Statistical in situ scanning electron microscopy investigation on the failure of oxide scales

Oxide scales play a pivotal role in obstructing surface chemical and electrochemical reactions,hence hindering chemo-mechanical effects such as liquid metal embrittlement of steels.Therefore,the critical conditions and failure mechanism of the oxide film are of major interest in the safe service of steels.Though in situ microscopic methods may directly visualize the failure mechanism,they are often challenged by the lack of statistically reliable evaluation of the critical conditions.Here,by combining in situ scanning electron microscopy with a tapered specimen tensile test in a single experiment,we uniquely achieve a mechanistic study with statistically reliable quantification of the critical strains for each step of the dynamic process of film rupture.This is demonstrated with the oxide films formed on a ferrite-martensite steel in liquid lead-bismuth eutectic alloy at elevated temperatures,with in situ results falling right into the predictions of the statistical analysis.Explicitly,the integrated experimental methodology may facilitate the materials genome engineering of steels with superior service performance.

Microstructure characteristics of 12Cr ferritic/martensitic steels with various yttrium

12Cr ferritic/martensitic steels with 0, 0.1 wt%, 0.2 wt% and 0.3 wt% theoretical yttrium(Y) additions were fabricated by vacuum inducting melting and casting method. Solubilities of Y in the 12Cr steels are0.027, 0.078 and 0.17 for 12Cr-0.1 Y, 12Cr-0.2 Y and 12Cr-0.3 Y, respectively. Phase transformations and microstructure characteristics under different heat-treatment schedules were investigated. The starting temperature of ferrite-to-austenite transformation A^(c1) are maintained about 850℃, but the finishing temperature of ferrite-to-austenite transformation A^(c3) are about 950, 970, 980 and 1000℃ for 12Cr-0 Y,12Cr-0.1 Y, 12Cr-0.2 Y and 12Cr-0.3 Y, respectively, which indicates that A^(c3) increases gradually with the addition of Y. Martensite accompanied with a few δ-ferrite is the dominant structure in all the steels. The amount of δ-ferrite shows a strong dependence with the Y content and austenitizing temperature. Area fraction of δ-ferrite increases with the content of Y, which is the ferrite favouring element. The minimum amount of δ-ferrite are achieved at 950℃ for 12Cr-0 Y, 12Cr-0.1 Y, 12Cr-0.2 Y and 1000℃ for 12Cr-0.3 Y.Besides, more carbides precipitate along the martensite laths and grain boundaries in the Y-bearing steel due to the redistribution of carbon between austenite and ferrite resulting from the ferrite favouring element of Y.

Role of titanium carbide and alumina on the friction increment for Cu-based metallic brake pads under different initial braking speeds

To understand the effect of abrasives on increasing friction in Cu-based metallic pads under different braking speeds,pad materials with two typical abrasives,titanium carbide(TiC)and alumina(Al_(2)O_(3)),were produced and tested using a scale dynamometer under various initial braking speeds(IBS).The results showed that at IBS lower than 250 km/h,both TiC and Al_(2)O_(3) particles acted as hard points and exhibited similar friction-increasing behavior,where the increase in friction was not only enhanced as IBS increased,but also enhanced by increasing the volume fraction of the abrasives.However,at higher IBS,the friction increase was limited by the bonding behavior between the matrix and abrasives.Under these conditions,the composite containing TiC showed a better friction-increasing effect and wear resistance than the composite containing Al_(2)O_(3) because of its superior particle-matrix bonding and coefficient of thermal expansion(CTE)compatibility.Because of the poor interface bonding between the matrix and Al_(2)O_(3),a transition phenomenon exists in the Al_(2)O_(3)-reinforced composite,in which the friction-increasing effect diminished when IBS exceeded a certain value.

Modifying the Properties of Tungsten Based Plasma Facing Materials with Single-Wall Carbon Nanotubes

Tungsten is one of the best candidates for plasma-facing components in fusion reactors owing to its unique properties. But disadvantages such as its brittleness and high ductile-to-brittle transition temperature have restricted its fusion energy application. Single-walled carbon nanotubes （SWCNTs） have the potential to be used as reinforcements due to their excellent mechanical properties. A new method of modifying the properties of tungsten by doping with SWCNTs was introduced. An efficient way of dispersing SWCNTs into the tungsten matrix with strong interfaces by heterocoagulation and ultrasonication was employed, and hot explosive compaction （HEC） technology was selected to compact and sinter the composite powders. The sintering properties, microstructure, densification effect, thermal conductivity, hardness and fracture toughness of the obtained SWCNTs/W bulk samples were tested, and compared with pure tungsten. The influences of SWCNTs on these properties and the main toughening mechanism of SWCNTs in a tungsten matrix were discussed.

Preparation of a diamond coating by the CVD method on the tungsten substrate and its resistance to D plasma irradiation

After the surface of tungsten(W)alloys were roughened by laser,chemical vapor deposition(CVD)of diamond coatings were deposited on three tungsten substrates of pure W,W-1 wt.%La2O3 and W-0.5 wt.%TiC.Under the same growth parameters,the presence of the second phase in the tungsten matrix impeded the growth rate of diamonds,so a completed diamond coating with the thickness of 45μm and a grain diamond size of 10μm was obtained only on pure tungsten substrate after running for 10 h.Scanning electron microscopy,X-ray diffraction and Raman spectroscopy tests proved that the obtained diamond coating was compact with a high purity and regular morphology.To verify the chemical and structural stability,the as-obtained diamond-coated tungsten materials were exposed to an ion flux of 1.4×10^(21)ions m^(−2)s^(−1) in D plasma for 30 min.After irradiation,neither delamination,dramatic coating failure nor entire erosion of the coating(graphitization)was observed.The diamond coating can be an effective protective layer to stop tungsten atoms from splashing into the plasma.

A Simulation Study on the Thermal Shock Behavior of Tungsten Mock-Up under Steady-State Heat Loads

In a fusion reactor,due to high heat flux（HHF） loads,the plasma facing components（PFCs） will suffer severe thermal shock.In this paper,the temperature distribution and thermal-stress field of tungsten armor under HHF loads were investigated by the method of finite element modeling and simulating.The orthogonal experiment and range analysis were employed to compare the influence degree of four representative factors：steady-state heat flux;thickness of tungsten armor;inner diameter of cooling tube and the coefficient of convection heat transfer（CCHF） of cooling water,on thermal shock behavior tungsten mock-ups,and then get an optimization model to conduct the transient heat flux experiment.The final simulation results indicated that the steady-state heat flux and the thickness of W armor are the main influential factors for the maximum temperature of mock-ups.Furthermore,the influence of transient thermal shock all mainly concentrates on the shallow surface layer of tungsten（about 500 μm） under different transient heat flux（duration 0.5 ms）.The results are useful for the structural design and the optimization of tungsten based plasma facing materials for the demonstration reactor（DEMO） or other future reactors.