Mechanical properties of tungsten alloys with Y2O3 and titanium additions

In this research the mechanical behaviour of pure tungsten (W) and its alloys (2 wt.% Ti���0.47 wt.% Y2O3 and 4 wt.% Ti���0.5 wt.% Y2O3) is compared. These tungsten alloys, have been obtained by powder metallurgy. The yield strength, fracture toughness and elastic modulus have been studied in the temperature interval of 25 ��C to 1000 ��C. The results have shown that the addition of Ti substantially improves the bending strength and toughness of W, but it also dramatically increases the DBTT. On the other hand, the addition of 0.5% Y2O3, is enough to improve noticeably the oxidation behaviour at the higher temperatures. The grain size, fractography and microstructure are studied in these materials. Titanium is a good grain growth inhibitor and effective precursor of liquid phase in HIP. The simultaneous presence of Y2O3 and Ti permits to obtain materials with low pores presence

Improved GaN-based HEMT performance by nanocrystalline diamond capping

As a wide-bandgap semiconductor, gallium nitride (GaN) is an attractive material for next-generation power devices. To date, the capabilities of GaN-based high electron mobility transistors (HEMTs) have been limited by self-heating effects (drain current decreases due to phonon scattering-induced carrier velocity reductions at high drain fields). Despite awareness of this, attempts to mitigate thermal impairment have been limited due to the difficulties involved with placing high thermal conductivity materials close to heat sources in the device. Heat spreading schemes have involved growth of AIGaN/GaN on single crystal or CVD diamond, or capping of fullyprocessed HEMTs using nanocrystalline diamond (NCD). All approaches have suffered from reduced HEMT performance or limited substrate size. Recently, a "gate after diamond" approach has been successfully demonstrated to improve the thermal budget of the process by depositing NCD before the thermally sensitive Schottky gate and also to enable large-area diamond implementation.

Profiling the temperature distribution in AlGaN/GaN HEMTs with nanocrystalline diamond heat spreading layers

Reduced performance in Gallium Nitride (GaN) based high electron mobility transistors (HEMTs) as a result of self-heating has been well-documented. A new approach, termed ���diamond-before-gate" is shown to improve the thermal budget of the deposition process and enables large area diamond without degrading the gate metal NCD capped devices had a 20% lower channel temperature at equivalent power dissipation.

Comportamiento mec��nico de la aleaci��n W+1%Y2O3 en funci��n de la atm��sfera y la temperatura

Mediante ensayos de flexi��n en tres puntos se compara y eval��a el comportamiento mec��nico de la aleaci��n W- 1wt%Y2O3 con el W puro fabricados ambos mediante HIP. Se ha obtenido la tenacidad de fractura, la resistencia a flexi��n y el l��mite el��stico en atm��sfera oxidante y de vac��o en un intervalo de temperaturas comprendido entre -196 ��C, ensayos de inmersi��n en nitr��geno l��quido, y 1200 ��C. Previamente, se ha medido la densidad, la dureza mediante ensayos Vickers y el m��dulo de elasticidad din��mico de los materiales. Adem��s, la dureza y el m��dulo de elasticidad se han comparado con los obtenidos mediante ensayos instrumentados de nanoindentaci��n. Finalmente se ha realizado un peque��o estudio de las superficies de fractura de las muestras ensayadas mediante microscop��a electr��nica de barrido para poder relacionar el modo de rotura de los materiales y las propiedades mec��nicas macrosc��picas con los micromecanismos de fallo involucrados en funci��n de la temperatura.

Ballistic performance of nanocrystalline and nanotwinned ultrafine crystal steel

Because of their remarkable mechanical properties, nanocrystalline metals have been the focus of much research in recent years. Refining their grain size to the nanometer range (<100 nm) effectively reduces their dislocation mobility, thus achieving very high yield strength and surface hardness���as predicted by the Hall���Petch relation���as well as higher strain-rate sensitivity. Recent works have additionally suggested that nanocrystalline metals exhibit an even higher compressive strength under shock loading. However, the increase in strength of these materials is generally accompanied by an important reduction in ductility. As an alternative, efforts have been focused on ultrafine crystals, i.e. polycrystals with a grain size in the range of 500 nm to 1 ��m, in which ���growth twins��� (twins introduced inside the grain before deformation) act as barriers against dislocation movement, thus increasing the strength in a similar way as nanocrystals but without significant loss of ductility. Due to their outstanding mechanical properties, both nanocrystalline and nanotwinned ultrafine crystalline steels appear to be relevant candidates for ballistic protection. The aim of the present work is to compare their ballistic performance against coarse-grained steel, as well as to identify the effect of the hybridization with a carbon fiber���epoxy composite layer. Hybridization is proposed as a way to improve the nanocrystalline brittle properties in a similar way as is done with ceramics in other protection systems. The experimental campaign is finally complemented by numerical simulations to help identify some of the intrinsic deformation mechanisms not observable experimentally. As a conclusion, nanocrystalline and nanotwinned ultrafine crystals show a lower energy absorption than coarse-grained steel under ballistic loading, but under equal impact conditions with no penetration, deformation in the impact direction is smaller by nearly 40%. This a priori surprising difference in the energy absorption is rationalized by the more important local contribution of the deviatoric stress vs. volumetric stress under impact than under uniaxial deformation. Ultimately, the deformation advantage could be exploited in the future for personal protection systems where a small deformation under impact is of key importance.

Grain size gradient length scale in ballistic properties optimization of functionally graded nanocrystalline steel plates

The last few years have highlighted the existence of two relevant length scales in the quest to ultrahigh-strength polycrystalline metals. Whereas the microstructural length scale ��� e.g. grain or twin size ��� has mainly be linked to the well-established Hall���Petch relationship, the sample length scale ��� e.g. nanopillar size ��� has also proven to be at least as relevant, especially in microscale structures. In this letter, a series of ballistic tests on functionally graded nanocrystalline plates are used as a basis for the justification of a ���grain size gradient length scale��� as an additional ballistic properties optimization parameter.