Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Newly Developed Austenitic Heat-Resistant Alloy

The hot deformation characteristics, microstructure evolution, and dynamic recrystallization (DRX) mechanism of the newly developed austenitic heat-resistant steel Fe–18Cr–10Ni–0.3Nb–2.5Cu were systematically investigated by thermal compression tests combined with microstructure characterizations. The activation energy (Q) map, Zener–Hollomon parameter (Z) map, and processing map were plotted according to the stress–strain curves to reveal the inherent connection between the three maps and the hot deformation characteristics of this alloy. The high η region in the processing map does not precisely correspond to the region where DRX developed. Nevertheless, the flow instability map accurately predicts the microstructure. The variation pattern of Z corresponded more closely to the hot deformation microstructure evolution than did the variation pattern of Q. The degree of DRX increases with decreasing Z. The optimal process parameters are 1000 °C/0.01 s⁻¹/0.8 and 1100 °C/10 s⁻¹/0.8 (temperature/strain rate/strain), and they result in complete DRX and a narrow range of Z values. The DRX mechanism at high strain rate is characterized by the combined enhancement of discontinuous DRX (DDRX), continuous DRX (CDRX), and twin-DRX (TDRX). The dominance of the particle-stimulated nucleation (PSN) mechanism at intermediate strain rate results in the formation of incompletely recrystallized microstructures with approximate orientation. Sufficient time at low strain rate promotes the development of DDRX and CDRX.

     

Hot workability analysis and processing parameters optimisation for 20CrMnTiH steel by combining processing map with microstructure

The processing map of 20CrMnTiH steel is developed by using the dynamic material model according to the hot compression experiments, performed on a Gleeble-3500 thermal simulator at the temperature range of 850–1150°C and the strain rate of 0.01–1?s^?1. Hot workability characteristics of 20CrMnTiH steel are analysed based on the developed processing map. The safe deformation regions with higher power dissipation efficiency η exhibit the dynamic recrystallisation (DRX) mechanism and show fine and homogeneous microstructure. The unstable regions with negative instability coefficient ξ occur at both lower temperature with all strain rates and at high temperature with high strain rate at the strain of 0.2. The area of instability gradually decreases with the increasing strain and only appears at lower temperature and higher strain rate when the strain is above 0.2. The unstable regions indicate the flow localisation by microstructure analysis. Combining with the developed processing map with DRX behaviour, the optimal values of hot processing parameters for 20CrMnTiH steel are obtained to achieve good hot workability and small grains sizes at the process parameters ranged at 1036–1070°C/0.1–1?s^?1 and 918–985°C/0.01–0.014?s^?1.     

Modeling the Flow Curve Characteristics of 410 Martensitic Stainless Steel Under Hot Working Condition

The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s⁻¹ to 1 s⁻¹. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener–Hollomon parameter (Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol. The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the experimental work.     

Elevated-temperature deformation at forming rates of 10−2 to 102 s−1

In the hot working at constant strain rate ( ) of Al and α Fe alloys at 0.5 to 0.9 T _M (absolute melting temperature), steady-state deformation is achieved in similarity to creep, which is usually at constant stress. After an initial strain-hardening transient, the flow stress becomes constant in association with a substructure which remainsequiaxed and constant in the spacing of sub-boundaries and of dislocations in both walls and subgrains. All these spacings become larger at higher temperature (T) and lower values as well as with lower stress, being fully consistent with the relationships established in creep. Because hot working can proceed to a much higher true strain in torsion (∼100) and compression (∼2) as well as in extrusion (∼20) and rolling (∼5), it is possible to confirm that grains continue to elongate while the subgrains within them remain equiaxed and constant in size. When the thickness of grains reaches about 2 subgrain diameters (d _s), the grain boundaries with serrations (∼d _s) begin to impinge and the grains pinch off, becoming somewhat indistinguishable from the subgrains; this has been called geometric dynamic recrystallization (DRX). In polycrystals as at 20 °C, deformation bands form and rotate during hot working according to the Taylor theory, developing textures very similar to those in cold working. In metals of lower dynamic recoverability such as Cu, Ni, and γ Fe, new grains nucleate and grow (discontinuous DRX), leading to a steady state related to frequently renewed equiaxed grains, containing an equiaxed substructure that develops to a constant character and defines the flow stress. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Effect of Cooling Rate on Microstructure Evolution and Plastic Flow of Ti-6Al-4V

The effect of cooling rate following subtransus solution treatment on microstructure evolution and plastic flow of Ti-6Al-4V was established. For this purpose, three types of tests were performed using samples of Ti-6Al-4V with an initial structure of equiaxed α in a matrix of β and a cooling rate of 11, 42, or 180 K/min: (i) Static cooling following solution treatment without or with a prestrain, (ii) constant-strain-rate hot compression during concurrent cooling, and (iii) static cooling to a specified temperature followed by constant-strain-rate isothermal hot compression. The volume fraction of equiaxed α developed during cooling was strongly dependent on cooling rate, but pre- or concurrent deformation resulted in relatively-small changes in this quantity. In addition, the cooling rate through its effect on the growth kinetics of equiaxed α had a noticeable effect on plastic-flow behavior under both isothermal and non-isothermal conditions. In both instances, the retention of the high-temperature microstructure (characterized by a low fraction of equiaxed α) during rapid cooling gave rise to lower flow stresses than samples with equilibrium equiaxed phase fractions. By contrast, when secondary α was formed during cooling, higher flow stresses were generated due to a Hall-Petch-like effect. The results were interpreted using models for the diffusional growth of equiaxed α, the onset of nucleation of secondary α, and predictions of the plastic-flow response of equiaxed two-phase microstructures based on a self-consistent approach. Unlike previous findings which indicated a large increase in the rate of dissolution of equiaxed α due to concurrent deformation/pipe diffusion during heating transients, the present work did not reveal a corresponding enhancement of growth during cooling.

     

Transitional Behavior for Dynamic Recrystallization in Nuclear Grade 316H Stainless Steel during Hot Deformation

The dynamic recrystallization (DRX) behaviors and their transformation process during hot deformation with various Zener–Hollomon (Z) values were investigated in nuclear grade 316H stainless steel, and the factors influencing DRX transformation, especially adiabatic heating, were evaluated in depth. During hot deformation, with the increase of the Z value, the degree of flow softening (DFS) showed a tendency to decrease first and then increase gradually. The analysis of the microstructure revealed that at low Z value (not exceeding 3.9 × 10¹⁹ s^–1) deformation conditions, DRX was massively activated and the recrystallization mechanism had a transition from continuous DRX (CDRX) to discontinuous DRX (DDRX) with the increasing Z values, leading to the transition of homogeneous grains to heterogeneous grains. Furthermore, with the reactivation of DRX at high Z value deformation conditions, the discontinuous DRX becomes the primary recrystallization mode. Adiabatic heating plays an important role in facilitating the reactivation of DRX and flow softening during hot deformation with low temperature or high strain rate (high Z values, above 6.1 × 10²¹ s^–1).

     

Dynamic Recrystallization Behavior and Microstructure Evolution of Bridge Weathering Steel in Austenite Region

The austenite dynamic recrystallization (DRX) behavior and microstructure evolution of a bridge weathering steel was systematically investigated at a deformation temperature range of 800–1100°C and strain rate of 0.1–10 s^?1 by using hot compression test and optical microscopy. The stress exponent and hot deformation energy were obtained by regression method to determine thermal deformation constitutive equation. The curve of stress versus strain is used, combined with high order polynomial fitting, to accurately determine the critical value of DRX. The relationships between critical strain, critical stress, and Z parameter of the bridge weathering steel were obtained by regression method. Moreover, the influence factors of DRX kinetics of the bridge weathering steel were studied in the light of the experimental results. It is shown that the strain rate has a more significant effect on the rate of DRX than that of the deformation temperature, and there is almost 0.85 orders of magnitude increment in the rate of DRX as the strain rate increases an order of magnitude. The dynamically recrystallized grain size can be decreased with decreasing the deformation temperature and increasing the strain rate during the austenite deformation.     

Comparison of the texture evolution of direct chill and continuous cast AA5052 hot bands during isothermal annealing

As-received commercial direct chill cast (DC) and continous cast (CC) AA5052 hot bands were isothermally annealed at different temperatures. The variations of electrical resistivity, hardness, microstructure, and texture of these hot bands during annealing were examined. Texture was investigated by X-ray diffraction. The variation in texture volume fractions with annealing time was quantitatively analyzed by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The orientation maps of the partially and fully recrystallized samples were characterized by electron backscattered diffraction (EBSD). It was found that electrical resistivity of the CC AA5052 hot band was higher than that of the DC AA5052 hot band. The electrical resistivity of CC AA5052 decreased more significantly than that of DC AA5052 during isothermal annealing. The DC AA5052 hot band was more easily recrystallized than CC AA5052 hot band during annealing. After complete recrystallization, DC AA5052 hot band possessed a stronger cube texture than the CC AA5052 hot band. The texture evolutions of DC and CC AA5052 can be expressed quantitatively by the JMAK equation after the volume fractions of the texture components are calculated by an improved integration method. Annealing temperature had an influence on the n values in the JMAK-type equations. The n values are small at low annealing temperatures.     

Recovery and Recrystallization Behaviors of Ni–30 Mass Pct Fe Alloy During Uniaxial Cold and Hot Compression

The recovery and recrystallization behaviors of the high-temperature γ-phase of carbon steel during deformation strongly affect the mechanical properties of steel. However, it is difficult to evaluate such behaviors at a high temperature. This study proposes the deformation behavior of the high-temperature γ-phase of low-carbon steel based on the quantitative observation of dislocation density and vacancies in the Ni–30 mass pct Fe alloy. This alloy was used because its stacking fault energy (60 to 70 mJ m^-2) is similar to that of low-carbon steel. Uniaxial compression tests were conducted at a strain rate of 10⁻³ s⁻¹ and 1473 K (1200 °C) for dynamic recrystallization and at 293 K (20 °C) for work hardening. The compression process was interrupted at different strain values to systematically investigate microstructural changes. The changes in work hardening, recovery, and recrystallization behaviors were obtained from the true stress–true strain curves of the uniaxial compression tests. Further, the microstructure changes during cold and hot uniaxial compression were investigated from the viewpoint of lattice defects by X-ray diffraction, positron annihilation analysis, transmission electron microscopy, and electron backscatter diffraction to comprehend the work hardening, dynamic recovery (DRV), and dynamic recrystallization (DRX). This study helps understand the DRV, DRX, and work hardening behaviors in the γ-phase of the Ni–30 mass pct Fe alloy during cold and hot compression.

     

Strain-induced grain evolution in polycrystalline copper during warm deformation

The evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct). Deformation was carried out by multipass compression with changing of the loading direction in 90 deg in each pass at temperatures of 473 K to 573 K (0.35 to 0.42 T _m ) under a strain rate of 10⁻³ s⁻¹. The flow stresses increase to a peak followed by a work softening accompanied mainly by dynamic recrystallization (DRX) at 523 K to 573 K. In contrast, the steady-state-like flow appears at 473K accompanied with the development of fine grains at strains as high as 4.2. The relationship of flow stress to the new grain size evolved can be expressed by a power law function with a grain size exponent of about −0.35, which is different from −0.75 for high-temperature DRX at above 0.5 T _m . At 473 K, misorientations of deformation-induced dislocation subboundaries increase with increasing strain, finally leading to the evolution of new grains. It is concluded that the dynamic grain formation at 473 K cannot result from DRX, but from the evolution of deformation-induced dislocation subboundaries with high misorientations and, concurrently, the operation of dynamic recovery.     

The effect of processing on the hot workability of Ti-48Al-2Nb-2Cr alloys

The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near-γ Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at. pct), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range of 1100 °C to 1300 °C at strain rates ranging from 1.67×10⁻¹/s to 1.67×10⁻⁴/s. The P/M materials consolidated by either hot isostatic pressing (“hipping”) or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability was observed in materials with equiaxed microstructures without any lamellar constituents.     

Hot deformation behavior and processing map of Mg-2Zn-1Al-0.2RE alloy

In this study,uniaxial hot compression tests were carried out between 200 and 400℃ over strain rates of0.001-1 s~(-1) to investigate the hot deformation behavior of Mg-2 Zn-1 Al-0.2 RE alloy with coarse grains.The average activation energy was measured to be 174.51 kJ/mol.In addition,a constitutive relation based on the Arrhenius equation was established.Dynamic recrystallization(DRX) kinetics were studied by Avrami equation to characterize the evolution of DRX volume fraction.DRX was favored at high temperatures of 300-400℃ and low strain rates of 0.001-0.01 s~(-1).According to dynamic material model and Prasad's instability criterion,a maximum power dissipation of 38% and 32% occurs at 400℃/0.001 s~(-1) and 400℃/0.01 s~(-1),respectively.According to the proce ssing map,330-400℃/0.001-0.01 s~(-1)was determined as the optimum deformation parameter range.     

Flow Curve Analysis of 17-4 PH Stainless Steel under Hot Compression Test

The hot compression behavior of a 17-4 PH stainless steel (AISI 630) has been investigated at temperatures of 950 °C to 1150 °C and strain rates of 10⁻³ to 10 s⁻¹. Glass powder in the Rastegaev reservoirs of the specimen was used as a lubricant material. A step-by-step procedure for data analysis in the hot compression test was given. The work hardening rate analysis was performed to reveal if dynamic recrystallization (DRX) occurred. Many samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall toward the steady-state stress. At low Zener–Hollomon (Z) parameter, this material showed a new DRX flow behavior, which was called multiple transient steady state (MTSS). At high Z, as a result of adiabatic deformation heating, a drop in flow stress was observed. The general constitutive equations were used to determine the hot working constants of this material. Moreover, after a critical discussion, the deformation activation energy of 17-4 PH stainless steel was determined as 337 kJ/mol.     

Hot Deformation Behaviors of Fe–30Mn–3Si–3Al TWIP Steel during Compression at Elevated Temperature and Strain Rate

The hot deformation behavior of twinning‐induced plasticity (TWIP) steel was investigated at 973–1373 K and strain rates of 0.01–20 s^?1 by hot‐compression experiments performed on a Gleeble‐3800 thermo‐simulation test system. Microstructural evolution during recrystallization in the hot deformed TWIP steels was investigated by metallurgical analysis. The hot‐flow behavior can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation. The hot‐deformation activation energy is 436.813 kJ mol^?1. Deformation bands are initially generated in the deformed austenitic grains during the dynamic recrystallization (DRX) of TWIP steel. With increasing temperature, the recrystallized grains emerge at the boundary junctions after the disappearance of the deformation bands. Subsequently, they gradually spread along the austenitic boundaries and exhibit a necklace shape. The dynamic recrystallized grains continuously grow until they finally reach equilibrium. The DRX mechanism of TWIP steel is a boundary bulge mechanism. The optimum hot‐working technology parameters (especially for rolling) for the TWIP steel is the deformation temperature range of 1223–1323 K, and strain rate range of 1–10 s^?1.     

Microstructural Evolution During the Hot Deformation of Ti-55Ni (at. pct) Intermetallic Alloy

The hot deformation behavior of Ti-55Ni (at. pct) alloy was studied using compression testing at 1173 K (900 °C) to 1323 K (1050 °C) and at the strain rates of 0.001 to 0.35 s⁻¹. The microstructure evolution was characterized using optical and scanning electron microscopy (SEM). The influences of hot-working parameters on the flow stress and microstructural features of this alloy were then analyzed. The results indicate that, depending on the temperature and strain rate, the dynamic recrystallization (DRX) is the dominate mechanism. Besides, the particle-stimulated nucleation (PSN) mechanism could partially recrystallize the structure. The PSN phenomenon is of significant importance for the Ti-55Ni (at. pct) that suffers from insufficient workability because of its high content of intermetallic phases. It is of interest that the discontinuous yielding phenomenon has been observed when the specimens were deformed at 1173 K (900 °C). Finally, the optimum parameters for hot working of Ti-55Ni (at. pct) alloy are determined as well.     

Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma titanium aluminide alloy

The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal, constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between 80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s⁻¹ on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains of γ + α ₂ + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling of the lamellae, and the formation of equiaxed particles of γ + β ₂ + α ₂ on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions.     

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of <Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript>+<Emphasis Type="Italic">γ</Emphasis> titanium aluminides

The stability of a lamellar structure consisting of α ₂ and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other α ₂+γ alloys.     

Effects of lanthanum on hot deformation behaviour of Mn-Cr-Mo bainitic rail steel

Effects of lanthanum(La) as micro-alloying element on the hot deformation behaviour of the high strength Mn-Cr-Mo bainitic rail steel were investigated under a range of deformation conditions. The results indicate that La increases the flow stress by 10-30 MPa through strengthening nanoscale strain induced precipitation(SIP) θ-(Fe,La)3 C during hot deformation. The hot deformation activation energy increases by 10-40 kJ/mol due to the "Zener effect" of SIP and the dynamic recrystallization(DRX) is retarded due to the competitive behaviour between SIP and DRX. Bainite plates in the DRX domain can be refined by adding La, resulting in the improvement of hot workability. The DRX domain with peak power dissipation efficiency of 52% is determined to be the optimal processing region for Mn-Cr-Mo-La bainitic rail steel.     

A Comparative Study on the Hot Working Behavior of Inconel 718 and ALLVAC 718 Plus

Hot compression tests were performed on Inconel 718 and ALLVAC 718 PLUS (718+) at temperatures and strain rates in ranges of 1223 K to 1373 K (950 °C to 1100 °C) and 0.001–1 s⁻¹, respectively. Discontinuous yield behavior was observed in the flow curves of both alloys. For both alloys, the drop in stress at the yield point (yield drop) was maximized at 0.01 to 1 s⁻¹. The alloy 718+ showed larger yield drop than 718 over the studied deformation conditions. The different yield behaviors were attributed to the various chemical compositions. The peak strain for both alloys increased in temperature range of 1223 K to 1273 K (950 to 1000 °C) and strain rates of 0.01 to 1 s⁻¹. This uncommon behavior was ascribed to the change in the mechanism of microstructural evolution from continuous to discontinuous dynamic recrystallization (DRX). The kinetics of DRX was described by the Avrami equation and the exponent was determined at different deformation conditions. The Avrami exponent increased in the middle values of Zener–Hollomon (Z) parameters, i.e., 29.3 < lnZ < 32.9 for 718 and 31.4 < lnZ < 34.5 for 718+. The unusual variation of the Avrami exponent was attributed to the change in the mechanism of DRX.

     

Microstructural evolution in wire-drawn Ti-22Al-26Nb powder

The microstructural evolution in Ti-22Al-26Nb, as it was drawn from hot isostatically pressed powder to a 0.13 mm wire, is followed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The primary α ₂ particles present in the starting powder persist throughout the drawing process, although they undergo both morphological and crystallographic changes. They are altered from 4 to 5-μm-diameter, equiaxed particles to 1 to 2-μm-diameter, highly elongated particles by the drawing process. In addition, these primary particles transform in situ to α ₂ + O phases. There is considerable oxygen pickup during the intermediate annealing cycles, but the increased oxygen has not deleteriously affected room-temperature tensile ductility.