Long-term Thermal Exposure Research Articles

Influence of long-term thermal exposure on the tensile properties of a high-temperature titanium alloy Ti-55

Stability of Ti-55 after a long-term thermal exposure was studied on samples taken from 25 mm rolled bars. In order to evaluate the variation of mechanical properties with service time, the tensile properties of this alloy after exposure at 500, 550 and 600°C for times up to 3000 h were investigated. The samples were exposed under two conditions: before and after machining the specimens. It was found that both yield strength and fracture strength increase after exposure. However, both the tensile elongation and reduction in area decrease with exposure time at all three exposure temperature. For the exposure of bars, even after 500 h at 600°C both the tensile elongation and reduction in area remain unchanged with exposure time. Examination of the microstructure by optical and electron microscopy shows that the factors that influence stability are: surface oxidation, decomposition of the residual beta phase and precipitation of the alpha-2 Ti 3X phase.

Injection moulding of inconel 718 aerospace components: J.Spirko et al. (Concurrent Technology Corp, USA.)

Inconel 718 was subjected to various heat treatments, i.e., solution heat treatment, standard ageing treatment and standard ageing plus 700 °C thermal exposure. The mechanical properties of the alloys were determined using tensile tests and Charpy pendulum impact tests at 650 °C and room temperature, respectively. The highest yield strength of 988 MPa was attained in the standard aged specimen, whereas a maximum impact toughness of 217 J cm−2 was attained in the solution-treated specimen. After thermal exposure, the mechanical properties of the specimens degrade. Both the yield strength and impact toughness decreased monotonically with increasing thermal exposure time. Subjected to a 10000-h long-term thermal exposure, the yield strength dramatically decreased to 475 MPa (almost 50% of the maximum strength), and the impact toughness reduced to only 18 J cm−2. The microstructures of the specimens were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Coarsening of γ′ and γ″ and the transformation of γ″ to δ-Ni3Nb was observed after thermal exposure. However, a complete transformation from metastable γ″ to δ-Ni3Nb was never accomplished, even after the 10000-h long-term thermal exposure. Based on the obtained experimental results, the effects of the microstructural evolution on the mechanical properties are discussed.

Correlation of grain-boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure

The effect of thermal exposure on grain-boundary precipitation in Al–Cu–Mg–Ag alloy was studied using quantitative transmission electron microscopy. Grain-boundary precipitate parameters, such as average size, number density and precipitate-free zone width, were measured. The effective diameter of precipitates, number of precipitates per grain-boundary area and area fraction of precipitates on the grain boundary were calculated. These data were applied to a grain-boundary fracture model to calculate grain-boundary fracture strain. The calculated fracture strains, in turn, were used to check the validity of two existing models of fracture toughness, which are based on grain-boundary nucleation of cracks and their propagation through precipitate-free zones. The fracture toughness model of Hornbogen and Graf closely agrees with the experimental results.

Influence of long-term thermal aging on the microstructural evolution of nuclear reactor pressure vessel materials: an atom probe study

Atom probe field ion microscopy (APFIM) investigations of the microstructure of unaged (as-fabricated) and long-term thermally-aged (~100 000 h at 280°C) surveillance materials from commercial reactor pressure vessel steels were performed. This combination of materials and conditions permitted the investigation of potential thermal aging effects. This microstructural study focused on the quantification of the compositions of the matrix and carbides. The APFIM results indicate that there was no significant microstructural evolution after a long-term thermal exposure in weld, plate and forging materials. The matrix depletion of copper that was observed in weld materials was consistent with the copper concentration in the matrix after the stress relief heat treatment. The composition of cementite carbides aged for 100 000 h were compared to the Thermocalc™ prediction. The APFIM comparisons of materials under these conditions are consistent with the measured change in mechanical properties such as the Charpy transition temperature.

γ′ Precipitate splitting in Nickel-based superalloys: A 3-D finite element analysis

The effect of Ta on the microstructure stability of powder metallurgy (PM) Ni-base superalloy was examined at standard heat treatment and after long-term thermal exposure. The results reveal that the previous particle boundary structure is eliminated with the addition of Ta. As Ta content increases, the γ' morphology changes from spheres to cuboids and begins to split, which is caused by the increase in the γ/γ' mismatch. Reversal coarsening of γ' particles in Ta-containing alloys are observed during exposure at 750 °C. The main reasons are related to γ/γ' lattice mismatch, elements partitioning and soft impingement. Besides, the increase of Ta content promotes the precipitation of σ phase at grain boundaries and in grains.

The effects of strain rate and long-term aging on the properties of (Fe,Ni) 3V LRO alloys

This study investigated the effect of strain rate and long-term thermal aging on the mechanical properties of the (Fe,Ni) 3 V long range ordered (LRO) alloys. Tensile tests covering strain rates from 10 −6 to 10 0 s −1 were performed on the ordered alloys at room temperature. The results indicate that there is little or no effect of strain rate on the tensile strength and ductility. The long-term thermal exposure at 600°C does not cause any significant change in the tensile properties. The deformation and fracture characteristics and with controlling mechanisms change as a function of strain rate and aging time.

Adhesive evaluation of LARC-TPI and a water-soluble version of LARC-TPI

A linear thermoplastic polyimide, LARC-TPI, developed several years ago at NASA Langley Research Center, showed promise as a high temperature adhesive for application in aircraft and spacecraft based on the material's toughness, flexibility, good thermal and thermo-oxidative stability, and other attractive properties. Since then, a commercial product manufactured and supplied by Mitsui Toatsu Chemicals, Inc, Tokyo, Japan, has become available. In addition, a water-soluble version containing N,N-dimethylethanol amine salt was supplied by United Technologies Research Center, East Hartford, CT, USA. The water-soluble version is highly attractive because of the safety requirements involved with organic solvents presently used for polyimide materials. This report details the results of a study to evaluate the two LARC-TPI materials as high temperature thermoplastic adhesives and primers for bonding titanium (6Al-4V) adherends. A limited characterization of the materials was performed using a diffuse reflectance-Fourier transform infrared spectroscopy technique. Thermomechanical analysis and torsional braid techniques were used to determine the glass transition temperature. The effects of long-term thermal exposure (up to 1000 h) at 204°C and 72 h water-boil on the strength of the adhesives was determined by simple lap shear tests.

Creep and Rupture Behavior of Weld-Deposited Type 16-8-2 Stainless Steel at 593°C

The creep and rupture behavior of weld-deposited Type 16-8-2 stainless steel at 593°C was investigated over the time range from 3.6 × 104 s to 2.5 × 107 s. Equations relating stress (σ) to the time to rupture (tr), the time to the onset of tertiary creep (tt), and the time to produce a given creep strain (tεc) were obtained: lntr=81.3456−12.176lnσlntt=80.0786−12.176lnσlntεc=Aεc−13.0lnσAεc=87.3633+1.32066lnεc+0.446428[lnεc]2+1.08131[lnεc]3+6.96513×10−3[lnεc]4 The experimental results indicate that the control of welding parameters (e.g. current, voltage, and travel speed) within reasonable ranges can yield weld deposits with consistent time-dependent properties. Limited data suggest that high temperature (1065°C) post-weld annealing significantly alters only the flow curve for plastic deformation, while long-term thermal exposure at an intermediate temperature (565°C) produces only minor changes in either the plastic deformation or creep behavior of the weld materials.