Creep of udimet 500 during thermal cycling: II. The time to failure

The effect of rapid temperature cycling on the time to failure of Udimet 500 superalloy was investigated. It was found that the time to failure is determined only by the minimum creep rate and not by the thermal history. For this reason the time to failure under thermal cycling conditions is longer than the time to failure calculated from isothermal data.     

Thermal Fatigue Behaviour of Co-Based Alloy Coating Obtained by Laser Surface Melt-Casting on High Temperature Alloy

Inthefieldofaviation,aswormgearvanesofanaircraftenginerevolveathighspeed,theyarebeatenbydustsandparticlesintheair.Afterservingforalimitedtime,theywouldgraduallyproducecorrosionpits,anditispossiblethatthesetinycorrosionpitsbecomecrackinitiation.Therefore,itisconsideredtocoatalayerofalloybylasermelt-castingonthedamagedplacesofvanes,inordertomakethepropertiesofthecastinglayerapproachorsurpassthoseofsubstrate,andgetthevanestoservenormally.Thuscostscanbereducedandapossiblecomprehensiveeconomicbenef…     

The effect of rapid thermal cycling on the creep properties of alpha iron

The effect of rapid, low amplitude thermal cycling on the creep properties of alpha iron was investigated. It was found that creep rates under thermal cycling conditions are lower than the creep rates calculated from steady-state isothermal creep experiments. This relative decrease of the creep rate was dependent on the dwell time and the applied stress. The maximum decrease in creep rate occurred when the time at the lower temperature was very short. In single cycle creep experiments it was found that an abrupt temperature decrease was followed by a delay period of zero creep rate. When the temperature was again increased, a period of inverse transient creep was observed. It is concluded that the inverse transient creep is responsible for the relatively lower creep rates observed under thermal cycling conditions. Investigations of the dislocation substructure with the electron microscope did not show any significant changes attributable to thermal cycling. The lowering of the creep rate is tentatively explained on the basis of dislocation pinning by point defects during the cooling part of the cycle which inhibits subsequent dislocation motion during the rest of the cycle and hence decreases the overall creep rate. D. EYLON, formerly with the Department of Materials Engineering, Technion     

Microstructure and thermal cycling behavior of nanostructured yttria partially stabilized zirconia (YSZ) thermal barrier coatings

Nanostructured yttria partially stabilized zirconia(YSZ) coatings were prepared by atmospheric plasma spraying(APS) using the conglomeration made by zirconia nanoparticle as the raw materials.The measurement methods,which consisted of scanning electron microscopy(SEM),transmission electron microscopy(TEM) and thermal cycling behavior,were used to character the morphology,composition and thermal oxidation behavior of the powder and the coatings.From the results,it was shown that the YSZ coating was the laminar structure,and the elements distribution in the bond and top coat were well-proportioned.The YSZ coatings were composed of fine grains with size ranging from 30 to 110 nm.The laminar layers with columnar grains were surrounded with unmelted parts of the nanostructured powder and some equiaxed grains.In the as-sprayed nanostructured zirconia coatings,there existed pores that were less than 1 μm.The cracks were observed on some of the crystal border.The cyclic oxidation experiment showed that the nanostructured coating had longer thermal cycling lifetime to exhibit the promising thermal cyclic oxidation resistance.The failure of the nanostructured TBC was similar to the failure of conventional APS TBC.     

Thermomechanical fatigue of a lead alloy

The effects of superimposing a thermal cycle onto a strain-controlled fatigue cycle have been studied for a lead-base alloy containing a minor amount of tin. These tests have been conducted at temperatures near half the melting point under conditions of cycling which included strain ranges from 0.3 to 3 pct and periods between 184 and 1040 seconds per cycle. Cracks were seen to initiate rapidly as grooves along grain boundaries and grow inward until a drop in the load-carrying capacity of the sample was observed when a significant fraction of the crack population exceeded approximately 300 μm in depth. The presence of an in-phase thermal cycle was seen to substantially reduce the number of cycles to failure relative to a reference isothermal test at the highest temperature of the thermomechanical cycle. For a given set of test conditions, the number of cycles to failure increased with the angle of phase lag of the mechanical cycle with respect to the thermal cycle. Decreasing frequency was found to increase the number of cycles to failure in thermomechanical fatigue (TMF), while the opposite was observed for the isothermal reference tests. Hold time was found to reduce the number of cycles to failure in TMF.     

Effect of thermal cycling on the transformation temperatures of TiNi alloys

The effect of thermal cycling on the transformation temperatures was investigated in TiNi alloys which were subjected to various types of thermo-mechanical treatments. In solution-treated specimens with various Ni-contents, the transformation temperatures were found to change with thermal cycling. However, in aged Ni-rich specimens and those annealed at a temperature lower than the recrystallization temperature after cold working, they were found to be constant even after thermal cycling. Transmission electron microscopy revealed that dislocations were introduced by thermal cycling in the solution-treated specimens and their density increased with increasing number of thermal cycles. However, in the specimens with constant transformation temperatures, no substantial change in the microstructures was observed with thermal cycling. On the basis of these results, it is concluded that the shift of the transformation temperatures with thermal cycling is due to the introduction of dislocations, but not due to any ageing effect nor due to a change in the degree of order during thermal cycling.     

Enhanced densification of cavitated dispersion-strengthened aluminum by thermal cycling

We report experimental data of creep cavity shrinkage for dispersion-strengthened-cast aluminum with about 23 vol pct submicron Al₂O₃ dispersoids, annealed isothermally or subjected to thermal cycling without applied stress. Thermal cycling is found to increase the rate of densification by a factor of 3 to 5.5 relative to isothermal annealing at the upper cycling temperature, allowing for recovery of full theoretical density in a shorter time. Isothermal densification is discussed in light of a diffusive cavity shrinkage mechanism, and a model considering thermal mismatch stresses is employed to rationalize the enhanced rate of densification observed during thermal cycling. Intermittent thermal-cycling densification is shown to improve creep life of dispersion-strengthened aluminum through the suppression of tertiary damage accumulation processes.     

Dependence of thermal residual stress on temperature in a SiC particle-reinforced 6061Al alloy

The thermal stresses (TS) in the matrix of a SiC _p /6061Al composite during thermal cycling were measured by X-ray diffraction. Also, the TS during thermal cycling and residual stress distribution (RSD) at room temperature in the two phases of composite were calculated by finite element modeling (FEM). The measured and calculated results indicated that the closed stress-temperature loop was formed during thermal cycling. The stress state in the matrix changed from tension to compression during heating and from compression to tension during cooling. Plastic deformation took place in the matrix of the composite during thermal cycling. The general change trend of TS with temperature during thermal cycling was in agreement between the experiment and calculation.     

The influence of a martensitic phase transformation on stress development in thermal barrier coating systems

Thermal barrier coatings (TBCs) provide thermal insulation and oxidation protection of Ni-base superalloys in elevated temperature turbine applications. Thermal barrier coating failure is caused by spallation, which is related to the development of internal stresses during thermal cycling. Recent microstructural observations have highlighted the occurrence of a martensitic bond coat transformation, and this finite-element analysis was conducted to clarify the influence of the martensite on the development of stresses and strains in the multilayered system during thermal cycling. Simulations incorporating the volume change associated with the transformation and experimentally measured coating properties indicate that out-of-plane top coat stresses are greatly influenced by the presence of the martensitic transformation, the temperature at which it occurs relative to the strength of the bond coat and attendant bond coat plasticity. Intermediate values of bond coat strength and transformation temperatures are shown to result in the highest top coat stresses. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

The effect of rapid thermal fluctuations on the creep rate in Inconel 718

The creep properties of Inconel 718 under thermal cycling conditions were investigated. It was found that the creep rates under thermal cycling conditions are lower than the creep rates calculated from results of isothermal creep experiments. It was also found that an abrupt temperature decrease during steady-state creep is followed by a certain period of zero creep rate. It was shown that the decrease of strain rate during creep under cycling conditions is due to this stopping effect.     

Assessment of the Performance of Sn–3.5Ag/Cu Solder Joint Under Multiple Reflows,Thermal Cycling and Corrosive Environment

The solder joint performance of Sn–3.5Ag/Cu combination was studied under multiple reflows, thermal cycling and exposure to the corrosive environment. Factorial experiment was carried out to assess the effect of individual parameters and the interaction of parameters on the shear strength of the solder joint. The results showed that the combination of thermal cycling and immersion in corrosive media resulted in the maximum decrease in the shear strength followed by the combination of multiple reflows and corrosive media. The shear strength reduced with the increase in immersion duration in corrosion medium. Factorial experiment was analyzed using analyis of variance (ANOVA). The results indicated that the individual parameters had a significant effect, whereas the effect of interaction of these parameters was less significant on the performance of the solder joint. Fracture surface indicated mixed mode of failure and the occurrence of fracture predominantly in the bulk solder.     

热循环温度场作用下1420Al-Li合金焊缝组织和拉伸性能能的演变规律

利用外约束型模拟空间热循环温度场试验设备对1420Al-Li合金焊缝进行了热循环（77～393K）试验，测量了热循环前后焊缝的拉伸性能，并观察了热循环前后焊缝的显微组织，讨论了热循环对焊缝组织和拉伸性能的影响。试验结果表明，经1000次或3000次热循环后，焊缝的强度和延伸率显著降低。热应力使焊缝显微组织产生损伤，主要表现为从晶界处向晶内发射位错，在晶界处形成位错塞积群，晶粒内位错密度逐渐升高。随着热循环次数的增加，组织损伤的不断累积导致在晶界处产生的应力集中程度增大，这是导致合金焊缝强度和延伸率下降的主要原因。     

热循环温度场作用下1420A1-Li合金焊缝组织和拉伸性能的演变规律

利用外约束型模拟空间热循环温度场试验设备对1420Al-Li合金焊缝进行了热循环(77～393 K)试验,测量了热循环前后焊缝的拉伸性能,并观察了热循环前后焊缝的显微组织,讨论了热循环对焊缝组织和拉伸性能的影响.试验结果表明,经1000次或3000次热循环后,焊缝的强度和延伸率显著降低.热应力使焊缝显微组织产生损伤,主要表现为从晶界处向晶内发射位错,在晶界处形成位错塞积群,晶粒内位错密度逐渐升高.随着热循环次数的增加,组织损伤的不断累积导致在晶界处产生的应力集中程度增大,这是导致合金焊缝强度和延伸率下降的主要原因.     

快速循环相变对GCr15SiMn钢组织和性能的影响

在Gleeble1500D热模拟试验机上对GCr15SiMn钢进行了不同道次的快速循环相变处理。结果表明,随循环次数的增加,试验钢组织趋于均匀,晶粒逐渐细化,冲击韧性和硬度得到不同程度的提高。相变硬化再结晶是晶粒细化的主要原因。采用适当的快速循环相变处理工艺可使GCr15SiMn钢的综合力学性能得到有效提升。     

The effect of thermal cycling on the structure and properties of a Co,Cr, Ni-TaC directionally solidified eutectic alloy

A Co, Cr, Ni matrix, TaC reinforced, directionally solidified eutectic alloy was tested under conditions of thermal fatigue. The material was cycled between 427 and 1093°C for up to 5000 cycles. The stress rupture properties are decreased by thermal cycling. This loss in stress rupture properties varies with the number of cycles; the initial loss in properties at about 200 cycles is relatively high. The stress rupture strength improves with further cycling up to 2000 cycles and decreases for a larger number of cycles. The TaC fibers in the cycled material developed surface serrations which increased with maximum cycling temperature and number of cycles. The formation of these serrations is attributed to the increasing solubility of the TaC fibers in the matrix with temperature. Since the easy growth planes of the TaC fibers were not coincident with the fiber faces exposed to the matrix, the TaC which was dissolved and reprecipitated during thermal cycling formed these serrations on the easy growth planes of the fibers. These serrations act as notches which reduce the tensile strength of the fibers. Formerly Graduate Student, Case Western Reserve University     

Response to thermal cycling of tool materials under steel thixoforming conditions

Abstract

The principal failure mechanism of steel thixoforming dies is thermal fatigue owing to forging pressures much lower than those encountered in conventional forging. This makes a properly designed thermal fatigue test the best method to identify suitable tooling materials for the steel thixoforming environment. Samples of X32CrMoV33 hot work tool steel and CrNiCo alloy were cycled thermally between 450 and 750°C, every 60 s for a total of 1500 cycles. While the thermal stresses generated at the surfaces of the two materials were very similar, their responses to thermal cycling were markedly different. The X32CrMoV33 steel was softened by nearly 40% after only 400 cycles, raising serious concerns over its temper resistance under steel thixoforming conditions. The extensive oxidation and subsequent spalling of oxide scales suffered by the X32CrMoV33 hot work tool steel is also a major shortcoming. The performance of the CrNiCo alloy, on the other hand, was judged to be satisfactory with a much thinner heat affected zone and a much better oxidation resistance. Lack of evidence for heat checking in this alloy after 1500 cycles is an encouraging sign.     

Quantifying thermomechanical fatigue of light-metal-matrix composites by mechanical spectroscopy

This article reviews recent progress in understanding the stress-relaxation mechanisms in metal-matrix composites (MMCs) subjected to thermomechanical fatigue. Mechanical loss, dynamic shear modulus, and permanent torsional-strain measurements have been performed with forced oscillations during thermal cycling. A transient mechanical-loss maximum, which is absent in the monolithic material, appears during cooling. It has been attributed to the development of plastic zones around the reinforcements by dislocation generation and motion, which result from the differential thermal contraction of the matrix and reinforcement. This damping maximum is strongly dependent on both measurement and material parameters. The reversible shear-modulus evolution during thermal cycling suggests that no interfacial debonding occurs. In unalloyed matrices, extended thermal-stress-induced pleasicity occurs, leading to a plateau in the shear modulus, which is recovered at low temperatures by plastic-zone overlapping and matrix strain hardening. Simultaneously measured strain-temperature loops exhibit both reversible and permanent plasticity during thermal cycling (strain ratcheting).     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Anti-implantation activity of antiestrogens and mifepristone

PURPOSE: Strong durable bonds between resin cements and metal alloys are critical to the success of resin-bonded, resin-veneered, or resin-retained prostheses. However, few comprehensive, comparative evaluations of materials or the fatigue effects of thermal cycling have been reported. The rate of strength loss may be a more important predictor of long-term success than bond strength. The purpose of this study was to investigate the effects of artificial aging by thermal cycling and resin cement type on the bond strengths to a base metal alloy. MATERIAL AND METHODS: This study investigated the effect of the number of thermal cycles (0, 1, 10, 100, 1,000, and 10,000) on the bond strengths of nine fixed prosthodontic resin cements. Specimens were assigned randomly to thermal cycle number/cement type test groups. Cylinders of a base metal alloy were bonded in an end-to-end configuration. One end of each bonded specimen was insulated, and the specimen was thermal cycled. Then, the bonds were tested in shear and bond strengths calculated. RESULTS: Two-way ANOVA revealed that the effects of cement type, the number of thermal cycles, and their interaction all significantly affected bond strength (p < .0001). Multiple range analysis showed that some cements had significant trends to lose bond strength with thermal cycling (p < .05), while others did not (p > .05). CONCLUSIONS: Both the type of resin cement and the amount of thermal cycling influenced bond strength to a base metal alloy. Some materials displayed more rapid loss of bond strength than others.     

Creep of udimet 500 during thermal cycling: I. The minimum creep rate

The effect of rapid temperature cycling on the minimum creep rate of Udimet 500 super-alloy was investigated. It was found that an abrupt temperature decrease during creep is followed by a delay period of zero creep rate. Due to this delay period the creep rate under thermal cycling conditions is lower than the creep rate calculated from the steady state creep rate at constant temperature.