Oxidation and the Effects of High Temperature Exposures on Notched Fatigue Life of an Advanced Powder Metallurgy Disk Superalloy
Oxidation and the effects of high temperature exposures on notched fatigue life were considered for a powder metallurgy processed supersolvus heat-treated ME3 disk superalloy. The isothermal static oxidation response at 704 C, 760 C, and 815 C was consistent with other chromia forming nickel-based superalloys: a TiO2-Cr2O3 external oxide formed with a branched Al2O3 internal subscale that extended into a recrystallized - dissolution layer. These surface changes can potentially impact disk durability, making layer growth rates important. Growth of the external scales and dissolution layers followed a cubic rate law, while Al2O3 subscales followed a parabolic rate law. Cr- rich M23C6 carbides at the grain boundaries dissolved to help sustain Cr2O3 growth to depths about 12 times thicker than the scale. The effect of prior exposures was examined through notched low cycle fatigue tests performed to failure in air at 704 C. Prior exposures led to pronounced debits of up to 99 % in fatigue life, where fatigue life decreased inversely with exposure time. Exposures that produced roughly equivalent 1 m thick external scales at the various isotherms showed statistically equivalent fatigue lives, establishing that surface damage drives fatigue debit, not exposure temperature. Fractographic evaluation indicated the failure mode for the pre-exposed specimens involved surface crack initiations that shifted with exposure from predominately single intergranular initiations with transgranular propagation to multi-initiations from the cracked external oxide with intergranular propagation. Weakened grain boundaries at the surface resulting from the M23C6 carbide dissolution are partially responsible for the intergranular cracking. Removing the scale and subscale while leaving a layer where M23C6 carbides were dissolved did not lead to a significant fatigue life improvement, however, also removing the M23C6 carbide dissolution layer led to nearly full recovery of life, with a transgranular initiation typical to that observed in unexposed specimens.
Oxidation behavior of several chromia-forming commercial nickel-base superalloys
Several commercially available Ni-base superalloys were exposed isothermally in air at temperatures between 750° and 1000°C and also under cyclic conditions at 1000°C. The kinetics of oxidation were determined and the scales were analyzed by electron microscopy and X-ray diffraction. Thin adherent chromia-rich scales formed on the alloys at 750°C after 1000 hr. Although Waspaloy showed the lowest weight gain in this test, it also showed the deepest internal corrosion due to oxidation of the grain-boundary carbides. At temperatures up to 1000°C the external scales were also chromia-rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile (TiO2) grains at the surface as well as forming TiO2 and TiN internally. The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxides below the chromia scale, although Astroloy when oxidized isothermally at 1000°C developed a semicontinuous internal layer of alumina due to its higher Al content. Under cyclic conditions Astroloy formed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.
A Historical Introduction to the Law of Obligations
1 PROLOGUE: THE PREHISTORY OF THE ENGLISH LAW OF OBLIGATIONS 2 STRUCTURAL FOUNDATIONS 3 UNITY AND FRAGMENTATION OF THE MEDIAEVAL LAW OF CONTRACT 4 TRESPASS, TRESPASS ON THE CASE, AND THE MEDIAEVAL LAW OF TORT 5 THE SUBSTANTIVE LAW OF TORTS 6 THE SUBSTANTIVE LAW OF CONTRACT PART 2 THE TRIUMPH OF TRESPASS ON THE CASE 7 TORT, PROPERTY, AND REPUTATION: THE EXPANSION OF THE ACTION ON THE CASE 8 THE RISE OF THE ACTION OF ASSUMPSIT PART 3 THE MODERN LAW OF TORT AND CONTRACT 9 TRESPASS, CASE, AND THE MORAL BASIS OF LIABILITY 10 THE LAW OF TORTS IN THE NINETEENTH CENTURY: THE RISE OF THE TORT OF NEGLIGENCE 11 THE LAW OF TORTS IN THE TWENTIETH CENTURY: EXPANSION AND COLLAPSE OF THE TORT OF NEGLIGENCE 12 FOUNDATIONS OF THE MODERN LAW OF CONTRACT 13 THE RISE OF THE WILL THEORY THE WILL THEORY AND THE CLASSICAL MODEL OF CONTRACT 14 THE DECLINE OF THE WILL THEORY: LEGAL REGULATION AND CONTRACTUAL FAIRNESS PART 4 UNJUST ENRICHMENT 15 UNJUST ENRICHMENT 16 LEGAL CHANGE AND LEGAL CONTINUITY