Mechanical properties of multicomponent titanium alloy

A β-titanium alloy, obtained by cooling a melt at a rate of ∼ 800°C/s, has been studied by the methods of scanning and transmission electron microscopy, short-duration and long-duration macroindentation, as well as uniaxial compression in a temperature range of 20–1000°C. The alloy in the solid state contains ultrafine dendritic crystals, in the interstices between which are nanosized particles of minor phases. The alloy may be regarded as heat-resistant alloy: it possesses a high thermal stability of mechanical properties, higher values of high-temperature strength and creep resistance.     

Fracture Behavior of Single-Crystal Alloys Under Thermocyclic Loading

Strength of Materials - Experimental study results for thermal fatigue fracture of sand-glass specimens from a ZhS32 single-crystal alloy with different crystallographic orientations are presented...     

Martensitic transformation behaviour of non-equilibrium heat treated Ni50·9Ti49·1 alloys

Abstract

Ni50·9Ti49·1 specimens were heat treated using a thermal simulator. The martensitic transformation behaviours of selected areas of the thermal simulating treated specimens were studied with resistivity temperature measurements. In the thermal simulating process specimens were heated by a large electric current to a given peak temperature (400, 500, 600, 800, 900 or 1100°C respectively) and immediately water cooled to room temperature. As the two ends of a NiTi alloy specimen were fixed in copper jigs, unequal heat treatment effect areas were formed in the specimen segments near its two ends. In the unequal area of an 800°C thermal simulating treated sample, a wide transformation temperature range phenomena appeared. The experimental results indicate that non-equilibrium heat treatment proves to be an effective method to fabricate transformation temperature gradient shape memory materials.     

Multidisciplinary analysis of the operational temperature increase of turbine blades in combustion engines by application of the ceramic thermal barrier coatings (TBC)

The improvement of the temperature resistance of the aircraft engine elements can be obtained by application of a single ceramic thermal barrier coating (TBC) (e.g. Noda [1]) or several composite layers (e.g. Sadowski [2]). Engine elements protected by TBC can work safely in elevated temperature range above 1000 °C. Continuous endeavour to increase thermal resistance of engine the elements requires, apart from laboratory investigations, also numerical study of the different aero-engine parts. The most important are turbine blades, where high temperatures and stress concentrations during thermal shocks or thermal fatigue can be observed during engine exploitation. The high temperatures and stress concentrations can act as the local sources of damage initiation and defects propagation in the form of cracks.The present paper deals with the solution of the transient temperature transfer problem in bare and thermal barrier coated alloy Inconel 713 for the temperature range up to 1000 °C. The computational fluid dynamics (CFD) part of analysis was performed by application of ANSYS Fluent code receiving the temperature field of combustion gas, whereas computational structural mechanics (CMS) part concerning the temperature distribution inside the turbine blade was done by ABAQUS. Finally, the efficiency of the TBC layer (0.5 mm thickness) protecting and cooling channels was discussed in order to explore the operational temperature increase in the aero-engines.     

Study on EB-PVD Zirconia Thermal Barrier Coatings for Gas Turbine Blade Protection

Thermal barrier coatings on hollow turbine blades of DD3 were studied. The DD3 single crystal alloy has excellent mechanical properties, equivalent to that of PWA 1480. The results show that ZrO₂ coatings consist of oriented columnar grains. The coating is composed of t' cubic and tetragonal phases and there is an alumina layer at the ZrO₂-NiCrAlY alloy interface after aging for 30 min. at 1050°C. No degradation occurred to the EBPVD ceramic coatings after 200 thermal cycles at 1100°C for 10 min., air cooling and 100 hours high temperature oxidation at 1100°C.     

High-temperature tensile and creep deformation of cross-weld specimens of weld joint between T92 martensitic and Super304H austenitic steels

The high-temperature mechanical behavior of cross-weld specimens prepared from a dissimilar weld joint between T92 martensitic and Super304H austenitic heat-resistant steels incorporating Ni-based weld metal was evaluated at temperatures up to 650 °C. For both high temperature tensile and creep tests, failure took place in T92 due to its faster degradation with temperature increase. The heat-affected zone of T92 played a critical role during creep deformation, resulting in type IV failure under the long-term creep condition. For the creep specimens, the location of failure shifted from the base metal region to the fine-grained heat-affected zone as the creep duration time increased from the short-term to the long-term condition. The massive precipitation of Laves phase on the grain boundaries of the fine-grained heat-affected zone during creep deformation was observed and found to be responsible for the accelerated void formation in the area leading to the premature failure.     

Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

Thermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.     

Effect of annealing temperature on the microstructure and mechanical properties of an as-rolled Mg-9wt.%Li-3wt.%Al-1wt.%Zn alloy sheet

This study investigated the effect of annealing temperature on the mechanical properties of an as-rolled Mg-9.26wt.%Li-3,03wt,%Al-1,10wt.%Zn （LAZ931） alloy sheet. The dual-phase （a ＋ 13） LAZ931 alloy plate of 3 mm in thickness were roiled （67% reduction） and then annealed at temperatures at 100℃-350℃. The alloy＇s ductility showed a sharp concave downward tendency as a function of annealing temperature, The elongation of the LAZ931 alloy sheet increased with annealing temperature up to 150℃, followed by a sharp decrease of the alloy＇s ductility as the annealing temperature higher than 150℃. The specimen exhibited an extremely low elongation （only -0,5%） at annealing temperature around 300℃. Formation of brittle AILi particles on boundary resulted in Li depletion zone near by grain boundary, transforming the Li depletion zone into a （hcp） layer. The combined effects including brittle AILi particles on boundary and the hcp α layer on boundary resulted in the brittlement of the high-temperature-annealing sample.     

Stability of amorphous Fe-W alloys in multilayer metallizations on silicon

Co-sputtered amorphous Fe_0.37 W_0.63 alloys were investigated for applications as diffusion barriers in multilayer metallizations on silicon. Interface reactions and recrystallization during thermal annealing at 400–800°C were studied by back-scattering spectrometry and X-ray diffraction. On SiO₂ substrates the recrystallization of these films occurs at approximately 700°C. On silicon the recrystallization is accompanied by the formation of a silicide layer containing FeSi₂ and WSi₂ phases. No detectable reaction is observed when the alloy film is amorphous. In contact with an overlay metal such as aluminum, copper, nickel or platinum the amorphous Fe_0.37 W_0.63 layer prevents direct interaction between the silicon substrate and an overlay metal film 1000 Å thick during thermal annealing for 30 min at 650°C. The lifetime of the barrier is limited by dissolution and compound formation at the interface and at grain boundaries of the overlay metal.     

High-velocity impact damage behavior of plain-woven SiC/SiC composites after thermal loading

This study investigates characteristics of foreign-object damage in plain-woven SiC/SiC composites after thermal loading. High-speed impact tests were conducted on virgin specimens, thermally exposed specimens, and thermally shocked specimens, in which the maximum temperature during thermal loading was 600 °C or 1000 °C. An oxide layer was generated on the specimen surface by thermal loading at 1000 °C. Damaged areas on the front and back surfaces induced by particle impact were independent of thermal loading. However, in specimens thermally loaded at 1000 °C, brittle failure, i.e. cone cracking without fiber pull-out, occurred due to oxidation of the fiber/matrix interfaces, and the ballistic limit velocity significantly decreased. Finally, the ballistic limit is predicted using static strength properties, and the effect of thermal loading on impact resistance is discussed.     

Stability of microstructure and mechanical properties of K17G cast nickel-base superalloy during prolonged aging

K17G cast nickel-base superalloy has an attractive combination of mechanical properties: high strength and good ductility as well as low density. In particular, after prolonged exposure at 750, 850 (both up to 10000 h) and 950°C (up to 1500 h), this alloy proved to have excellent phase stability and no sigma phase was found. The effects ofprolonged exposure on high temperature tensile and stress rupture properties were not serious and decreased gradually with aging time. The change in properties of specimens taken directly from the turbine blades followed those ofthe cast-to-size specimens. The rupture lives, tensile and rupture ductilities were approximately one third lower compared with those ofthe cast-to-size specimens, but still remained at reasonably high levels.     

Failure Investigation of a Furnace Tube Support

A furnace tube support failed after 6 months service at 850 °C. The support was an HK alloy, a member of the heat-resistant cast alloy family (H-Series) steels. The H-series steels are widely used in the petrochemical industry for components requiring enhanced high-temperature properties. Microstructural changes occurring at high temperature clearly affect the mechanical properties. The property degradation in HK-40 steel furnace tube support subjected to high temperature was caused by the formation of sigma phase. The investigation included metallurgical analysis, materials characterization, and mechanical analysis.     

Effects of high temperature mechanical damage on physical properties of unidirectional Ti/SiC metal matrix composite

Abstract

A miniaturised test system was used to investigate how the thermal and electrical properties of a unidirectionally reinforced titanium alloy (Ti–6Al–4V)/SiC (SM1140+)metal matrix composite change with mechanical damage at elevated temperature. Thermal conductivity and expansion measurements were obtained in the longitudinal and transverse direction both before and after short term strength and creep tests and at intervals during tests to assess changes in interface characteristics as functions of mechanical or thermal damage. The mechanical tests included monotonic stress–strain and ramp creep at temperatures between 500 and 650°C. The changes in thermal properties were compared with model predictions for the dependence of thermal properties on interface characteristics. The agreement was good for thermal expansion changes but not for thermal conductivity. This was ascribed to the nature of the damage at the interface that probably still allowed thermal transport but not mechanical load transfer.     

Effect of hot deformation and heat treatment of VT-8 alloy on its endurance

We study the effect of various types of hot deformation (forging, stamping, and upsetting) with subsequent heat treatment on the mechanical and fatigue characteristics and the crack resistance of VT-8 titanium alloy. It is shown that the maximum value of σ_-1 for smooth specimens is attained by forging in the β-region at a temperature of 1050°C; for specimens with notches andK _th, the maximum value of σ_-1 is guaranteed by upsetting in the β-region at the same temperature. Karpenko Physicomechanical Institute. Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 5, pp. 71–76, September–October, 1996.     

Investigating effect of temperature and time of metalised layer sintering by SMPP method on tensile strength and thermal shock resistance

Abstract

Sintered metal powder process is one of the high technology methods in ceramic–metal joining processes. Improvement in joining zone properties is very important in this method. The present study reveals the effect of metalised layer sintering temperature and time, and applied layer thickness on tensile strength and thermal shock resistance of alumina–copper joint. The results reveal that primary sintering for holding time duration of 90 min at a temperature of 1530°C and applied layer thickness of 50 μm with proper different stages of plating and brazing leads to a tensile strength of 120 MPa in the joining zone. The specimens, which were joined in this condition, were thermal shock resistance.     

The diffusion barrier effect of a vanadium layer in the formation of nickel silicides

The formation of nickel silicides and vanadium silicides and the diffusion barrier effect of a vanadium layer in the formation of nickel silicides were studied for annealed metal films deposited on silicon substrates by depth profiling using secondary ion mass spectrometry and X-ray diffraction. Nickel films more than 2500 Å thick react with silicon after annealing at 400 °C for 30 min in a vacuum. A vanadium layer 250 Å thick between nickel and silicon shows a barrier effect in the nickel silicide formation after annealing at 400 °C for 30 min. The barrier effect of a vanadium layer 250 Å thick becomes imperfect after annealing at 500 °C for 30 min.     

Study of high temperature mechanical behavior of the thermally oxidized Ti‐6Al‐4V alloy

The influence of oxidation of a Ti‐6Al‐4V alloy at 800 °C on its tensile properties at 600 °C has been studied. Specimens of this alloy were oxidized at 800 °C for 0.5, 1, 5, 10, 20 and 40 h. Tensile tests at 600 °C were carried out and the fracture surfaces were also examined. Oxidation of the specimens resulted in the formation of an oxide layer that spalled and another oxide layer that adhered to the substrate. Oxide formation increased with increase in duration of oxidation. In this investigation, density curves of the oxide layer as a function of duration of oxidation at 800 °C were used to identify a parabolic oxide growth law. The results of this study revealed coherence between the experimental data and calculations based on the Pilling‐Bedworth law. The mechanical strength of the Ti‐6Al‐4V alloy did not vary significantly with oxidation, but reduction in cross sectional area with increase in oxide layer thickness, as well as the slope of the stress‐strain curve decreased beyond the ultimate tensile strength. Fracture of the tensile tested specimens was predominantly ductile with microcavities. At certain regions of the oxide layer, brittle fracture with radial cracks was observed indicating intergranular fracture.     

Directional solidification of Ni base superalloy IN738LC to improve creep properties

Abstract

The high Cr, Ni base superalloy IN738LC has been directionally solidified on both laboratory and industrial scales using Bridgman and liquid metal cooling (LMC) methods respectively. In the Bridgman experiments, cylindrical rods were grown using a graphite chill with temperature gradient G = 5·0 K mm^-1 and a water cooled copper chill with G = 8·5 K mm^-1, and a wide range of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h^-1. In the LMC rigs, several turbine blades were grown using a wide range of withdrawal rates of R = 120, 225, 330, 420, and 630 mm h^-1. Grain and dendritic structures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions. Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 650°C and creep tests at 152 MPa and 982°C, 340 MPa and 850°C, and 586 MPa and 760°C. It was shown that at R = 600 mm h^-1 with water cooled copper chill, directionally solidified rods with a well orientated dendritic structure and better segregation pattern gives higher tensile properties at 25°C and creep properties at 340 MPa and 850°C. Tension and creep tests of turbine blades showed that although the yield and tensile strength of directionally solidified specimens are in the range of conventionally cast ones, the creep properties of the blades have been significantly improved using the LMC process.     

Fatigue crack growth characteristics of inertia friction welded Ti17 alloy

Fatigue crack growth rates (FCGR) of the inertia friction welded Ti17 alloy joint was studied at room temperature (RT) and 400 °C at a stress ratio of 0.1. Microstructural analysis and mechanical tests were also carried out. The results show that at RT, FCGR of weld metal with recrystallisation microstructure was higher compared with the base metal and the thermo‐mechanically affected zone. At 400 °C, however, the difference of the FCGR became insignificant. Fractographic observation showed that the failure of the base metal was dominated by slip band in transgranular mode at both RT and 400 °C, whereas crack in weld metal grew in intergranular mode at RT but in transgranular mode at 400 °C. The combined effects of lower yield strength and bigger yield‐ultimate tensile strength difference may be responsible for the higher fatigue crack growth (FCG) resistance of inertia friction welded Ti17 alloy at elevated temperature.     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.