Hellmuth Klingelhöffer

Modelling the orientation and direction dependence of the critical resolved shear stress of nickel-base superalloy single crystals

Abstract The orientation and direction dependence of the critical resolved shear stress was determined experimentally for the chromium-rich superalloy SC 16 at 650, 750 and 850°C and a constant strain rate of 10−3 s−1. The results are used to establish an extended Schmid law for octahedral slip in the temperature and orientation range in which cross-slip pinning of dislocation pairs in the γ′ phase takes place. Normal Schmid behaviour was assumed for orientations near [111], for which cube slip was activated on a macroscopic level. Differences between some commercial superalloys are worked out and can be attributed to morphology and volume fraction of the γ′ phase. The orientation dependence and asymmetry effects increase in the order NIMONIC 105, SC 16, Rene N4. The orientation range where macroscopic cube slip can be expected increases in the same order. A close inspection of the parameters which are responsible for non-Schmid behaviour suggests that, in addition to cross-slip pinning, a matrix effect must be operating as well, partly counteracting the behaviour expected for mono-phase γ′ crystals.

In situ deformed specimen grating replication technique in moiré interferometry

High-sensitivity moire interferometry with the in situ deformed grating replication technique is adopted to investigate the deformations of a nickel-based polycrystal superalloy under uniaxial tensile loading. By multiple replication of several desired loading levels and observing them in moire interferometry, nearly real time deformation measurements can be conducted flexibly even when the testing machine is far away from the optical set-up. As an alternative to deformed specimen grating replication caused by mechanical loading, a thermally induced deformed specimen grating can also be replicated at an elevated temperature and deformation fields can be obtained by observing the replicated pieces in moire interferometry at room temperature. Thermally induced deformations of a microelectronic assembly are given as an example.

Multi-Axial Thermo-Mechanical Fatigue of a Near-Gamma TiAl-Alloy

A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (-TiAl) alloys. Structural components in aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermo-mechanical fatigue (TMF) behaviour of this -TiAl alloy have been examined at 400 – 800oC with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-of-phase (OP). The effects of TMF on the microstructure were also investigated. For the same equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at the same equivalent mechanical strain amplitude compared to the other types of tests.

Thermo-mechanical Fatigue of the Nickel-Base Superalloy Nimonic 90

The thermo-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850°C), and 180°-out-of-phase-tests (180° OP) where the maximum mechanical strain coincides with the minimum temperature (400°C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of Rε = - 1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180° OP tests is longer compared to identical IP tests. The stress / mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180° OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations.

Time Dependence of γ/γ' Lattice Mismatch in Creep-Deformed Single Crystal Superalloy SC16 at 1173 K

The profiles of 001 and 002 reflections have been measured at 1173 K as a function of time by means of X-ray diffraction (XRD) on tensile-creep deformed specimens of single crystal superalloy SC16. Decrease in line width (full width at half maximum: FWHM) by about 7 % and increase in peak position by about 3x10-4 degrees was detected after 8.5x104 s. Broadening of the 002 peak profile indicated a more negative value of the lattice misfit after the same time period. The results are discussed in the context of the anisotropic arrangement of dislocations at the γ/γ’ interfaces during creep and their rearrangement during the thermal treatment at 1173 K.

Failure estimation of thermo-mechanically loaded hot parts in turbochargers

Components directly exposed to the exhaust-gas streams of combustion engines are increasingly thermo-mechanically loaded. A keystone for dimensioning is the calculated failure prediction. Within the FVV research project No. 916 (“Hot Parts”), a model for the calculated simulation of thermo-mechanical fatigue was developed for the cast iron material SiMo 4.05 in the division Mechanical Behaviour of Materials at the Federal Institute for Material Research and Testing (BAM). Using this model, the location and the time of the occurrence of first cracks in exhaust-gas turbocharger housings may be estimated in good approximation.

Temperature dependence of lattice distortion in strongly creep-deformed single crystal superalloy SC16

Specimens of single crystal superalloy SC16 have been pre-deformed at 1223 K to strain of 15 % under tensile load of 150 MPa. The profiles of the 001 superlattice reflection of the γ’ precipitate phase as well as the profiles of the 002 reflections of the fcc matrix phase and of γ’-precipitate phase have been measured between RT and 1073 K by means of X-ray diffraction (XRD). From these measurements the lattice distortion of the γ’ precipitates and the lattice misfit between γ and γ’ phase have been determined as a function of temperature. The width of the 001 reflection as well as the lattice misfit decreased as a function of temperature. The results are ascribed to the anisotropic arrangement of dislocations at the γ/γ’ interfaces and to the different thermal expansion coefficients of the γ- and γ’-phase. A comparison of lattice misfit with previous measurements on moderately strained specimens suggests that the measured changes in lattice structure are predominantly created in stage I of creep-deformation.

Versagensabschätzung Thermomechanisch Belasteter Heissteile in Turboladern

Komponenten, die unmittelbar den Abgasstromen von Verbrennungsmotoren ausgesetzt sind, werden zunehmend thermisch und mechanisch belastet. Ein wesentlicher Baustein zur Auslegung ist dabei die rechnerische Versagensprognose. Im Rahmen des FVV-Forschungsvorhabens Nr. 916 („Heisteile“) wurde in der Fachgruppe Werkstoffmechanik an der Bundesanstalt fur Materialforschung und -prufung (BAM) fur den Eisengusswerkstoff SiMo 4.05 ein Modell zur rechnerischen Simulation thermomechanischer Ermudung entwickelt. Hiermit konnen Ort und Zeitpunkt des Auftretens erster Risse an Abgasturboladergehausen in guter Naherung abgeschatzt werden.

Low Cycle Fatigue of the Single-Crystal Nickel-Base Superalloy CMSX-4 — Anistropy and Effect of Creep Damage*

Abstract Low cycle fatigue of the single-crystal nickel-base superalloy CMSX-4 was investigated in the temperature range 700—950°C under strain controlled cyclic loading. The points of interest were the crystallographic anisotropy of low cycle fatigue behaviour and the effect of creep damage on low cycle fatigue life. For the testing conditions used the low cycle fatigue life of CMSX-4 is strongly orientation dependent as well as very sensitive to the formation of rafted γ/γ’-microstructure.

THERMOMECHANICAL DEFORMATION BEHAVIOUR OF IN 738 LC AND SC 16

ABSTRACT This article describes the study of uniaxial and biaxial thermo-mechanical fatigue (TMF) response of IN 738 LC and the initial experiments of single crystal superalloy SC 16. A life prediction assessment is proposed based on the inelastic work Σσ ij Δe ij in at N f /2. It is shown that the J 2 -theory is applicable to TMF-loadings. Initial experiments on single crystal superalloy SC 16 prove that there is a non-uniform strain distribution in the plastic region along the circumference of [001] orientated specimens. These findings must be weighed when performing TMF-tests.