Papers by Keyword: Primary Creep

Abstract: Creep characteristics of alloys and compounds have been evaluated mainly by the minimum creep rate or the steady-state creep rate, and by its stress and temperature dependences. In some cases, however, direct comparison of the minimum creep rate or the steady-state creep rate are not practically easy due to difficulties of experiment, i.e., a long duration of primary stage of creep deformation. The minimum creep rates are not always precise representative value, which is directly evaluated from experiments. It should be valuable, if one could estimate the minimum creep rate from creep curve in primary stage. I have proposed a method of quantitative evaluation of creep curve based on the evaluation of strain rate change and its strain dependence during creep [1-3]. The value that reflects a shape of creep curve is named “Strain Acceleration and Transition Objective-Index (SATO-Index)” [4]. SATO-Index and related differential equation show a strain dependence of strain rate and lead entre creep curve by numerical integration. This concept provides quantitative information of shape of each creep curve, and information of the entire creep curve. In this paper, examples of evaluation and extrapolation of creep rate from primary stage in compression are presented. It is concluded that the extrapolation with the concept of SATO-Index reasonably provides imaginal minimum creep rate. Usability of extrapolation of creep curve by the concept is presented.

Abstract: The “Phase-Field Model of Dislocations” (PFMD) was used to simulate shearing of gamma-prime precipitate arrays in single crystal turbine blade superalloys. The focus of the work has been on the cutting of the L12 ordered precipitates by a<112>{111} dislocation ribbons during Primary Creep. The Phase Field Model presented incorporates specially developed Generalised Stacking Fault Energy (–surface) data obtained from atomistic simulations. The topography of this surface determines the shearing mechanisms observed in the model. The merit of the new –surface, is that it accounts for the formation of extrinsic stacking faults, making the model more relevant to creep deformation of superalloys at elevated temperatures.

Abstract: Primarystageofcreepprocessisaperiodwhenthecreeprateisdecreasingduetomi-crostructuredevelopment.Thedislocationdensityevolvestothedynamicequilibriumvaluerelatedtotheappliedstress.Inlow-strainratecreepexperiments,similarstageofdecreasingstrainrateisobservedalongwithapparenttransitiontosteadystate.Nevertheless,deeperanalysisofthevariousparametersofthestageshowsthatthesubstanceoftheprocessmustbedifferent.Themainreasonsare:i)thestrainistoosmallfordislocationsubstructurerearrangement,ii)stressandtemperaturedependenciesofthetransitionstrainandrelaxationtimeoftheprocessdoesnotcorrespondtoanyoftheknownprimarycreepprocesses,iii)thestrainreachedduringthestageisalmostfullyrecoverable,sothatthedeformationisratheranelasticthanplasticone.Thetransitionstrainobservedinlowstrainrateexperimentsthusshouldbetreatedasaspecial“pre-primary”stage,whiletheapparentsecondary(steadystate)stageisinfactpartoftheprimarycreepstage.

Abstract: Fully lamellar structures of powder metallurgy (PM), investment cast, and directionally solidified (DS) TiAl alloys containing β stabilizer were produced after stepped cool heat treatment, and interface β precipitates were formed after aging at 950°C. In addition, a columnar grain structure combined with a fully lamellar structure aligned with the load direction and interface β precipitates were formed by directional solidification and subsequent heat treatments. Creep test results of PM TiAl indicate that controlling the initial microstructures is also critical for balancing the primary and steady-state creep resistance during short and long-term tests. DS TiAl alloy exhibits a significant reduction of the primary strain and creep rate compared to polycrystalline TiAl due to the unique DS microstructure. Therefore, a DS microstructure with proper lamellar orientation and controlled interface β precipitation is the ideal if maximum time to a relatively small (<0.5%) strain is the design criterion of merit.

Abstract: The primary creep behavior at ambient temperature of typical h.c.p., b.c.c. and f.c.c. metals and alloys of annealed state was surveyed and the deformation mechanism of CP-Ti was discussed through transmission electron microscopy. Only h.c.p. metals and alloys demonstrated significant creep at ambient temperature. Arrays of straight screw dislocations of b=1/2<2110> and <2110> direction were observed in the crept CP-Ti.