High Temperature Toughness Research Articles

Oxygen solutes-regulated low temperature embrittlement and high temperature toughness in vanadium

The plasticity of body-centered cubic (BCC) metals is sensitive to interstitial trace impurities. Owing to high oxygen affinity, vanadium (V) shows a tendency of embrittlement with increasing oxygen concentration. However, how oxygen solutes affect the ductile-to-brittle transition (DBT) behavior of V remains unclear. In this study, we investigate the DBT behavior of V with different oxygen solute concentrations using small-punch test. As oxygen content increases, the DBT temperature (DBTT) rises incrementally accompanied by a widening of the semi-brittle transition zone. The reduction of the lower fracture energy plateaus indicates a classical low-temperature embrittlement, while the rising of the upper fracture energy plateaus manifests a remarkable high-temperature toughening. Below DBTT, owing to the strong pinning effect of oxygen-vacancy complexes on dislocations, only a limited number of slip systems were activated with very low dislocation density, and all screw dislocations are immobile. Above DBTT, owing to the intensive interactions between dislocation and oxygen-vacancy complexes, frequent dislocation cross-slips trigger multiple slip systems and accelerate dislocation multiplication and storage, all of which contribute to the high-temperature toughness. These findings clarify the effect of oxygen solute on the DBT of V and guide the design of high-performance refractory metals.

A Review of High-Temperature Toughness Improvement Strategies for Medium Entropy Alloys

A Review of High-Temperature Toughness Improvement Strategies for Medium Entropy Alloys

Effect of Transition Metal Layer on Bending and Interfacial Properties of W/TiN/Ta-Laminated Composite

The widespread applications of W in the fusion reactor are limited by its low-temperature brittleness, recrystallization brittleness, and irradiation-induced brittleness. Many toughening methods were used to improve the brittleness of W, such as adding second-phase particles, adding W fibers, preparing laminated composite, and so on. Among these, preparing laminated W-based composites has been proven to effectively improve both the low-temperature and high-temperature toughness of W. In this study, W/M/TiN/Ta-laminated composites with transition metal layer (M) were synthesized through the spark plasma sintering (SPS) at three different temperatures. The effects of nano-scale (Ni, Ti, and Cr) and micron-scale (Ni, Ti, and V) transition layers on the bending and interfacial properties of the W/M/TiN/Ta composite were studied via an electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). Compared with W/TiN/Ta, the flexural strength and strain of W/Ninm/TiN/Ta were increased by 25.6% and 17.6%, respectively. Ni, Ti, and V micron transition layers can improve the combination of the W-TiN interface and decrease the joining temperature. The micron V layer has the best strengthening effect. The flexural strength of W/V/TiN/Ta reached 1294 MPa, much higher than W/Ta's 1041 MPa.

Effect of nickel on the formation of γ/γ’ microstructures in WC/Co-Ni-Al-W

Cuboidal and coherent precipitates form in ternary cobalt-aluminium‑tungsten systems and are described to increase high temperature toughness of superalloys for gas turbines. γ’ is a metastable phase in the form of Co3(Al,W) which is stabilized by the substitution of cobalt with nickel. Strengthening of cobalt binder (γ) in cemented carbides by precipitation hardening is of interest as it potentially increases hardness and wear resistance without sacrificing tensile strength or toughness of the composite materials. Infiltration of preliminary produced cobalt-base and cobalt‑nickel-base superalloys into tungsten carbide powder beds yielded a γ/γ’ binder microstructure. It was shown that nickel stabilizes the γ’ phase in superalloys as well as in cemented carbides, but carbon may lead to the formation of an additional κ phase with E21 structure.

Simulation and Experimental Research on Carbide Tool’s Rake Face Wear of Cutting 2.25Cr1Mo0.25V Material

Due to the high temperature toughness and low thermal conductivity of 2.25Cr1Mo0.25V material, the rake face of the tool can easily wear out during the cutting process, which reduces the service life of the tool. In this paper, by analyzing the contact behavior, heat exchange conditions and tool wear mechanism during the cutting process, a 3 D finite element simulation model of tool wear in accordance with the actual cutting process is established. Combined with the cutting wear experiment, the validity of the model is verified. The influence of cutting speed, cutting depth, feed rate and the interaction between them on rake face wear was studied by designing a simulation scheme. The simulation results show that: With the increase of cutting speed and feed rate, the wear depth of the tool's rake face gradually increase, but for cutting depth, the depth of wear gradually decreases. The interaction of cutting speed and feed rate has a significant influence on the rake face wear, the interaction between cutting speed and feed rate, and the interaction between feed rate and cutting depth are not significant. The research results provide a theoretical basis for improving the service life of the tool and provide technical guidance for the selection of cutting parameters in the actual production.

Carbides and Carbon Control in MC – Reinforced Superalloys

Polycrystalline alloys based on cobalt and elaborated by classical foundry are known since the middle of the last century. They are currently still used, notably for geometrically complex components working at high temperature. Beyond the oldest ones reinforced by chromium carbides, new principles of carbides–strengthened cobalt–based alloys have recently appeared. MC–type refractory mono–carbides allow maintaining the melting start temperature at a high level when they are present as single carbide phase. Their high temperature stabilities and script–like morphologies also favor high mechanical properties at elevated temperatures. Optimized MC–carbides fractions can be obtained with carbon contents, close to 0.4 wt.%C, for achieving significant strengthening without threat for the low and high temperature toughness. Controlling the carbon content is thus of prior importance. Unfortunately the most common metallographic apparatus used for measuring the chemical composition of alloys – the Energy Dispersion Spectrometers (EDS) attached to Scanning Electrons Microscopes (SEM) – are not able to analyze, with sufficient accuracy, carbon with so low contents. A simple indirect method using both EDS and SEM is proposed here to get some information about the carbon content in several MC–reinforced Co–based superalloys.

High-temperature toughening in ternary medium-entropy (Ta1/3Ti1/3Zr1/3)C carbide consolidated using spark-plasma sintering

ABSTRACT We report for the first time the effect of temperature on the mechanical properties of (Ta1/3Ti1/3Zr1/3)C carbide. Flexural strength and fracture toughness were investigated using the three-point bending technique in argon. Using commercially available carbide powders with an equimolar ratio and performing spark plasma consolidation at 1973°C, we obtained a bulk single-phase medium-entropy carbide ceramic with the lattice parameter a= 4.458 Å. The flexural strength and fracture toughness at room temperature reached on average 700 MPa and 3.2 MPa m1/2, respectively. At 1800°C, local decomposition of the medium-entropy carbide took place as a structure with a local chemical gradient was observed after high-temperature tests, which increased fracture toughness by 30% (to 4.4 MPa m1/2).

Tri-modal microstructure in high temperature toughening and low temperature strengthening treatments of near-β forged TA15 Ti-alloy

For Ti-alloy tri-modal microstructure can be obtained by the near-β forging (water quenching, WQ)+high temperature toughening and low temperature strengthening treatment (HLT) method. However in actual production, it is hard to accomplish water quenching after near-β forging immediately. In this paper a short time of air cooling (AC) after near-β forging was introduced to consider the temperature drop during the forgings transferring in actual production. For TA15 alloy the formation process of tri-modal microstructure in near-β forging (AC+WQ)+HLT was studied. Under the given near-β forging condition, the influences of the subsequent toughening and strengthening treatment on the obtained tri-modal microstructure were revealed as well as on the corresponding mechanical properties. Then the reasonable HLT route for tri-modal microstructure with excellent comprehensive properties was determined.

Small-scale fracture toughness of ceramic thin films: the effects of specimen geometry, ion beam notching and high temperature on chromium nitride toughness evaluation

For the implementation of thin ceramic hard coatings into intensive application environments, the fracture toughness is a particularly important material design parameter. Characterisation of the fracture toughness of small-scale specimens has been a topic of great debate, due to size effects, plasticity, residual stress effects and the influence of ion penetration from the sample fabrication process. In this work, several different small-scale fracture toughness geometries (single-beam cantilever, double-beam cantilever and micro-pillar splitting) were compared, fabricated from a thin physical vapour-deposited ceramic film using a focused ion beam source, and then the effect of the gallium-milled notch on mode I toughness quantification investigated. It was found that notching using a focused gallium source influences small-scale toughness measurements and can lead to an overestimation of the fracture toughness values for chromium nitride (CrN) thin films. The effects of gallium ion irradiation were further studied by performing the first small-scale high-temperature toughness measurements within the scanning electron microscope, with the consequence that annealing at high temperatures allows for diffusion of the gallium to grain boundaries promoting embrittlement in small-scale CrN samples. This work highlights the sensitivity of some materials to gallium ion penetration effects, and the profound effect that it can have on fracture toughness evaluation.

Room and high temperature toughening in directionally solidified B4C–TiB2 eutectic composites by Si doping

Directionally solidified В4С–TiB2 eutectics doped with different Si contents were prepared by floating zone method. Silicon doping (up to 3vol.%) promotes a more coherent interface between B4C matrix and TiB2 inclusions, a decrease of the spacing between TiB2 (when solidification rate is constant), and formation of a more uniform morphology with thinner inclusions. These features lead to simultaneous enhancement of Vickers hardness (HV), fracture toughness (K1c) and bending strength (σ) when compared with Si-free В4С–TiB2 eutectics. For our materials, the average HV, K1c and σ were 45.2GPa, 7.04MPam1/2, and 460MPa at room temperature or 487MPa at 1600°C, respectively. The values of σ at indicated temperatures are approximately twice higher than for the Si-free В4С–TiB2 eutectic composites.

Sequential approximate optimization-based robust design of SiC–Si3N4 nanocomposite microstructures

A simulation-based robust design optimization methodology to predict the most suitable microstructures of SiC–Si 3N 4 nanocomposites for desired high-temperature toughness is presented. The focus is on finding robust nanocomposite microstructures with maximum toughness at two temperatures: 1500°C and 1600°C. Within this context a sequential approximate optimization algorithm under uncertainty is applied to six different test problems addressing different aspects of robust microstructure generation. During optimization, statistical uncertainties inherent to the computational microstructural generation are quantified and introduced in the optimization framework. The results show that the SiC volume fraction, the number of Si 3N 4 grains, the grain size distribution of the Si 3N 4 grains, and the grain size of the SiC particles have varied effects on the microstructure toughness at different temperatures. At 1500°C, the preferred microstructure is the one with higher Si 3N 4 volume fraction, whereas at 1600°C, the preferred microstructure is the one with higher SiC volume fraction.

Working Capacity Enhancement of Cutting Tools in Terms of Machining of Difficult-to-cut Materials by Complex Application of Wear-resistant Coating and Hard Alloy

The paper describes the results of research of cutting properties of hard alloys WC-Co-carbides with Ti-(Ti,Al)N-TiN coating when turning of structural steels and hard-to-machine nickel alloys. Due to combination of high temperature toughness of carbide and wear resistant coating tools cutting properties increase in several times.

A31 Focused ion beam machining of silicon carbide(M4 processes and micro-manufacturing for science)

Focused ion beam can remove hard materials such as diamond, sapphire, and silicon carbide (SiC). SiC is very useful for glass molding because of high temperature toughness and high hardness, however, fine patterning of SiC is very difficult. Therefore, ion beam machining properties of SiC were investigated and 3D fabrication of SiC using dwell time machining method was also carried out. The machining properties, submicron patterns and 3D patterns of SiC were obtained.

Creep effects on crack growth in a Mo–Si–B alloy

Crack growth behavior during monotonic and cyclic loading at elevated temperature is affected by creep and/or by environment and, therefore, high-temperature toughness and fatigue response can be temperature and loading-rate dependent. This paper reports on the effect of loading rate on fracture toughness at high temperatures for a two-phase Mo–Si–B alloy; the observed response is understood by examining the interaction of the advancing crack with the microstructure, and the evolution of microstructure ahead of the crack tip as a consequence of the crack-tip field. Parallel studies were also performed under cyclic loading conditions by subjecting compact tension specimens to sinusoidal and trapezoidal loading waveforms. In certain cases, the microstructure ahead of the crack tip revealed several instabilities (recrystallization, grain growth and creep cavitation). Finite element analysis revealed strain localization ‘pockets’ ahead of the crack tip, which are thought to provide the driving force for the observed microstructural instabilities.

Surface Modification by Scratching Test for Laser Cleaning WC-Co in Liquid Polymer

Tungsten carbide including cobalt, WC-Co, shows good performance for high temperature toughness and is expected when applying to a high temperature mold but it is difficult to cut into ultra-fine precision because of its brittleness. In this study, WC-Co surface is treated by a pulsed laser irradiation to softening and cleaning in a machining liquid and cutting test by Vickers indenter. Under the case of Methyl n-Nonanoic acid, the surface wetness shows same behavior for with and without laser cleaning. In this case, the surface fracture by the cutting is extended and a lot of damages are generated along the cutting track for both as polished surface and laser cleaning surface. Under the case of n-Undecyl Alcohol and Palmitic acid n-Butyl Ester, the surface wetness is improved by the laser cleaning and the surface fracture is inhibited by the laser cleaning. As results of the FT-IR measurements of Palmitic acid n-Butyl Ester on the WC-Co surface, the peak position of C-H stretching mode (2900-2950 cm-1) shifted from a lower wavenumber (on the surface of as polished) to a higher wavenumber (on the laser cleaning surface). This result indicates that the Palmitic acid n-Butyl Ester interacts with the WC-Co surface and influences to the surface fracture of the laser cleaning WC-Co surface, where it is restrained by the surface adsorption.

An investigation of a new near-beta forging process for titanium alloys and its application in aviation components

In this paper, the authors present a new near-beta forging process, in which materials are heated at about 15 °C below the beta transus, to improve the combined properties of titanium alloys. Materials processed by the near-beta forging process, followed by rapid water-cooling, then high temperature toughening and low temperature strengthening heat treatments, produce a new kind of microstructure for titanium alloys. This new tri-modal microstructure consists of about 15% equiaxed alpha, 50–60% lamellar alpha and transformed beta matrix. Materials with tri-modal microstructure show a high low cycle fatigue property, high creep-fatigue interaction life, high fracture toughness and a high service temperature without decreasing ductility and thermal stability. The experimental fundamentals of the new process and the strengthening and toughening mechanism have been discussed. A critical issue in the practical application of the near-beta forging process is the control of temperature. A new metallographic inspection method was proposed to solve this problem. This new near-beta forging process has been reliably applied to produce several aeroengine compressor disks, rotators, and other airplane components.

Conditions for Preparation of Oxide Components and Their Effect on Properties of Al2O3 – ZrO2 – SiC Composite

Results of a study of phase formation in the Al2O3 – ZrO2 – SiC system sintered under vacuum and in a reducing medium at 1350 – 1550°C are reported. Conditions for preparation of Al2O3 and ZrO2 powders from hydroxides are specified and the effect of specific surface, temperature, and holding time on the reaction between oxide and carbide components is considered. Results of microstructural, x-ray phase, thermogravimetric, and chemical analyses of precursor materials and end products are discussed. Optimum composition (20% SiC, 15% ZrO2, 65% Al2O3 ) and dispersity of the mixture for obtaining a composite with a strength of about 200 MPa by a method other than high-temperature toughening are determined.

Effect of thermal residual stress on the high temperature toughening behaviour of TiB 2/SiC w composites

SiC-whisker-reinforced TiB 2 composite was produced by hot pressing techniques. The composite exhibited superior mechanical properties at room temperature compared to those of monolithic TiB 2, especially in respect of the fracture toughness, K IC, which was approximately 115% higher than that of TiB 2 ceramic without SiC whiskers, the maximum values of the fracture toughness and flexural strength being 7.3 MPa m and 640 MPa, respectively. Considerable improvement in the high-temperature fracture toughness of the composite was observed up to 1200°C. Based on the thermal residual stress analysis and fracture surface observations, the fracture toughness at high temperature was correlated with the change in inequality between the whisker fracture strength and the compressive clamping forces on the whiskers caused by thermal expansion mismatch between the whiskers and the matrix. The results showed that the number of whiskers pulled out depends greatly on the test temperature, the whisker aspect ratio and the interfacial friction coefficient. At high temperature the whiskers are mainly pulled out, whilst at low temperature the whiskers are mainly fractured.

Toughness and microstructural degradation at high temperature in SiC fiber-reinforced ceramics

The fracture behavior of three different SiC fiber-reinforced ceramics at ambient and elevated temperature in air is studied. The fracture properties were obtained by flexure tests on notched beams and the increase in fracture resistance with crack length was determined using an equivalent elastic crack approach. The microstructural changes during elevated temperature exposure, which could be divided into interfacial and fiber degradation, were analyzed in each material and related to the fracture micromechanisms and to the degradation in toughness and fracture resistance. Finally, the different strategies to improve the high temperature toughness of fiber-reinforced ceramics are briefly discussed.

A new high-temperature deformation strenthening and toughening process for titanium alloys

A new high-temperature deformation strengthening and toughening process for titanium alloys, which consists of near beta transus forging, rapid water quenching, high-temperature toughening and low-temperature strengthening treatment, has been developed. The materials processed by this method produce a new tri-modal microstructure, which shows a high low-cycle fatigue property, high fatigue-creep interaction life, high fracture toughness and a high service temperature without the decreasing ductility and thermal stability. The fundamentals of the new process and the strengthening and toughening mechanism have also been discussed. It is proved that the high-temperature deformation strengthening and toughening process is applicable not only to alpha-beta alloys, but also to alpha, near alpha and near beta titanium alloys.