatment and cold rolling processes were drafted in order to improve the poor yield strength of the steel. The results show that the yield strength of the steel increases from 320 MPa to 445 MPa, and the tensile strength increases from 680 MPa to 795 MPa, but the uniform elongation decreases from 65% to 55%. Nb addition can strongly hinder the growth of recrystallized grains, moreover Nb atoms react with C atoms to form nanoscale NbC precipitations, and these precipitations can block the dislocation motion, and then the yield strength and initial work hardening ability of Fe–25Mn–3Si–3Al steel is clearly improved. Furthermore, the strain-induced twinning is still a major deformation mechanism for the Nb-microalloying TWIP steel, and the twinning induced plasticity (TWIP) effect ensures a satisfactory ductility for the steel. Finally, the modified TWIP steel obtains a better match between the strength and plasticity by the joint action of precipitation strengthening and TWIP effect.     

Hot ductility of directly cast C–Mn–Nb–Al steel

Abstract

Tensile samples of a C–Mn–Nb–Al steel (BS 4360: 50D grade) have been cast in situ and either directly tested in the temperature range 850–1200°C, or were allowed to cool through the transformation, re–solution treated, and then tested in the same temperature range. The hot ductility of the directly tested cast material was found to be superior to that of the reheated material. Carbon extraction replicas taken close to the fracture surfaces showed large differences in the distribution of sulphide inclusions and NbCN precipitates along the γ boundaries. The directly cast material had sulphide inclusions and NbCN precipitates present in the form of coarse particles situated close to the interdendritic boundaries. A significant proportion of these coarse sulphide inclusions and NbCN eutectics, produced during solidification, redissolved on reheating at 1330°C, and subsequently precipitated in a much finer form at the γ grain boundaries, reducing hot ductility. It appears likely that the very marked segregation which occurred during solidification enhanced the interdendritic regions with sulphur to such an extent that the sulphideformed was (Mn, Fe)S, which in gradually changing to the equilibrium precipitate, depleted the surrounding matrix of manganese. The low manganese level accompanying these inclusions allowed a greater degree of solution of the sulphides to occur on reheating and accounted for the subsequent fine precipitation at the boundaries.

MST/361     

Strain rate-dependent hardening with dislocation-twin interaction of Fe–Mn–Al–C steel using crystal plasticity

A crystal plasticity finite element model with dislocation-twin interaction was developed to study the strain rate-dependent hardening of Fe–Mn–Al–C twinning-induced plasticity steel. Microstructural state variables including twinning space and dislocation density were incorporated to describe the mechanical twins hindering gliding dislocations. In situ scanning electron microscope tension and electron backscatter diffraction tests were conducted as validation and supplement. Predicted stress and strain hardening rate at various strain rates agree well with the experimental results. The increasing strain hardening stage is attributed to the dynamic competition between deformation twinning and dynamic recovery of dislocations. The intergranular deformation heterogeneity associated with the competitive activities of deformation mechanisms was also studied. The results indicate a larger contribution of slip to overall hardening than twinning.     

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α′ (bcc)- and ε (hcp)-martensite in the γ (fcc) matrix during plastic deformation was analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. Depending on the content of the alloying elements different phase transformations γ → ε, γ → α′ (TRIP effect), or the formation of deformation twins (TWIP effect) occurred. Additions of Al increased the stacking fault energy (γ_fcc) and suppressed the γ → ε transformation while Si decreased γ_fcc and sustained the γ → ε transformation. These steels with reduced densities of about 7.3 Mg m^?3 exhibit high tensile ductility up to 95% with true tensile strength of about 1100 MPa. The excellent plasticity induced by twinning or phase transformation up to extremely high strain rates of about <disp-formula><graphic href="splitsection2-m1.tif"/></disp-formula> results in an extraordinary shock resistance and allows for deep drawing and backward extrusion operations of parts with complex shapes.     

Quantification of deformation induced α’-martensite in Fe–19Cr–3Mn–4Ni–0.15C–0.15N austenitic steel by in situ magnetic measurements

An in situ magnetic device was employed to quantify the deformation induced martensite in a Fe–19Cr–3Mn–4Ni–0.15C–0.15N (wt-%) steel during tensile testing in the temperature range of ?40 to 22°C. The new device consists of an electromagnetic field which serves to magnetise the martensite phase as it forms during tensile loading and a second coil to detect the effective electrical potential difference induced by the magnetisation of tensile specimens. To implement the in situ measurement system, a correlation was necessary between the induced electrical potential difference and the deformation induced martensite fractions during uniaxial static tensile tests. The correlation procedure was found to require only the quantification of deformation induced martensite content in a tensile specimen strained until fracture using an ex situ magnetic saturation unit.     

Severe plastic deformation of an as-cast hypoeutectic Al–Si alloy

Different equal channel angular pressing (ECAP) processing routes have been employed to investigate the flow plane microstructures in a hypoeutectic Al–7wt%Si. In the as-cast condition, this alloy exhibits equiaxed primary aluminum dendrite cells embedded in an Al–Si eutectic constituent. The observed microstructures have been compared to the predicted distortion of a volume element expected during idealized ECAP. The effect of different processing routes on the microstructure refinement, degree of homogenization of second phase particles, and associated mechanical properties are discussed.     

Effect of Al content on stacking fault energy in austenitic Fe–Mn–Al–C alloys

Effect of Al content on the stacking fault energy (SFE) was investigated in the austenitic Fe–25Mn–(1.16–9.77)Al–0.68C (at%) alloys by X-ray diffraction line profile analysis and thermodynamic estimation, and was discussed on the basis of anomaly in shear modulus caused by the antiferromagnetic transition. The experimental results show that the stacking fault probability decreases with increasing Al content, the observed SFE increases linearly when Al content is lower than 6.27 at%, and markedly as it is more than 6.27 at%. The thermodynamic estimation indicates that the non-magnetic component of SFE increases faster than the observed one with increasing Al content in the antiferromagnetic state, and both are almost equal in the paramagnetic state. The magnetic order increases SFE in the antiferromagnetic state, and the magnetic component of SFE depends on the average magnetic moment and Néel temperature. The increases in the localized magnetic moment and the decrease in the Néel temperature are caused by the addition of Al atoms into the austenitic Fe–Mn alloys and are accompanied by the anomaly in shear modulus, which affects SFE in the antiferromagnetic state. The anomalous drop in shear modulus leads to the inconsistency for the variations of the observed SFE and non-magnetic component with Al content in the antiferromagnetic state.     

An investigation to the microstructural evolution of Fe–29Mn–5Al dual-phase twinning-induced plasticity steel through annealing

The potential effects of twinning induced plasticity have been taken into consideration to further improve the mechanical properties of advanced high-strength steels. Accordingly the high-Mn twinning-induced plasticity (TWIP) steels with austenite–ferrite dual-phase microstructure have been developed. In the present study, the influence of cold rolling and post-annealing treatments on the microstructural evolution and mechanical behavior of a group of dual-phase TWIP steel as a function of annealing time have been investigated. The mechanical behavior of processed materials has been examined through applying a set of low strain rate (0.001 s⁻¹) compression tests at room temperature. The austenite recrystallization characteristics during various annealing conditions are explained through proper microstructural examinations. The results indicate that as the recrystallization proceeds with annealing time the related yield stress deceases. The rapid drop of yield stress in short annealing periods is related to the onset of recrystallization. The yield stress variation diminishes as the annealing duration increases. This is attributed to the formation of austenite side-plates which may balance the softening effects of restoration processes.     

In-situ EBSD study of deformation behavior of Al–Si–Cu alloys during tensile testing

This study deals with the microstructural aspects of the deformation behavior in Al–Si–Cu alloy A380. This has been carried out with in-situ tensile testing coupled with EBSD analysis. The alloy specimens having different microstructures with two different secondary dendrite arm spacing (SDAS) of 9 μm and 27 μm were produced by the unique gradient solidification method. The study of misorientation distribution and texture evolution was performed with different tools in EBSD analysis. The texture was not significantly affected by deformation in both types of alloy specimens. With increase in the deformation, the microstructures are characterized by degradation of EBSD patterns and generation of substructures including low angle boundaries (LABs) and high angle boundaries (HABs). In both the microstructures with low and high SDAS, the boundaries were concentrated around eutectic phases; however this behavior was more pronounced at higher SDAS. The increase in the fraction of LABs with deformation was much higher in the microstructure with higher SDAS than with lower SDAS. This localized strain concentration was especially attributed to the large and elongated eutectic Si particles and Fe-rich intermetallics. The lower mechanical properties obtained at higher SDAS are the result of inhomogeneous strain distribution in the microstructure.     

Thermo-mechanical behavior of the Al–Si alloy coated hot stamping boron steel

Hot tensile tests of boron steels with and without an Al–Si coating were performed using a Gleeble 3500 test system, at temperatures of 700–850 °C and strain rates of 0.01–1/s. The phase and microstructure of the coating in as-coated and press-hardened conditions were observed under scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis and X-ray diffraction (XRD). Experimental results indicate that the Al–Si coating gave an unignorable influence on the thermo-mechanical properties of the boron steels. The ultimate tensile strength (UTS) of the Al–Si coated boron steel was almost equal to that of the uncoated under the lower strain rate at the same deformation temperature. At a higher strain rate, the UTS value appeared to be lower than that of the uncoated. Moreover, the UTS difference increased with the decreasing deformation temperature. The ductility of the Al–Si coated steel was lower than that of the uncoated under the described test conditions. Following the tensile tests, extensive cracks were visible in the Al–Si coating layer. SEM observation showed that microcracks and voids appeared after austenization, which may act as nucleation sites for the cracks. The cracks first propagated in the direction perpendicular to the coating/substrate interface and were identified as Type I cracks. The propagation was hindered by the substrate when these cracks reached the coating/substrate interface. This occurred because the interfacial bonding strength between the coating and the substrate was lower than the substrate strength. Following this initial failure, the cracks turned to propagate paralleled to the coating/substrate interface. In addition with the shear stress resulting from the substrate yielding, Type II cracks formed. Eventually, the cracked coatings were accompanied by interface decohesion from the substrate. The width and density of the cracks were found to increase with the decreasing deformation temperature and rising stain rate.     

Microstructure characterisation of Fe–21Cr–15Ni–Nb–V non-magnetic austenitic stainless steel during hot deformation

Hot deformation behaviour of Fe–21Cr–15Ni–Nb–V stainless steel was investigated by isothermal compression in the temperature range of 950–1150°C with a strain rate of 0.01–10?s^?1. The results showed that complete recrystallisation occurred beyond 1050°C, resulting from the pinning effect of (Nb, V)(C, N). The nucleation of dynamic recrystallisation (DRX) was performed by the bulging, sub-grain swallowing and twinning mechanism. With increasing strain rate, new twinning was transformed into the Σ3 regeneration mechanism in the partial DRX region, while an opposite transformation was observed in the complete DRX region. In the partial recrystallisation region, grain rotation resulted in the formation of 110 orientation. In the complete recrystallisation region, the texture tended to distribute randomly at a high strain rate, and the grain growth was accompanied by the emergence of stable 100 orientation.     

Hot tensile deformation behaviors and constitutive model of an Al–Zn–Mg–Cu alloy

The hot tensile deformation behaviors of an Al–Zn–Mg–Cu alloy are studied by uniaxial tensile tests under the deformation temperature of 340–460 °C and strain rate of 0.01–0.001 s⁻¹. The effects of deformation temperature and strain rate on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The Arrhenius-type constitutive model is developed to predict the peak stress under the tested deformation condition. The results show that: (1) The true stress–true strain curves under all the tested deformation conditions are composed of four distinct stages, i.e., elastic stage, uniform deformation stage, diffusion necking stage and localized necking stage. The flow stress decreases with the increase of deformation temperature or the decrease of strain rate. (2) The elongation to fracture increases with the increase of deformation temperature. Under the tested conditions, the strain rate sensitivity coefficient varies between 0.1248 and 0.2059, which indicates that the main deformation mechanism is the lattice diffusion-controlled dislocation climb. (3) The localized necking causes the final fracture of specimens under all the deformation conditions. Microvoids coalescence is the main fracture mechanism under relatively low deformation temperatures. With the increase of deformation temperature, the intergranular fracture occurs. (4) The peak stresses predicted by the developed model well agree with the experimental results, which indicate the validity of the developed model.     

Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ～790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ～700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ～225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ～0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.     

Effect of deformation of austenite and cooling rates on transformation microstructures in a Mn–Cr gear steel

The effect of austenite deformation and cooling rates on continuous cooling transformation microstructures for a Mn–Cr gear steel were investigated using a Gleeble 1500 thermomechanical test system. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, leading to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The deformation enhances the stability of austenite against bainite transformation, which results in an increase in amount of martensite/austenite (M/A) constituent with deformation at some cooling rates studied. Moreover, cooling rate also affects amount of M/A constituent. With decrease of cooling rate, amount of M/A constituent increases at first, but decreases subsequently till disappears eventually.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.