Nickel-base Superalloy IN738LC Research Articles

Tertiary Creep in the Cast Nickelbase Superalloy IN738LC. Effects of Pretreatments

Tertiary Creep in the Cast Nickelbase Superalloy IN738LC. Effects of Pretreatments

Observations of deformation and fracture heterogeneities in a nickel-base superalloy using electron back scattering patterns

Electron back scattering patterns have been used to characterise the changes in structure that occur during creep deformation of a directionally solidified nickel-base superalloy IN738LC. A novel approach to the evaluation of pattern diffuseness, which is a measure of dislocation content of the material, in terms of a characteristic magnification has been proposed. There is an approximately linear relationship between this magnification factor and creep strain for 〈100〉 oriented crystals stressed parallel to the solidification direction and this is consistent with a strain-softening model of creep of superalloys. Analysis of transversely stressed specimens, both to give crystal orientations and to assess pattern diffuseness, shows that grain boundary damage correlates with large mis-matches in creep strain between adjacent grains.

On the threshold stress for dislocation creep in particle strengthened alloys

Abstract Current models describing dislocations by-passing particles by climb do not satisfactorily account for the creep deformation of alloys containing high particle volume fractions such as nickel-base superalloys. An alternative approach involving cooperative climb over groups of particles leads to a resistance to deformation, rather than a threshold stress, that is insensitive to the particle size but which depends strongly on applied stress. Detailed calculations have been carried out for the nickel-base superalloy IN738LC and these are in close agreement with measurements of the friction stress. The theoretical conclusions are generally compatible with the creep behaviour of nickel-base superalloys.

Precipitation of β phase in the γ’ particles of nickel-base superalloy

Cast nickel-base superalloy IN738LC was studied to identify the fine precipitate in coarse γ particles. The spot patterns of the single-crystal coarse γ phase and the ring patterns of the polycrystalline fine precipitates were simultaneously gained by selected-area electron diffraction in the same photograph. The results showed that not only is this fine precipitate not Ni3AlCx, but also it is not γ phase; it is theβ phase with bcc structure and its lattice parameterα ≈ 2.89 A.

Effect of environment on the low cycle fatigue behaviour of cast nickel-base superalloy IN738LC

Low cycle fatigue tests were conducted on cast nickel-base superalloy IN738LC in vacuum and air and on specimens coated with a layer of NaCl and Na 2SO 4 at 900°C and two different strain rates. The fatigue life, determined in terms of the cyclic plastic strain, decreased markedly as the severity of the environment increased, ie in the order vacuum, air, NaCl + Na 2SO 4. The marked strain-rate effect found in air and saline environments disappeared when the experiments were conducted in a vacuum of 1.3 × 10 −2 Pa. Cyclic plastic deformation had the effect of producing coarsening which was associated with cyclic softening. Crack nucleation occurred in favourably oriented grains in vacuum while for air and for NaCl + Na 2SO 4 coated specimens the cracks always initiated at hot-corroded grain boundaries near the surface or generally at oxide spikes in surface-connected grain boundaries. The crack propagation from these nucleation sites was essentially transgranular for all environments studied.

An investigation on the creep and fracture behavior of cast nickel-base superalloy IN738LC

The creep-rupture properties of cast nickel-base superalloy IN738LC were studied over the temperature range 750 to 950 °C. Our results show that primary and steady-state creep should not be regarded as distinct stages and that they have basically the same deformation mechanism. The dependence of the steady-state creep rate,es, on stress,δ, and on temperature,T, for this superalloy can be described ases = Aδ nexp(−Qc/RT).n = 8.3 - 9.8 andQc = 570 - 730 kJ mol−1 at high stress levels, whereasn = 4.1 - 4.9 andQc = 370 - 420 kJ mol−1 at low stress levels. The observations of dislocation structures during steady-state creep confirm that the creep mechanism is different in the high and low stress regimes. The observations of the microstructure show that the initial acceleration in creep rate during the tertiary stage is connected with changes in the size and distribution ofγ′ particles during creep. Rupture occurs by the propagation of oxidized intergranular cracks which initiate at the specimen surface, and the rate of crack propagation is controlled by the deformation behavior of the superalloy.

Microstructural contributions to friction stress and recovery kinetics during creep of the nickel-base superalloy IN738LC

The recovery kinetics and friction stress, σ 0, associated with high temperature creep of the directionally solidified nickel-based superalloy IN738LC have been determined by using a numerical method of analysis to extrapolate data from stress drop experiments. The variations of σ 0 and recovery kinetics with initial particle size, temperature and stress and throughout the creep life have been measured. It was found that during the creep test σ 0 remained fairly constant or decreased slightly while the recovery kinetics increased significantly, scaling with the increasing creep rate. High voltage transmission electron microscopy of specimens stopped at various stages in their creep lives revealed that the dislocation spacing at the semicoherent γ/γ′ interfaces decreased with increasing strain and it is proposed that this affects the creep rate through changes in recovery kinetics.

Low cycle fatigue behaviour of cast nickel-base superalloy IN738LC at room temperature

The cyclic stress/strain response and the low cycle fatigue life of cast nickel-base superalloy IN738LC were studied. Fully reversed strain-controlled tests were performed at room temperature and at two different strain rates. Optical and electron microscopy were used to study the processes of deformation and cracking during cycling. A power-law relationship between life-time and total (plastic as well as elastic) strain range was obtained, which is not influenced by the strain rate and by the frequency. Cracking was generally initiated at the surface microporosities and propagated along interdendritic paths. During cyclic deformation, only cyclic hardening occurred at room temperature.

Low cycle fatigue behaviour of cast nickel-base superalloy IN-738LC in air and in hot corrosive environments

The low cycle fatigue behaviour of as-heat-treated IN-738LC was studied in air and hot corrosive environments at 900°C and at two different strain rates. The fatigue lives markedly decrease as the environment becomes more severe and as the strain rate increases. Cyclic plastic deformation had the effect of producing γ′ coarsening which was associated with cyclic softening. The cracks were always initiated at hot corroded grain boundaries near the surface or generally at oxide spikes in surface-connected grain boundaries. The crack propagation from these nucleation sites is largely transgranular for both air and hot corrosive environments. The transition fatigue lives for IN-738LC are low and change with the environment, strain rate and temperature.

AlおよびPt-AlコーティングしたNi基超合金IN-738LCの耐燃焼ガス腐食性

Pack cementation process has been applied for aluminide and platinum-aluminide coating on a nickel-base superalloy IN-738LC. Burner rig test using light oil containing 0.33%S was carried out for the evaluation of hot corrosion resistance.Aluminide coatings have two coating layers. The outer layer is high-Al NiAl and the inner layer consists mainly of low-Al NiAl. Hot corrosion resistance of the aluminide coatings can be improved by increasing the thickness of the outer layer.In the burner rig tests at 850°C or below, the aluminide coatings whose outer layer thicknesses are about 80μ are more hot corrosion resistant than the platinum-aluminide coatings made by electroplating of platinum of 7μ thickness and aluminizing at 1120°C. This difference is attributed to the preferential attack of the PtAl2 dispersed in the outer layer of the platinum-aluminide coatings.In the burner rig test at about 1000°C, the platinum-aluminide coatings are more hot corrosion resistant than the aluminide coatings, because of dissolution of the undesirable PtAl2 particles into the matrix and the increased Pt concentration in the coating layer, both due to the rapid diffusion of atoms at the high temperature.