Deformation characteristics of isothermally forged UDIMET 720 nickel-base superalloy

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10⁻³ to 1 s⁻¹ and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ′ solvus temperature was accompanied with grain boundary separation. The hot working window was determined to be in the interval 1100 °C to 1150 °C. Thermomechanical behavior of the material was modeled using the power-law, the Sellars-Tegart, and an empirical equation. The flow stress values showed a nonlinear dependence of strain rate sensitivity to strain rate. The analysis indicated that the empirical method provides a better constitutive equation for process modeling of this alloy. The apparent activation energy for deformation was calculated and its variations with strain rate and temperature are discussed.     

Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10⁻⁴ to 10⁰ s⁻¹) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10⁻⁴ to 10⁻² s⁻¹ and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing γ + α phases, resulting in excellent ductility even at high strain rates.     

Influence of initial texture on the microstructural instabilities during compression of commercial α-Titanium at 25 °C to 400 °C

Cylindrical specimens of textured commercial pure α-titanium plate, cut with the cylinder axis along the rolling direction for one set of experiments and in the long transverse direction for the other set, were compressed at strain rates in the range of 0.001 to 100 s~’ and temperatures in the range of 25 °C to 400 °C. At strain rates ≥ 1 s⁻¹ ’, both sets of specimens exhibited adiabatic shear bands, but the intensity of shear bands was found to be higher in the rolling direction specimens than in the long transverse direction specimens. At strain rates ⪯0.1s ⁻¹ the material deformed in a microstructurally inhomogeneous fashion. For the rolling direction specimens, cracking was observed at 100 °C and at strain rates ⪯0.1 s⁻¹. This is attributed to dynamic strain aging. Such cracking was not observed in the long transverse specimens. The differences in the intensity of adiabatic shear bands and that of dynamic strain aging between the two sets of test specimens are attributed to the strong crystallographic texture present in these plates.     

Hot deformation mechanisms in a powder metallurgy nickel-base superalloy IN 625

The hot working behavior of the nickel-base superalloy IN 625 produced by hot extrusion of a powder metallurgy (P/M) compact has been studied by compression testing in the temperature range 900 °C to 1200 °C and true strain rate range 0.001 to 100 s⁻¹. At strain rates less than about 0.1 s⁻¹, the stress-strain curves exhibited near steady-state behavior, while at higher strain rates, the flow stress reached a peak before flow softening occurred. The processing maps developed on the basis of the temperature and strain rate and strain dependence of the flow stress exhibited three domains. (1) The first domain occurs at lower strain rates (<0.01 s⁻¹) and temperatures higher than about 1050 °C. The peak efficiency and the temperature at which it occurs have increased with strain. The microstructure of the specimen deformed in this domain exhibited extensive wedge cracking. (2) The second domain occurs in the intermediate range of strain rates (0.01 to 0.1 s⁻¹) and temperatures lower than 1050 °C, and in this domain, microstructural observations indicated dynamic recrystallization (DRX) of γ containing δ precipitates and carbide particles resulting in a fine-grained structure. (3) The third domain occurs at higher strain rates (> 10 s⁻¹) and tempe ratures above 1050 °C, with a peak efficiency of about 42 pct occurring at 1150 °C and 100 s⁻¹. Microstructural observations in this domain revealed features such as irregular grain boundaries and grain interiors nearly free from annealing twins, which are typical of DRX of homogeneous γ phase. The instability map revealed that flow instability occurs at strain rates above 1 s⁻¹ and temperatures below 1050 °C, and this is manifested as intense adiabatic shear bands. These results suggest that bulk metal working of this material may be carried out in the high strain rate domain where DRX of homogeneous γ occurs. On the other hand, for achieving a fine-grained product, finishing operations may be done in the intermediate strain rate domain. The wedge cracking domain and the regime of instability must be totally avoided for achieving defectfree products.     

Characterization of hot deformation behavior of brasses using processing maps: Part II. β Brass and α-β brass

The hot deformation behaviors of β brass in the temperature range of 550°C to 800°C and α-β brass in the temperature range of 450°C to 800°C have been characterized in the strain rate range of 0.001 to 100 s⁻¹ using processing maps developed on the basis of the Dynamic Materials Model. The map for β brass revealed a domain of superplasticity in the entire temperature range and at strain rates lower than 1 s⁻¹, with a maximum efficiency of power dissipation of about 68 pct. The temperature variation of the efficiency of power dissipation in the domain is similar to that of the diffusion coefficient for zinc in β brass, confirming that the diffusion-accommodated flow controls the superplasticity. The material undergoes microstructural instability in the form of adiabatic shear bands and strain markings at temperatures lower than 700°C and at strain rates higher than 10 s⁻¹. The map for α-β brass revealed a wide domain for processing in the temperature range of 550°C to 800°C and at strain rates lower than 1 s⁻¹, with a maximum efficiency of 54 pct occurring at about 750°C and 0.001 s⁻¹. In the domain, the α phase undergoes dynamic recrystallization and controls the hot deformation of the alloy, while the β phase deforms superplastically. At strain rates greater than 1 s⁻¹, α-β brass exhibits microstructural instabilities manifested as flow rotations at lower temperatures and localized shear bands at higher temperatures.     

Mechanism of cyclic softening and fracture of an Ni-Base γ′-Strengthened alloy under low-Cycle fatigue

Evolution of the microstructure and its impact on the mechanical behavior of a γ′-strengthened Ni-base alloy under low-cycle fatigue (LCF) at 760 °C, 871 °C, and 982 °C were studied. A cyclic softening was observed, the extent of which increased with temperature. This was attributed to the coherency loss of the primary γ′ particles and partial shearing and dissolution of the secondary γ′ particles. The precipitates and carbides were resistant to coarsening during thermal exposure alone or LCF at the lower two temperatures; the primary γ′ particles, however, lost coherency and spheroidized during LCF at 982 °C. Fatigue cracking initiated at the surface was mainly responsible for failure, although cavities were found in localized areas of a few test specimens. The creep damage was attributed to an accumulation of plastic strains of high amplitude at inhomogeneities, either macroscopic or microscopic.     

Influence of temperature transients on the hot workability of a two-phase gamma titanium aluminide alloy

The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase γ-titanium aluminide alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated with special reference to the influence of temperature transients immediately pre-ceding plastic deformation. Specimens were soaked at 1321 °C or 1260 °C, cooled directly to test temperatures of 1177 °C and 1093 °C, and upset under conditions of constant strain rate and tem-perature. Plastic flow behavior and microstructure evolution occurring in tests involving prior tem-perature transients were compared with those occurring in specimens which were directly heated to the test temperature and upset under identical deformation conditions. Flow curves associated with prior exposure at 1321 °C exhibited very sharp peaks and strong flow softening trends compared to those obtained under isothermal conditions,i.e., involving no temperature transients. During cooling from 1321 °C, the metastable α phase undergoes limited or complete decomposition into α/α₂ + γ lamellae, depending on the final temperature (1177 °C/1093 °C). Subsequent hot deformation leads to partial globularization of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 1177 °C/1093 °C preserves the fine, equiaxed microstructure, through dynamic recrystallization of the γ grains. Cracking observed in specimens deformed at 1093 °C and 1.0 s⁻¹ after exposure at 1321 °C has been attributed to the low rate of globularization as well as the occurrence of shear localization. Plastic flow behavior observed in this work is compared with that observed in several single-phase and two-phase gamma titanium aluminide alloys in order to identify mechanism(s) responsible for flow softening.     

Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196 °C to 1200 °C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600 °C and 800 °C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600 °C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The workhardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200 °C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600 °C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900 °C) is thought to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating.     

Microstructural Features of Flow

Cylindrical specimens of commercial pure titanium have been compressed at strain rates in the range of 0.1 to 100 s^-1 and temperatures in the range of 25 °C to 400 °C. At strain rates of 10 and 100 s^-1, the specimens exhibited adiabatic shear bands. At lower strain rates, the material deformed in an inhomogeneous fashion. These material-related instabilities are examined in the light of the “phenomenological model” and the “dynamic materials mode.” It is found that the regime of adiabatic shear band formation is predicted by the phenomenological model, while the dynamic materials model is able to predict the inhomogeneous deformation zone. The cri- terion based on power partitioning is compctent to predict the variations within the inhomo- geneous deformation zone.     

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 °C to 1200 °C and strain rate range 0.001 to 100 s^•1. The dynamic materials model has been used for developing the pro-cessing maps which show the variation of the efficiency of power dissipation given by [2m/ (m + 1)] with temperature and strain rate, withm being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125 °C and 0.3 s^•1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) de-fects, cracks, or localized shear bands. At 100 s^•1 and 1200 °C, the material exhibits inter-crystalline cracking, while at 0.001 s^•1, the material shows wedge cracks at 1200 °C and PPB cracking at 1000 °C. Also at strain rates higher than 10 s^•1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.     

Effect of Aging Treatment on the Microstructure and Resistivity of a Nickel-Base Superalloy

The γ′ precipitation behavior of age-hardened WASPALOY, aged at 998 K, 1073 K, and 1148 K (725 °C, 800 °C, and 875 °C) for times ranging from 0.5 to 263.5 hours, were evaluated via analysis of ultra small angle X-ray scattering (USAXS) curves and scanning electron microscopy (SEM) micrographs. The USAXS spectra revealed a single precipitate size distribution at the earliest aging times, which evolves into a bimodal precipitate size distribution at later aging times. The primary precipitate radius displayed t ^1/3 coarsening dependence for aging at 1073 K and 1148 K (800 °C and 875 °C); however, the primary radius increased with t ^0.4 dependence at 998 K (725 °C), most likely due to mixed growth and coarsening. A figure of merit, η′, consisting of two terms, one associated with precipitate size and volume fraction and the other with compositional fluctuations, was proposed. η′ shows direct empirical correlations with changes in the measured electrical resistivity.     

Comparison of deformation induced precipitation behaviour in Alloy 718 under two microstructural conditions

Alloy 718 samples under two initial microstructural conditions, viz., solution annealed to form only γ phase (ST) and aged to precipitate only δ particles (DELTA), were deformed in tension till fracture in the temperatures range from 200°C–700°C. From the comparison of the evolved microstructure of deformed and undeformed specimens that have been subjected to similar thermal history, deformation induced precipitation could be identified. Deformation in the range of 550°C to 650°C promoted the precipitation of γ′ and γ″ phases in both structures. In case of DELTA alloy, the γ′ precipitation was found to precede the γ′ phase precipitation while no such preference for precipitation could be identified in ST specimens. This difference in the precipitation behaviour and the sequence of precipitation has been explained on the basis of the relative concentration of solutes in the matrix of the starting microstructures of ST and DELTA specimens.     

Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196°C to 1200°C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600°C and 800°C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600°C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The work-hardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200°C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600°C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900°C) is though to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating. Z. JIN, formerly Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory     

Hot Deformation Characteristics of Functionally Graded Steels Produced by Electroslag Remelting

In this work, a hot compression test was carried out at 1173 K to 1473 K (900°C to 1200 °C), with a strain rate of 0.01 to 1 s⁻¹ up to ~50 pct height reduction on functionally graded steel (FGS) specimens comprised of ferritic, bainitic, austenitic, and martensitic layers (αβγMγ). The stress-strain curves are strongly dependent on temperature and strain rate. Compressive flow stress varied from 40 to 105 MPa depending on the applied temperature and strain rates. Variation in steady-state flow stress with temperature and strain rates was studied. The strain-rate-sensitivity exponent (m) and deformation activation energy (Q) for the αβγMγ composite under studied condition were 0.106 and 354.8 KJ mol⁻¹, respectively, which are within the values of boundary layers of ferrite (304.9 KJ mol⁻¹) and austenite (454.8 KJ mol⁻¹) layers. Given the alternative microstructure of the αβγMγ FGS, a range of deformation mechanisms from dynamic recovery to dynamic recrystallization maybe prevails, where the intensity of each mechanism depends on temperature and strain rates. In accordance with the experimental results, an empirical power-law equation was developed over the range of temperatures and strain rates investigated. The equation accurately describes temperature and strain-rate dependence of the flow stress.     

Creep deformation of Ni3Al-Mo in situ composites

High temperature γ′(Ni₃A1)-α(Mo) in situ composites solidified with growth rates ranging from 0.12 cm/h to 2.4 cm/h, were creep tested at temperatures 710 °C, 830 °C, and 950 °C under a tensile stress of 300 MPa. Creep rupture lives of rapidly grown alloys were comparable to those of γ′/γ-δ and Nitac eutectics whereas strain to rupture was up to three times greater. In comparison, creep rupture lives of slowly grown alloys were more than ten times and strain to rupture about two times smaller than in rapidly grown alloys. In slowly grown alloys, failure occurred by formation of shear bands caused by cooperative shear of matrix and fibers whereas no shear bands were seen in rapidly grown samples. Shear band formation was due to pile-up stresses and parallel orientation of matrix and fiber slip systems, the latter resulting from a change in the crystallographic phase relationship as growth rate decreased. There was evidence that shear band formation depended on a threshold stress. The creep behavior of rapidly grown alloys was in qualitative agreement with predictions obtained from a linear visco-elastic model composite strengthened by the “mean matrix stress”.     

The relationship between microstructural and plastic instability in AI-4.0 Wt Pct Cu alloy

The investigation of the effect of plastic deformation on the stability of theθ′ precipitates in an aluminum-4.0 wt pct copper alloy was performed. The alloy was produced by directional solidification, with Ti added as a grain refiner. Hot compression tests were performed at 200 °C in the strain rate range of 10_-3 to 10_-5 s₁ and equivalent strain up to 0.7 on specimens that had been initially heat treated, also at 200 °C, in order to obtain a uniform distribution of theθ0′ precipitates within the matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the plastically deformed specimens revealed a very heterogeneous distribution of strain. Also, the regions with localized strain contained randomly distributedθ precipitates of nearly equiaxed shape without any preferred orientation relationships to the matrix. Thus, the plastic deformation initiated the transformationθ′ →θ. The flow stress was reduced in the regions in which this transformation had occurred, which further accentuated the localization tendency of the strain. The combined process,θ′ →θ transformation/strain localization, thus developed in an avalanching way.     

Characterization and modeling of the high temperature flow behavior of aluminum alloy 2024

Isothermal flow curves were determined for aluminum alloy 2024-0 at temperatures of 145 to 482 °C and at constant true-strain rates of 10^-3 to 12.5 s^-1 using compression tests of cylindrical specimens. The average pressure was corrected for friction and for deformation heating to determine the flow stress. At 250 °C and above, the isothermal flow curves usually exhibited a peak followed by flow softening. At 145 °C the flow curves exhibited strain hardening. For 250 °C≦ T<= 482 °C, 10^-3 s^-1 ≦ ≦ 12.5 s^-1, and ε ≦ 0.6 the flow behavior was represented by the constitutive equation σ =K (T, ε) where logK andm are simple functions of temperature and strain. The as-deformed microstructures generally supported the idea that flow softening in Al 2024-0 is caused by dynamic recovery. At the higher temperatures and strain rates, however, fine recrystallized grains were observed in local areas near second phase particles and at as-annealed grain boundaries. At 482 °C, there was evidence of re-dissolution of the CuMgAl₂ precipitate. Formerly Visiting Associate Professor, Wright State University, Dayton, OH 45435 Formerly a Mechanical Systems Engineering Student at Wright State University Formerly a Materials Engineering Student at Wright State University Formerly Director, Metallurgy Program, National Science Foundation, Washington, DC     

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were performed at a strain amplitude of ±0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of γ′ and carbides); and microstructure B, in a double-aged condition with γ′ of 18-nm diameter and M₂₃C₆ carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered γ′ that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 × 10^-5 to 3 × 10^-3 s^-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.     

Creep of precipitation-hardened nickel-base alloy single crystals at high temperatures

Single crystals of a γ′ precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F. It was found that shear of the γ′ precipitate by pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep. This contrasts with 1400°F creep behavior where shear of γ′ is dominated by α/3 (112) dislocations in primary creep, but by pairs of α/2 (110) dislocations in steadystate creep. The orientation dependence of the steady-state creep rate at 1575°F is explained by the nature of dislocation junction reactions for the different orientations. Crystals along the [001]-[1•11] boundary have the greatest creep resistance because of the formation of stable dislocation networks at the matrix-particle (γ⊃ interfaces, whereas the lower creep resistance of crystals oriented along the [001]-[0•11] boundary is a consequence of the low probability for the formation of stable junction reactions. Finally, evidence, in the form of resolvable α/2 (110) dislocation pairs within the γ′ precipitate, is presented for a reduction in the local antiphase boundary energy of γ′ at high temperatures.