Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Effect of strain rate on spatio-temporal behavior of Portevin–Le Châtelier bands in a twinning induced plasticity steel

Uniaxial tensile tests at three strain rates are performed with the aid of the digital image correlation (DIC) technique to experimentally investigate the spatio-temporal behavior of PLC bands in a twinning induced plasticity (TWIP) steel. The whole strain fields of tensile specimens are acquired throughout the tests. Significant serration crests corresponding to band nucleation are observed on the true stress vs. true strain curves derived from DIC results beyond a critical true strain. The work hardening exponent (n-value) increases from ∼0.08 to ∼0.5 when true strain increases to the critical true strain, and beyond that, the n-value exhibits serrations with increasing true strain. Two typical nucleation modes of Type-A Portevin–Le Châtelier (PLC) bands are observed in all tests. Nucleation and propagation of PLC bands are described in details based on these two nucleation modes of Type-A PLC bands. The PLC band orientation, which indicates the angle between the normal direction of a PLC band and tensile direction, fluctuates during propagation, and the fluctuation amplitude increases during the development of a localized necking band from a PLC band before fracture. In particular, the effect of strain rate on the kinematics of Type-A PLC bands (band strain, band width and band propagating speed etc.) in the TWIP steel is quantitatively analyzed, and a new algorithm based on the DIC results is presented which includes the elongating effect of tensile specimens during deformation to show the actual kinematics of Type-A PLC bands.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

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.     

Constitutive behaviour in as quenched Al–5Cu–0·4Mn alloy during hot deformation

The overburning temperature of the ZL205A (Al–5Cu–0·4Mn) alloy is first determined by differential scanning calorimetry analysis. Then, the solid solution temperature of ZL205A was determined by metallurgical microstructure observation. Isothermal compression tests of the as quenched ZL205A were conducted in temperature from 25 to 500°C and the strain rate from 0·001 to 1 s^?1. The deformation behaviour of the as quenched ZL205A was investigated. The prediction of the flow stresses were studied using artificial neural network. The average absolute relative error between the predicted flow stresses and experimental results is 4·4%, which demonstrates that the network proposed in the present paper has high precision. Therefore, it can be chosen as a thermomechanical model to treat the distortion problems of components during quenching process.     

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.     

Constitutive analysis of compressive deformation behavior of ELI-grade Ti–6Al–4V with different microstructures

In this study, a constitutive analysis of the flow responses of Ti–6Al–4V under various strain rates [(e)\dot] \dot{\varepsilon } was conducted by separately quantifying the hardening and softening effects of microstructure, interstitial solute and deformation heating on the total stress. For this purpose, a series of compression tests on an extra-low interstitial grade alloy with equiaxed, lamellar, or bimodal microstructures was performed at 10 ^{- 3} ￡ [(e)\dot] ￡ 10  \texts ^{- 1} 10^{ - 3} \le \dot{\varepsilon } \le 10\;{\text{s}}^{ - 1} until the metal fractured, and the results were compared to those of the commercial grade alloy. The thermal stress σ^* increased with an increasing interstitial solute concentration; the athermal stress increased in the order of equiaxed, lamellar, and bimodal microstructures. Load–unload–reload tests revealed that the flow softening at a relatively high [(e)\dot] \dot{\varepsilon } was likely caused by deformation heating rather than by microstructure change; thus flow softening was attributed to a decrease in σ^*. Finally, a mechanical threshold stress model was extended to capture those observations; the modified model can provide a reasonable prediction of flow stress in Ti–6Al–4V with different microstructures and interstitial solute concentrations.     

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.     

Constitutive modeling of cyclic plasticity deformation and low–high-cycle fatigue of stainless steel 304 in uniaxial stress state

For constructing a theory that adequately describes the effects of cycling loading, we initially analyze an experimental plastic hysteresis loop of the stainless steel SS304 and allocate on it three backstress types responsible for yield surface center displacement. Evolutionary equations per each backstresses type are formulated based on the equation of plasticity flow theory at combined (isotropic-kinematic) hardening. Material functions (parameters) closing the theory are defined, and basic experiment and identification methods of material function are formulated. Comparison of design results and experiments testifies their reliable compatibility.     

X-ray diffraction and TEM analysis of Fe–Al alloy layer in coating of new hot dip aluminised steel

Abstract

The phase structure in the Fe-Al alloy layer of a new hot dip aluminised steel has been researched by means of electron probe microanalysis, X-ray diffraction and TEM, etc. The test results indicated that the Fe-Al alloy layer of the new aluminised steel was composed of Fe₃Al, FeAl, a few Fe₂Al₅ and α-Fe (Al) solid solution. There was no brittle phase containing higher aluminium content, such as FeAl₃ (59.18 wt-%Al) and Fe₂Al₇ (62.93 wt-%Al). The tiny cracks and brittlement, formerly caused by these brittle phases in the conventional aluminium coated steel, were effectively eliminated. There was no microscopic defect (such as tiny cracks, pores or loose) in the coating. This is favourable for resisting high temperature oxidation and corrosion of the aluminised steel.     

Small-angle neutron scattering analysis of Mn–C clusters in high-manganese 18Mn–0.6C steel

Nanometer-scale particles (Mn–C clusters) were analyzed quantitatively using small-angle neutron scattering in 18Mn–0.6C (wt.%) austenite high-manganese steel. The size, number, and volume fraction of the particles were determined as a function of strain (0, 5, 15, 30, 45, 50%) at different temperatures (25 and 100 °C). The diameter of the cluster ranges from 2 to 14 nm in the matrix. The total volume fraction of the cluster significantly increases from 2.7 × 10^− 6 to 8.7 × 10^− 6 as the strain increases. Such clustering phenomenon is correlated to the serration behavior under loading in high-manganese steels.     

Role of severe plastic deformation on the formation of nanograins and nano-sized precipitates in Fe–Ni–Mn steel

In this research, the effect of severe plastic deformation (SPD) on the formation of nano-scaled grains and precipitation of nano-sized particles which consequently control mechanical properties of Fe–Ni–Mn alloy was investigated. Fe–Ni–Mn martensitic steels show excellent age hardenability but suffer from embrittlement after aging. Discontinuous coarsening of grain boundary precipitates, resulting in the formation of precipitate free zone (PFZ) along prior austenite grain boundaries, has been found as the main source of embrittlement in the previous studies. In this paper, severe plastic deformation has been carried out on the Fe–10Ni–7Mn steel to improve its mechanical properties. It is found that substantial improvement of tensile properties in cold-rolled steels occurs at thickness reductions larger than 60% where formation of ultrafine grains is realized. According to transmission electron microscopy (TEM) observations, formation of nano-scaled grains less than 100 nm along with the copious precipitation of nanometer-sized precipitates take place in the severely-deformed steels.     

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.     

Effect of strain path on recrystallisation kinetics during hot rolling of Al–Mn

Abstract

Hot rolling of an aluminium–1% manganese alloy has been carried out. Wedge shaped specimens were rolled in two pass schedules, of either two forward passes or a forward and a reverse pass to the same overall net strain. Through thickness marker pins were inserted to allow the investigation of plastic flow during the different rolling schedules. The reversed rolling technique allowed the determination of the effect of a strain path change on the recrystallisation kinetics during hot rolling. Following subsequent annealing, quantitative metallography indicated that the forward–forward specimens showed faster recrystallisation kinetics than the forward–reverse specimens, and produced a finer recrystallised grain size following equivalent thermomechanical treatments differing only in strain path. A through thickness microstructural gradient was found in all materials.     

Effect of manganese content on hot deformation behaviour of Fe–(20/27)Mn–4Al–0.3C non-magnetic steels

The influence of Mn content on flow behaviour and constitutive equations of Fe–(20/27)Mn–4Al–0.3C austenitic steels was investigated by uniaxial hot compression tests. The recrystallisation fraction was also measured using electron backscattered diffraction. Although dynamic recrystallisation is slightly delayed by the increased Mn content, the peak stress still has a pronounced decline especially during the deformation at relatively low temperature. The reason should be ascribed to the enhanced stacking fault energy, by which workhardening rate before the onset of dynamic recrystallisation is diminished significantly with the reinforced cross-slip process. The hot deformation activation energy is decreased under the condition of higher Mn addition, while the strain rate sensitivity of flow stress almost remains unchanged.     

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.     

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     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

Deformation mechanism transition in Fe–17Mn–0.4C–0.06V TWIP steel with different strain rates

The dynamic tensile behaviour and deformation mechanism of the Fe–17Mn–0.4C–0.06V twinning-induced plasticity (TWIP) steel were investigated over a wide range of strain rates from 10^?4 to 10³ s^?1. With increasing strain rate, the stacking fault energy increased due to the increase of adiabatic heating temperature, ΔT. At 10^?4 to 10¹ s^?1, the transformation-induced plasticity (TRIP) effect coexisted with the TWIP effect and weakened with increasing strain rate. With the increase of strain rate in the range of 10^?1 to 10¹ s^?1, the TWIP effect strengthened gradually and intersected deformation twins were formed. When the strain rate was higher than 10¹ s^?1, the TRIP effect disappeared and the twinning was inhibited since the adiabatic heating effect elevated.