Precipitation and deformation behaviour of cast alloy 718 during creep and thermal exposure

Abstract

The precipitation, deformation, and fracture behaviour of cast alloy 718 during creep rupture tests was investigated, in comparison with thermal exposure tests. Inhomogeneous deformation bands appeared during monotonic or cyclic deformation of alloy 718. The bands were identified as mechanical twins, which are known to be responsible for crystallographic failure during creep rupture at and below 649°C. However, crystallographic failure was observed at temperatures up to 760°C in the present study. No crystallographic failure was observed at and above 816°C. Precipitation of δ phase was observed on deformation bands following creep rupture tests at and above 704°C. The difference in failure mode below 760°C and above 816°C is assumed to be caused by the precipitation of δ phase on the bands. A few discrete δ particles on the bands during 704°C creep rupture tests were not sufficient to prevent decohesion along the twin/matrix interface, and therefore crystallographic failure still occurred. In contrast with little or no precipitation below 704°C, needlelike or platelike δ phase precipitated at and above 816°C. It is postulated that the precipitation of δ phase restricted successive deformation. Since δ phase precipitates on {111} planes where major deformation occurs, this phase usually grows according to the following orientation relationship: (010)_δ∥(111)_γ, [100]_δ∥[11¯0]_γ. Restriction of deformation by the precipitation of δ phase caused the change in failure behaviour at and above 816°C.     

Rotary bending high-cycle fatigue behavior of DD32 single crystal superalloy containing rhenium

Smooth and notched specimens of single crystal superalloy DD32 were subjected to rotary bending high-cycle fatigue (HCF) loading at different temperatures. The experimental results demonstrate that fatigue strengths of the smooth and notched specimens reach the maxima and the minimum notch sensitivity displays at 760 °C. DD32 alloy exhibits excellent HCF properties compared to SRR99 alloy under the same test condition. As for the smooth specimens, slip bands moving through γ and γ′ phases as well as dislocation bowing are the main deformation modes. As for the notched specimens, the deformations are carried out by dislocation loop bowing and shearing of PSBs mode at intermediate temperatures; at 900 °C, the minimum fatigue strength results from dislocation climbing deformation and the degradation of γ′ precipitates. The fine secondary γ′ precipitates advantage the recovery of dislocations and further deformation of the fatigue specimens.     

Intermediate temperature brittleness and directional coarsening behavior of nickel-based single-crystal superalloy DD6

The microstructure of the nickel-based single-crystal superalloy DD6 after tensile deformation has been studied by transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS). The samples were strained to fracture at room temperature, 650 °C, 850 °C and 1020 °C along the [001] orientation. The results indicate that the yield strength at 650 °C is superior to that at room temperature (20 °C), 850 °C and 1020 °C, but low ductility was observed at 650 °C. It is demonstrated that the intermediate temperature brittleness (ITB) behavior was caused by the change of the deformation mechanism at intermediate temperature. At high temperature, the γ′ precipitates coarsening directionally along the direction perpendicular to the stress axis. This can be attributed to the directional diffusion of the chemical elements.     

Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C

The stress rupture properties and deformation mechanisms of K4750 alloy at 650 °C, 700 °C, 750 °C and 800 °C were investigated. As the decrease of temperature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ = 3166.455 − 119.969 × LMP and the fitting coefficient was 0.98. After testing, the dislocation configurations of all stress rupture samples were investigated by transmission electron microscopy (TEM). The temperature and stress had a significant impact on the deformation mechanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given temperature, the deformation mechanism gradually transformed from Orowan looping to stacking fault shearing. Based on experimental results, the threshold stress at 650 °C, 700 °C, 750 °C and 800 °C for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress, γ' phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had a long stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dominant mechanism, the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism, the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

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

Abstract

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

High strain rate superplasticity in powder metallurgy aluminium alloy 6061 + 20 vol.-%SiCpcomposite with relatively large particle size

Abstract

The possibility of high strain rate superplasticity (HSRS) was examined over a wide range of temperatures in a powder metallurgy aluminium alloy 6061/SiC_p composite with a relatively large SiC particle size of ~8 μm. A maximum tensile elongation of 350% was obtained at 600°C and 10^-2 s^-1. Tensile elongations over 200% were obtained in a narrow temperature range between 590 and 610°C at high strain rates of 10^-2 and 10^-1 s^-1. The current testing temperature range could be divided into two regions depending on the rate-controlling deformation mechanism. Region I is in the lower temperature range from 430 to 490°C, where lattice diffusion controlled dislocation climb creep (n = 5) is the rate-controlling deformation process, and region II is in the higher temperature range from 520 to 610°C, where lattice diffusion controlled grain boundary sliding controls the plastic flow. An abnormally large increase in activation energy was noted at temperatures above 590°C, where large tensile elonga tions over 200% were obtained at high strain rates. This increase in activation energy and high tensile ductility may be explained in terms of presence of a liquid phase created by partial melting, but such evidence could not be provided by the current differential scanning calorimetry (DSC) test. This may be because the DSC is not sensitive enough to detect the small amount of liquid phase.     

Deformation of polycrystalline Ti2AlC under compression

Deformation of polycrystalline Ti₂AlC under room and high temperature compression was investigated. The results demonstrated that Ti₂AlC was damage tolerant at room temperature and the samples were shear fractured upon failure. At high temperatures Ti₂AlC deform plastically. The brittle-to-ductile-transition temperature (BDTT) of Ti₂AlC was between 1000 °C and 1050 °C. The microstructure and fracture surfaces were examined using scanning electron microscopy. Due to insufficient number of dislocation systems, the room-temperature deformation was constitute of kinking and delaminating of laminated Ti₂AlC grains, basal plane dislocation slip, formation of voids and cavities in the vicinity of main crack. At high temperatures below BDTT, the deformation was a combination of cavities formation and intergranular sliding. At temperatures above BDTT, the deformation was mainly plastic flow. Received: 15 January 2001 / Reviewed and accepted: 12 April 2001     

Tensile elongation of lean-alloy austenitic stainless steels: transformation-induced plasticity versus planar glide

An Fe–13Cr–3.4Mn–0.47C lean-alloy stainless steel was made austenitic by solution annealing at 1250°C. Tensile tests between 20 and 200°C indicated enhancement of ductility at higher temperatures. At 200°C where planar glide, manifested as deformation twinning, was the dominant deformation mechanism, a uniform tensile elongation of 102% was achieved. At 20°C where deformation-induced α′-martensitic transformation replaced deformation twinning as the dominant deformation mechanism, tensile elongation was significantly impaired. The tensile elongation contribution by the planar glide was estimated to be at least four times that of the α′-TRIP (transformation-induced plasticity) mechanism. The results indicate that inexpensive lean-alloy austenitic stainless steels exhibiting pronounced α′-formation at room temperature could become highly formable at higher temperatures.     

Isothermal Dilatometric Study of Sintering in Kaolin

Solid-state sintering for kaolin samples was studied by dilatometric measurements in the isothermal regime in the temperature range from 600 °C to 1100 °C. The relative expansion was measured for a period of 10h. For the temperatures up to 850 °C, we observed only a small shrinkage (less than 0.5 %), most of which took place within the first 3h of the measurements. For the temperatures above 850 °C, a significant shrinkage occurred for the whole measured time interval and reached up to 2.7 %. Anomalous behavior—a decrease in the shrinkage with the temperature—was observed in the range from 700 °C to 850 °C. The dilatometric measurements are supplemented by porosity distribution measurements. The standard spherical-grains microscopic model was applied to determine that for the initial stages of the sintering process, grain boundary diffusion was the dominant mechanism at lower temperatures (600 °C to 850 °C), whereas lattice diffusion was dominant at higher temperatures (900 °C, 1050 °C, and 1100 °C).     

Effect of Firing Temperature on the Structure and Properties of YBa2Cu3O7−x Films by TFA-MOD Method

YBa₂Cu₃O_7−x (YBCO) films were fabricated on LaAlO₃ (LAO) substrate under various firing temperatures (760–870 °C) in the crystallization process by metalorganic deposition (MOD) method using trifluoroacetates. The effect of firing temperature on the structure and properties of YBCO films was systematically investigated. According to the XRD and SEM images, the films fired at low temperature (760–800 °C) showed poor electrical performance due to rough surfaces and impurity phases. However, the films fired at 850 °C showed the highest critical temperature of 90 K and the highest J _c of 3.1 MA/cm² which attribute to the formation of a purer YBCO phase, fewer pores, and stronger biaxial texture.     

Effects of chemical composition and thermomechanical treatment on austenite to ferrite transformation in 12 wt-%Cr steels

Abstract

The kinetics of austenite to ferrite transformation was studied in 11–12 wt-%Cr steels having an essentially austenitic microstructure at hot rolling temperatures (750–1050°C). The effects of chemical composition, high temperature γ/δ phase balance, and deformation before the transformation were assessed. The phase transformation was monitored using dilatometry, metallography, and hardness measurements. Small variations in chemical composition, particularly in the nickel and manganese content, resulted in significant differences in the kinetics of the transformation. These are a result of changes in the Ac₁ temperature, pre-existing δ ferrite content at high temperature, and probably the solute drag effect. Deformation at low temperatures of 850–750°C accelerated the transformation. The magnitude of this effect wasfound to depend on the degree of deformation and the cooling rate above the transformation temperature. Using a reduction of 30%, the cooling rate that resulted in a specific fraction of ferrite in the final structure was increased threefold. The results suggest that if the steel composition, particularly the nickel and manganese content, can be adjusted within narrow limits, controlled rolling together with controlled, retarded cooling can be applied to produce 11–12 wt-%Cr steels with adequate mechanical properties and excellent weldability, without the need for tempering.     

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.     

Hot-deformation characteristics of Waspaloy

Abstract

Specimens of wrought Waspaloy have been reheated to above the γ' solvus temperature and tested in plane-strain compression at constant equivalent tensile strain rates in the range 0.5–50 s^?1, and at initial test temperatures in the range 960–1070°C. The majority of tests were conducted with the tools and test environment at 850°C. Dynamic recrystallization was observed under all test conditions, but was only complete by the strain limit of 2·7 in tests at the highest temperatures. Static (metadynamic) recrystallization initiated rapidly after deformation. Flow stress, either at the peak or at a constant strain, is related exponentially to strain rate, and gives an activation energy of 475 kJ mol^?1 when related to the instantaneous temperature, which changed rapidly during deformation. For practical purposes, the stress-strain-strain-rate-temperature relationships may be treated as equations of state.

MST/96     

Post-forming tensile properties of superplastic Ti–6Al–4V alloy

Abstract

A study has been made of the influence of uniaxial superplastic deformation on the ambient temperature tensile properties of Ti–6Al–4V sheet. Material was deformed to various strains up to 200% at temperatures from 850 to 970°C at strain rates in the range 1·1?18 × 10^{;amp;#x2212;4}s^?1 (0·7?11% min^?1). Tests were also performed on statically annealed material to separate the effects of high temperature exposure and superplastic deformation. Mechanical property changes were complex and depended on the relative contributions from the strengthening and softening mechanisms occurring during either superplastic deformation or heat cycling. Structural features influencing mechanical properties were phase size and morphology, dislocation density, and crystallographic texture. The strength after superplastic deformation was always less than that of as-received material but a significant reduction in strength was attributable to heat cycling. In some cases, the strength of the superplastically deformed material was greater than that after heat cycling.

MST/593     

Hot deformation of ultrafine-grained Al6063/Al2O3 nanocomposites

Ultrafine-grained (UFG) Al6063 alloy reinforced with 0.8 vol% nanometric alumina particles (25 nm) was prepared by reactive mechanical alloying and direct powder extrusion. Transmission electron microscopy and electron backscatter diffraction analysis showed that the grain structure of the nanocomposite composed of nanosize grains (<0.1 μm), ultrafine grains (0.1–1 μm) and micronsize grains (>1 μm) with random orientations. Mechanical properties of the material were examined at room and high temperatures by compression test. It was found that the yield strength of the UFG composite material is mainly controlled by the Orowan mechanism rather than the grain boundaries. The deformation activation energy at temperature ranges of T < 300 °C and 300 °C ≤ T < 450 °C was determined to be 74 and 264 kJ mol⁻¹, respectively. This observation indicated a change in the deformation mechanism at around 300 °C. At the higher temperatures, significant deformation softening was observed due to dynamic recrystallization of non-equilibrium grain boundaries. The reinforcement nanoparticles, however, renders the high strength of the material at the elevated temperatures mainly by dislocation pinning.     

Structure and properties of solid state diffusion bonding of 17-4PH stainless steel and titanium

Abstract

Solid state diffusion bonded joint between titanium and 17-4 precipitation hardening stainless steel was carried out in the temperature range of 800–1050°C in steps of 50°C for 30 min and also at 950°C for 30–180 min in steps of 30 min under a uniaxial pressure of 3·5 MPa in vacuum. Bonded samples were characterised using light microscopy, field emission scanning electron microscopy and X-ray diffraction technique. Up to 850°C for 30 min, FeTi phase was formed at the diffusion interface; however, α-Fe+λ, χ, Fe₂Ti and FeTi phases and phase mixtures were formed above 850°C for 30 min and at 950°C for all bonding times. Maximum tensile strength of ～326 MPa, shear strength of ～254 MPa and impact toughness of ～24 J were obtained for the diffusion couple processed at 1000°C for 30 min and 30–180 min time interval at 950°C, and maximum tensile strength ～323 MPa, shear strength ～243 MPa and impact toughness of ～22 J were achieved when bonding was processed for 120 min. The residual stress of the bonded joints increases with the increase in bonding temperatures and times.     

Effect of hot working variables on microstructure and brittle to ductile transition in Ti–46.5Al–3Nb–2Cr–0.2W gamma titanium aluminide

Abstract

The microstructure and mechanical properties of a γ-TiAl alloy (Ti–46.5Al–3Nb–2Cr–0.2W, in at.-%) were studied in two conditions: (a) after conventional forging in the +γ phase field and (b) after subsequent isothermal forging in the ₂+γ phase field. Tensile tests were conducted in the temperature range 800–1000°C and strain rate range of 10^-3–10^-1 s^-1. The microstructure of the alloy in condition 1 was non-homogeneous consisting of about 90 vol.-% of small γ grains (grain size of 3 to 20 µm) and 10 vol.-% of coarse grains or lamellar regions. The alloy in this condition showed a brittle to ductile transition at about 950°C and extensive cavitation during deformation above the transition temperature. The microstructure in condition 2 was much more uniform and finer, and the transition temperature was decreased to 850°C. The alloy in condition 2 showed better deformability and cavitation resistance than that in condition 1 and superplastic behaviour at temperatures 900–1000°C.     

Influence of HIP processing on microstructure and mechanical properties of superalloy Udimet 720LI

Abstract

Inert gas (high purity argon) atomised powder of composition conforming to that of the superalloy Udimet 720 of low interstitial grade was hipped at 1200°C/120 MPa/3 h. The hipped alloy has shown near theoretical density and consisted of equiaxed grains with an average diameter of ～45 μm. While primary γ′-Ni₃ (Ti, Al) precipitates with an acicular morphology were found at the grain boundaries, finer secondary γ′ precipitates with near cuboidal morphology were present in the austenite γ matrix. The yield strength (YS) of the as hipped alloy was found to be the same as that of the wrought alloy heat treated for creep applications (termed as creep resistant alloy) at room temperature (RT) as well as at 650°C. However, the ultimate tensile strength (UTS) and ductility were found to be higher than those of the wrought creep resistant alloy. On the other hand, the YS and UTS of the as hipped material were lower than those of the wrought alloy heat treated for high strength applications (termed as high strength alloy), although the ductility of the former was comparable to that of the latter. In order to improve the strength, the hipped alloy was subjected to a heat treatment consisting of solution treatment followed by two-step aging. Extensive precipitation of fine and coarse γ′ precipitates with cuboidal morphology during duplex aging treatment has led to a considerable improvement in YS and UTS of the alloy, although the corresponding ductility dropped moderately at RT and at 650°C. Fractography of the tensile tested specimens has shown ductile transgranular mode of fracture in the as hipped alloy at RT and at 650°C, while the hipped+heat treated alloy exhibited a mixed mode of fracture at those temperature. The stress rupture properties of the as hipped alloy compare well with those of the wrought alloy and have been found to improve significantly after heat treatment. The present investigation reveals that the hipped superalloy Udimet 720LI has substantial potential for use in the development of near net shaped components for aerospace applications.     

Oxide scale stresses in polycrystalline Ni200

An X-ray diffraction (sin² ψ) method has been successfully used to measure the oxidation stresses at room temperature in annealed and electropolished samples of polycrystalline Ni200 coupons oxidized in the temperature range 760 to 982° C for 4 h. The stresses on the free surface of the oxide (σ ₁₁ andσ ²²) were compressive and the average stress through the thickness normal to the oxide layer was found to be tensile. Surface stresses on the oxides formed at temperatures up to 927° C were found to be isotropic and both surface stresses and the average normal stress increased with increasing temperature of oxidation. At 982° C, the surface stresses were lower and this was attributed to the deformation and fracture of oxide layer resulting in stress relaxation.