J. Daniel Whittenberger

Creep and tensile properties of several oxide dispersion strengthened nickel base alloys

The room temperature and 1365 K tensile properties and 1365 K tensile creep properties at low strain rates were measured for several oxide dispersion strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr and ODS Ni-16Cr-4J5Al. Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process. All alloys appear to possess a threshold stress for creep. It is believed that the threshold stress is associated with diffusional creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystalline ODS alloys. Threshold stresses for large grain size ODS Ni-20Cr and Ni-16Cr-4J5A1 type alloys are dependent on the grain aspect ratio. Because of the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated that the threshold stress may be the design-limiting creep strength property.

Tensile and creep properties of the experimental oxide dispersion strengthened iron-base sheet alloy MA-956E at 1365 K

A study of the 1365 K tensile properties, creep characteristics and residual room temperature properties after creep testing of the experimental oxide dispersion strengthened iron-base alloy MA-956E (Fe-20Cr-4.5Al-0.5Ti-0.5Y2O3) was conducted. The 1365 K tensile properties, particularly ductility, are strongly dependent on strain rate. It appears that MA-956E does not easily undergo slow plastic deformation. Rather than deform under creep loading conditions, the alloy apparently fails by a crack nucleation and growth mechanism. Fortunately, there appears to be a threshold stress below which crack nucleation andJor growth does not occur.

Elevated temperature mechanical properties of the iron base oxide dispersion strengthened alloy ma 956 bar

Abstract1144 to 1477 K elevated temperature tensile, stress rupture, and creep tests and residual room temperature tensile tests following creep exposures were conducted on the iron-base oxide dispersion strengthened alloy MA 956, nominally Fe-20Cr-4.5Al-0.5Ti-0.5Y203. While the majority of the testing was in the longitudinal bar direction, a few tests in the long transverse bar direction were also conducted. Under slow strain rate conditions in the longitudinal direction, MA 956 deforms via a crack nucleation and growth mechanism eventually leading to sudden fracture. The longitudinal direction is stronger than the long transverse direction. Small amounts (∼0.1 pct) of prior creep strain do not degrade subsequent room temperature tensile properties.

Elevated temperature compressive properties of Zr-modified NiAl

Small Zr additions are known to substantially affect the deformation behavior and strength of po-lycrystalline NiAl, yet little information is currently available regarding the high-temperature prop-erties of such alloys. Utilizing prealloyed powder technology, a series of four NiAl alloys have been produced containing from 0.05 to 0.7 at. pct Zr. The creep behavior of these alloys was characterized in compression between 1000 and 1400 K at strain rates ranging from ∼0.1 to 10-9 s-1. All the Zr-modified alloys were significantly stronger than binary NiAl under lower temperature and faster strain-rate conditions; however, the single-phase materials (Zr ≤ 0.1 at. pct) and binary NiAl had similar strengths at high temperatures and slow strain rates. The two-phase NiAl-Ni2AlZr alloys containing 0.3 and 0.7 at. pct Zr had nearly identical strengths. While the two-phase alloys were stronger than the single-phase materials at all test conditions, the degree of microstructural damage in the two-phase alloys due to internal oxidation during testing appeared to increase with Zr level. Balancing the poor oxidation behavior with the consistent strength advantage of the two-phase alloys, it is concluded that optimum elevated-temperature properties could be obtained in Heusler-strength-ened NiAl containing between 0.1 and 0.3 at. pct Zr.

Effect of strain rate on the fracture behavior at 1366 K of the bcc iron base oxide dispersion strengthened alloy MA 956

The tensile behavior of the oxide dispersion strengthened iron-base alloy MA 956 was investigated as a function of strain-rate ranging from 3.3×10−2 to 8.3×10−8 s−1 at 1366 K. All tests were conducted in the longitudinal direction on specimens machined from bar stock. Because of the microstructure of this alloy, all specimens were either single crystals or bicrystals with the boundary parallel to the gage length. Testing revealed that the strength was rather insensitive to strain-rate, the tensile ductility decreased with decreasing strain-rate, and for strain-rates ≤8.3×10−5 s−1, the alloy fractured in brittle manner. Evidence of transgranular cracking perpendicular to the applied stress was observed at all strain-rates; failure at strain-rates ≤8.3×10−5 s−1 was due to cracks which grow by the joining together of cavities ahead of the running crack. This alloy appears to possess a critical stress intensity factor for rapid crack growth.

Elevated temperature compressive steady state deformation and failure in the oxide dispersion strengthened alloy MA 6000E

A study of the compressive flow strength-strain rate behavior of the oxide dispersion strengthened (ODS) alloy MA 6000E has been conducted between 1144 and 1366 K. Specimens taken in the longitudinal, long transverse, and short transverse bar directions were tested at strain rates ranging from 2.1 × 10−5 s−1 to 2.1 × 10−7 s−1. The inherent compressive strength of MA 6000E was essentially independent of orientation. Testing at the higher temperatures and slower strain rates produced large scale cracking. Such cracks formed and propagated in bands in which slip had dissolved and redistributed the γ’ precipitates. Steady state deformation could be described through use of a threshold stress model of creep where threshold stresses were calculated as functions of temperature and orientation from the relatively fast flow stress-strain rate data and the assumption that the effective stress exponent was 3.5.

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.

Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys

A study was undertaken to determine if oxide dispersion strengthened (ODS) Ni-base alloys in wrought bar form are subject to a loss of room temperature tensile properties after elevated temperature creep similar to that found in a thin gage ODS alloy sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types. Tensile type test specimens were creep exposed in air at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, the appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation in the microstructure were interpreted as creep degradation effects. This work has shown that many ODS alloys are subject to creep damage. Degradation of tensile properties occurs after very small amounts (≲0.2 pct) of creep strain; ductility being the most sensitive property. The amount of degradation is dependent on the creep strain and is essentially independent of the alloy system. All the ODS alloys which were creep damaged possessed a large grain size (>100 μm). Creep damage appears to be due to diffusional creep which produces dispersoid free bands around boundaries acting as vacancy sources. Low angle and, possibly, twin boundaries were found to act as vacancy sources. The residual tensile properties of two alloys were not affected by prior creep parallel to the extrusion axis. One of these alloys, DS-NiCr(S), was single crystalline. The other alloy, TD-Ni, possessed a small, elongated grain structure which minimized the thickness of the dispersoid free bands produced by diffusional creep.

Effect of Simulated Earth Reentry Exposure on Mechanical Properties of Several Oxide Dispersion Strengthened and Superalloy Sheet Materials

The effects of simulated multiple reentry into the Earth’s atmosphere on the mechanical properties of several high temperature metallic sheet materials were evaluated. The materials included five thin gage (nominally 0.025 or 0.037 cm) oxide dispersion strengthened (ODS) alloys and two thin gage (nominally 0.037 cm) superalloys. Multiple reentry conditions were simulated through cyclic Plasma Arc Tunnel (PAT) exposure. PAT exposure generally consisted of 100 cycles of 600 s duration at 1255, 1366, or 1477 K in a Mach 4.6 airstream with an impact pressure of nominally 800 N/m2. PAT exposure generally produced a uniform oxide scale, oxide pits or intergranular oxidation, Kirdendall porosity, and alloy depletion zones except for the aluminum containing ODS alloys. Only a uniform oxide scale was formed on the aluminum containing ODS alloys. In some cases, the oxide scales contained copper, apparently from the PAT electrodes, thus PAT exposure may not be truly representative of reentry. Within the limits of the PAT exposures evaluated in this study, PAT exposure did not significantly affect the mechanical properties of the thin gage alloys evaluated. Thus, it appears that the microstructural changes produced by Plasma Arc Tunnel exposure have little influence on mechanical properties.