Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10⁻³ to 1 s⁻¹. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10⁻²s⁻¹, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10⁻¹s⁻¹ for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.     

The vanadium-platinum constitution diagram

The system V-Pt was investigated over the entire composition range by metallography, X-ray diffraction and electron microprobe studies. There are at least four equilibrium intermediate phases in this system and they are stable to progressively higher temperatures with increasing vanadium concentration. The phases which have been observed are: γ^′, cubic, Cu₃Au type; θ, tetragonal, TiAl₃ type; δ, orthorhombic, MoPt₂ type; ζ, orthorhombic, AuCd type; and β, cubic, Cr₃Si type (A15). The gg^′ phase is possibly metastable. A very stable ribbon-like growth of ζ phase in the fcc platinum terminal solid solution has been observed in alloys containing about 43 at. pct V. The platinum terminal solid solution forms a congruent melting maximum at about 1805°C. A eutectic reaction occurs at 1720° ± 10°C and a peritectic reaction is indicated at 1800° ± 10°C. Vanadium is soluble in the fcc platinum terminal solid solution up to about 57 at. pct at 1720°C. Platinum dissolves only to the extent of about 12 at. pct at 1800°C in bcc α-V.     

Thermomechanical fatigue of particulate-reinforced aluminum 2xxx-T4

Thermomechanical fatigue (TMF) and isothermal fatigue of unreinforced and SiC_p-reinforced aluminum 2xxx-T4 alloy were examined. Thermomechanical fatigue experiments were conducted underT _min = 100 °C,T _max = 300 °C andT _min = 100 °C,T _max = 200 °C conditions, and isothermal experiments were conducted at 200 °C and 300 °C. Based on stress range, substantial improvements in fatigue life were observed with reinforcement under both isothermal and thermomechanical loading conditions. Based on strain range, the TMF lives of the reinforced material increased in out-of-phase (OP) loading and remained unchanged in in-phase (IP) loading. A decrease in isothermal fatigue lives of the reinforced material compared to those of unreinforced material was observed in both 3 × 10⁻³ s⁻¹ and 3 × 10⁻⁵ s⁻¹ experiments at 200 °C and in 3 × 10⁻³ s⁻¹ experiments at 300 °C. Crack growth mechanism maps were constructed to identify crack growth behavior of the unreinforced and the reinforced materials. The TMF OP conditions were more favorable to transgranular cracking. Mixed (transgranular and intergranular) crack growth occurred in TMF IP experiments. Evidence of void formation at grain boundaries, crack deflection due to particles, and oxide penetration at the crack tips is demonstrated using scanning electron microscopy (SEM) and Auger spectroscopy analysis.     

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.     

Structural superplasticity at higher strain rates of hypereutectoid Fe-5.5Al-1Sn-1Cr-1.3C steel

A fine-grained ultra-high-carbon steel—UHC steel—containing 1.35 wt pct carbon, 5.5 wt pct aluminum, 1 wt pct tin, and 1 wt pct chromium exhibits fine-structure superplasticity in the temperature regime between 775 °C and 900 °C at higher strain rates up to 10⁻² s⁻¹. Thermomechanical processing was performed in order to achieve a fine-grained equiaxed microstructure consisting of κ-carbides of about 0.7 to 2.5 μm in size finely distributed within the ferritic Fe(Al, Sn, Cr) solid solution matrix with a linear intercept grain size of 3 to 5 μm. Superplasticity occurred in the strain rate regime of 10⁻⁴<- ≤10⁻² s⁻¹ with m values of 0.5 to 0.6 (stress exponent n=1.6 to 2). Tensile elongations of more than 900 pct were recorded. From thermal activation analysis, activation energies of Q=230 to 243 kJ/mole were determined, which clearly reveal a contribution of the alloying elements Al and Sn to the lattice diffusion of iron. The governing deformation mechanism is grain boundary sliding accommodated by dislocation climb controlled by lattice diffusion sustained by chemical diffusion. At very high strain rates of ≳2 · 10⁻² s⁻¹, the strain-rate-sensitivity exponent decreases to about 0.2≤m≤0.27, which indicates class II solid solution behavior of this material.     

Tensile behavior of inconel alloy X-750 in air and vacuum at elevated temperatures

Investigations carried out on the hot tensile properties of Inconel alloy X-750 at 700 °C in air and vacuum at different strain rates, in the range of 1 × 10⁻⁷ to 1.2 × 10⁻⁶ s⁻¹, have shown that testing in air had a weakening effect on properties. Creep ductility in vacuum (p ₀₂ = 2.7 × 10⁻⁵ Pa) did not change appreciably with strain rate, but ductility varied markedly when tested in the air. Further, the ductility minimum occurred at 625 °C in air whereas considerable improvements in the creep ductilities were observed at 575 °C and 625 °C in the vacuum. The results indicated that the environmental interaction during testing enhanced the rate of cavitation damage causing premature failure in the material.     

Plastic deformation kinetics of fine-grained alumina

The plastic deformation kinetics of a commercial fine-grained alumina with ∼300 ppm MgO and grain sizes from 1.4 to 2.9 μm were determined in tension at 1475 °C to 1600 °C and strain rates from 10⁻⁵ to 10⁻³ s⁻¹, employing stress relaxation (SR) as the principal test mode. The constants in the Weertman-Dorn (W-D) equation were determined and had the following values: A=2.9±0.5×10⁹, n=2.2±0.1, p=1.9±0.1, Q=492±3 kJ/mole, and threshold stress σ ₀=0. These constants are in accord with grain boundary sliding (GBS) accommodated by dislocation glide and climb with Al³⁺ ion lattice diffusion as the rate-controlling mechanism.     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

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.     

Characterization of hot deformation behavior of brasses using processing maps: Part I. α Brass

The constitutive flow behavior of α brass in the temperature range of 500°C to 850°C and strain rate range of 0.001 to 100 s⁻¹ has been characterized with the help of a power dissipation map generated on the basis of the principles of the Dynamic Materials Model. The map revealed a domain of dynamic recrystallization in the temperature range of 750°C to 850°C and in the strain rate range of 0.001 to 1 s⁻¹, with a maximum efficiency of power dissipation of about 54 pct. The optimum hot working conditions are 850°C and 0.001 s⁻¹, and these match with those generally employed in industrial practice. In the temperature range of 550°C to 750°C and strain rates lower than 0.01 s⁻¹, the efficiency of power dissipation decreases with decreasing strain rate, with its minimum at 650°C. In this regime, solute drag effects similar to dynamic strain aging occur to impair the hot workability. The material undergoes microstructural instabilities at temperatures of 500°C to 650°C and at strain rates of 10 to 100 s⁻¹, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands.     

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.     

The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity

The diffusivity and solubility of oxygen in liquid tin and solid silver in the temperature range of about 750° to 950°C (1023 to 1223 K) and the diffusivity of oxygen in solid nickel at 1393°C (1666 K) were determined using the electrochemical cell arrangement of cylindrical geometry: Liquid or Solid Metal + O (dissolved) | ZrO₂ + (3 to 4%)CaO | Pt, air The diffusivity and solubility of oxygen in liquid tin are given by:D _O(Sn) = 9.9 × 10⁻⁴ exp(−6300/RT) cm²/s (9.9 × 10⁻⁸ exp − 6300/RT m²/s) andN _O ^S (Sn) = 1.3 × 10⁵ exp(−30,000/RT) at. pct The diffusivity and solubility of oxygen in solid silver follow the relations:D _O(Ag) = 4.9 × 10⁻³ exp (−11,600/RT) cm²/s ( 4.9 × 10⁻⁷ exp − 11,600/RT m²/s) andN _O ^S (Ag) = 7.2 exp (−11,500/RT) at. pct The experimental value for the preexponential in the expression forD _O(Ag) is lower than the value calculated according to Zener’s theory of interstitial diffusion by a factor of 11. The diffusivity of oxygen in solid nickel at 1393°C (1666 K) was found to be 1.3 × 10⁻⁶ cm²/s (1.3 × 10⁻¹⁰ m²/s). Formerly Graduate Student, Department Formerly Graduate Student, Department Formerly Graduate Student, Department This paper is based upon a This paper is based upon a This paper is based upon a This paper is based upon a     

Elastic properties of a crossply SiC f /Ti composite at elevated temperatures

Using electromagnetic acoustic resonance (EMAR), we measured the complete set of elastic constants and internal friction of a silicon-carbide-fiber-reinforced Ti-6Al-4V-matrix cross-ply composite between room temperature and 1000 K. This material exhibits tetragonal symmetry and possesses six independent C _ij . By changing the magnetic-field direction, we selected two vibration groups among the eight vibration groups of a rectangular parallelepiped. The elastic constants showed anomalous temperature dependence around 650 K and internal friction Q ⁻¹ increased rapidly above that temperature; neither the C _ij nor Q ⁻¹ returned to the before-heating values. We attributed this to disbonding between the matrix alloy and SiC, and between SiC and the carbon core within the fiber caused by oxidation of carbon, which is supported by the microstructure observation. We present the temperature derivatives of the composite C _ij and estimate those of the fiber C _ij by comparing the matrix-C _ij derivatives.     

Microtexture evolution during annealing and superplastic deformation of Al-5 pct Ca-5 pct Zn

The microtexture and grain boundary misorientation distributions (i.e., mesotexture) of the superplastic alloy Al-5 pct Ca-5 pct Zn have been investigated in the as-processed condition, after annealing at 520 °C (for times ranging from 7 minutes to 90 hours) and after tensile straining in the transverse direction (TD). Three different superplastic straining conditions were considered: 550 °C/10⁻² s⁻¹, 550 °C/10⁻¹ s⁻¹, and 400 °C/10⁻² s⁻¹. Microtexture data were obtained by means of computer-aided electron backscatter diffraction analysis methods. The retention of the deformation texture of the as-received material and the development of an increasingly bimodal grain boundary misorientation distribution following static annealing are consistent with the occurrence of recovery and continuous recrystallization. During superplastic straining, deformation texture components are also retained, but with a change in the grain boundary misorientation distribution toward random, indicating that grain switching occurs during grain boundary sliding (GBS). At the midlayer, however, a change from an initial texture component near the Cu-type texture component toward the Brass texture component, {011}〈211〉, was observed even as the misorientation distribution became more random. This change in texture component is associated with the occurrence of single slip during superplastic flow.     

Plastic Flow Properties and Microstructural Evolution in an Ultrafine-Grained Al-Mg-Si Alloy at Elevated Temperatures

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 μm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10⁻⁴ to 10⁻¹ s⁻¹. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains.     

High-temperature deformation and failure of an orthorhombic titanium aluminide sheet material

The high-temperature deformation and failure behavior of an orthorhombic titanium aluminide sheet alloy (fabricated by diffusion bonding of six thin foils) was established by conducting uniaxial tension and plane-strain compression tests at 980 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. The stress-strain response was characterized by a peak stress at low strains followed by moderate flow softening. Values of the strain-rate sensitivity index (m) were between 0.10 and 0.32, and the plastic anisotropy parameter (R) was of the order of 0.6 to 1.0. Cavity nucleation and growth were observed during tensile deformation at strain rates of 10⁻³ s⁻¹ and higher. However, the combined effects of lowm, low cavity growth rateη, and flow softening were deduced to be the source of failure controlled by necking and flow localization rather than cavitation-induced fracture prior to necking.     

Diffusion of managenese and silicon in liquid iron over the whole range of composition

The measurement of the diffusivities of manganese and silicon in molten binary ferroalloys over the whole range of composition was undertaken to clarify existing but conflicting data at lower concentrations, to present new data at higher concentrations and to indirectly confirm the behavior of both systems observed in thermodynamic studies. The experiments were carried out under argon atmosphere in a Tammann furnace. The diffusion couples were held in 5 mm ID alumina tubes (98 pct Al₂O₃). Electron probe microanalysis of the samples led to a diffusion-penetration curve for the system under consideration. Results obtained over the whole range of composition showed a slight negative deviation for the Fe−Mn system and a very large positive deviation for the Fe−Si system. At lower concentrations (0 to 4 pct Mn), the temperature dependence of managanese diffusivity for the Fe−Mn binary alloy in the temperature range 1550° to 1700°C is as follows:D _Fe−Mn=1.8×10⁻³ exp (−13,000/RT) cm²/sec The concentration dependence of manganese diffusivity for the same system at 1600°C may be expressed asD _Fe−Mn={5.48−0.0137 (%Mn)+0.000276 (%Mn)²}×10⁻⁵ cm²/sec The temperature dependence of silicon diffusivity for the Fe−Si binary system in the temperature range 1550° to 1725°C at various concentrations is as follows:D _Fe−Si=2.8×10⁻³ exp (−11,900/RT) cm²/sec at 20 pct SiD _Fe−Si=2.1×10⁻³ exp (−13,200/RT) cm²/sec at 12.5 pct SiD _Fe−Si=5.1×10⁻⁴ exp (−9,150/RT) cm²/sec at 2.2 pct Si FELIPE P. CALDERON, formerly Graduate Student. University of Tokyo, Tokyo, Japan. This paper is based on a portion of a thesis submitted by FELIPE P. CALDERON in partial fulfillment of the requirements for the degree of Doctor of Engineering at University of Tokyo.     

On the effect of plastic deformation on the coarsening of ϑ-phase precipitation in an Al-Cu alloy

In the present investigation, the focus is on dynamic coarsening of the equilibrium ϑ phase in an Al-4wt pct Cu alloy. For this purpose, specimens containing a uniform ϑ particle distribution have been produced and deformed in compression at two different temperatures (200 °C and 250 °C) and strain rates in the ranges of 200 °C to 250 °C and 10⁻⁵ to 10⁻² s⁻¹, respectively. The particle size distribution measurements performed in a scanning microscope in back-scattered mode demonstrated a double peak behavior depending on temperature: at the lowest test temperature, the dynamic coarsening is enhanced at the highest strain rate, while at 250 °C, the coarsening seems to be affected by crushing of small and medium size particles during straining.     

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.