Alloy HfNbTaTiZr Research Articles

Chemical short-range order increases the phonon heat conductivity in a refractory high-entropy alloy

We study the effects of the chemical short-range order (SRO) on the thermal conductivity of the refractory high-entropy alloy HfNbTaTiZr using atomistic simulation. Samples with different degrees of chemical SRO are prepared by a Monte Carlo scheme. With increasing SRO, a tendency of forming HfTi and TiZr clusters is found. The phonon density of states is determined from the velocity auto-correlation function and chemical SRO modifies the high-frequency part of the phonon density of states. Lattice heat conductivity is calculated by non-equilibrium molecular dynamics simulations. The heat conductivity of the random alloy is lower than that of the segregated binary alloys. Phonon scattering by SRO precipitates might be expected to reduce scattering times and, therefore, decrease thermal conductivity. We find that, in contrast, due to the increase of the conductivity alongside SRO cluster percolation pathways, SRO increases the lattice heat conductivity by around 12 %. This is expected to be a general result, extending to other HEAs.

Atomistic Simulations of the Shock and Spall Behavior of the Refractory High-Entropy Alloy HfNbTaTiZr

AbstractUsing molecular dynamics simulation, we study the effect of a shock wave on the refractory high-entropy alloy HfNbTaTiZr. A single-crystalline sample, shocked along the [001] direction, is considered. The initial compression leads to only weak dislocation activity and a bcc $$\rightarrow$$ → hcp transformation in some regions of the sample. After the shock wave is reflected from the free back surface of the sample, hcp transforms back to bcc, and twins are formed in the bcc phase. The sample spalls under the high tensile pressures developing after wave reflection. In this stage, we observe dislocation activity from the twin boundaries and inside the nanograins generated by twinning. Under the large tensile stresses, some fcc phase appears together with disordered amorphous regions where voids nucleate and lead to spall. The fracture surfaces follow the twin boundaries set up in the compression phase. The spall strength is similar to the one found in simulations of other bcc metals at similar strain rates. Similar simulations for the equiatomic HfNbTaZr HEA show the same qualitative behavior, with twins and reduced dislocation activity, but without phase transformations.

Orientation dependent plastic localization in the refractory high entropy alloy HfNbTaTiZr at room temperature

A statistical and quantitative perspective on the incipient slip localization behavior of the HfNbTaTiZr refractory high entropy alloy is presented, after monotonic tensile testing at room temperature. High-resolution digital image correlation (HR-DIC) data collected in the scanning electron microscope (SEM) is coupled with electron back-scattered diffraction (EBSD) over millimeter-scale fields of view containing over a hundred crystal orientations. Two populations of grains are identified with respect to their slip localization: a vast majority of the grains exhibit a high density of apparently straight slip bands of low intensity, while the remaining grains show few but very intense and wavy slip bands at the mesoscopic scale. The slip localization behavior is strongly correlated to the crystal orientation with respect to the loading direction. This behavior is discussed with regard to dislocation glide and cross-slip processes.

Characterization of Lattice Defects in Refractory Metal High‐Entropy Alloy HfNbTaTiZr by Means of Positron Annihilation Spectroscopy

Lattice defects in refractory metal high‐entropy alloy HfNbTaTiZr are investigated using positron lifetime spectroscopy combined with coincidence Doppler broadening (CDB) spectroscopy. Annealing of HfNbTaTiZr alloy at 1000 °C results in full recovery of defects and formation of single‐phase random solid solution characterized by the bulk positron lifetime of 141 ps. This value is in accordance with the theoretical estimation. Quenching of the alloy from high temperature prevents recovery of thermally equilibrium vacancies and enables to retain a high concentration of vacancies in the sample at ambient temperature. For single vacancies in the HfNbTaTiZr alloy, a characteristic positron lifetime of 212 ps is measured, which is significantly shorter than the value estimated by theoretical calculations. It indicates that there is considerable ion relaxation around vacancies. The CDB spectroscopy investigations reveal that vacancies in HfNbTaTiZr alloy are not distributed randomly but are preferentially coupled with Hf solutes.

INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density.

Defects in Thin Layers of High Entropy Alloy HfNbTaTiZr

Defects in Thin Layers of High Entropy Alloy HfNbTaTiZr

Microstructure evolution and mechanical property of a precipitation-strengthened refractory high-entropy alloy HfNbTaTiZr

The mechanical properties and microstructure evolution of a precipitation-strengthened refractory high-entropy alloy, HfNbTaTiZr, with equiaxed grains are reported. The as-cast alloy shows a high yield strength of 356 MPa at 1200 °C, and 1597 MPa with good ductility at room temperature. The enhancement in strength is attributed to precipitation strengthening. Afterwards, microstructure evolution was investigated under several annealing treatment conditions: 1000 °C for 24 h, 1200 °C for 24 h, 1400 °C for 24 h, and 1450 °C for 168 h. The hcp-1 phase appears at 1000 °C and gradually dissolves up to 1200 °C. When the temperature rises to 1400 °C, the hcp-1 phase disappears, and fine nanoprecipitates begin to form. After annealing at 1450 °C for one week, however, this alloy exhibits phase decomposition. With the formation of two kinds of new fcc phase, fine nanoprecipitate only remains in bcc matrix, and another new hcp-2 phase forms in the grain.

Microstructure and Room Temperature Mechanical Properties of Different 3 and 4 Element Medium Entropy Alloys from HfNbTaTiZr System.

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.

Spark plasma sintering of gas atomized high-entropy alloy HfNbTaTiZr

Abstract

Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion

Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion

Strength enhancement of high entropy alloy HfNbTaTiZr by severe plastic deformation

Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of ≈80 nm was processed by high pressure torsion. The development of microstructure, lattice defects and mechanical properties with increasing strain was examined. Grain refinement of HfNbTaTiZr alloy deformed up to the equivalent strain e ≈ 50 resulted in a significant enhancement of strength while keeping sufficient ductility. However, further straining e > 100 led to a decrease of strength and the loss of ductility due to the decomposition of solid solution facilitated by vacancies introduced by severe plastic deformation.

Influence of the Surface Condition of Specimens on the Size of Coherently Diffracting Domains

Metallic sample preparation process from cutting to polishing introduces defects into the subsurface layers by plastic deformation. Methods of bulk sample characterization are usually sensitive to the microstructure and presence of defects e.g. dislocations, grain boundaries, etc. near the surface. The X-ray diffraction technique (XRD) is a unique method used primarily for phase identification and quantification of crystalline materials. The extending of the method by the whole pattern fitting using Rietveld refinement analysis enables also utilisation for other purposes such as determination of the size of coherently diffracting domains (CDD). This property quantifies the degree of disorder in material and is usually correlated with mean crystallite size, i.e. subgrain size. However, since the penetration depth of the laboratory X-ray radiation is limited to the order of tens of micrometers (depending on the material), the observed result is highly affected by the density of defects such as dislocations in the surface area of the specimen and thus by their surface condition. The effect of the different final steps of grinding and polishing on the observed size of CDD was studied and discussed for two materials - aluminum Al7075 alloy and high entropy alloy HfNbTaTiZr. The reasonable finishing for the optimal results was found for both materials.

Microstructure and properties of a refractory high-entropy alloy after cold working

A refractory high-entropy alloy HfNbTaTiZr was successfully rolled at room temperature up to 86.4% reduction in thickness (true thickness strain is −2.3). This represents the first successful attempt to cold roll a HEA with a BCC crystal structure. The microstructure and properties of the rolled sheets were determined in the as-rolled condition and after annealing at 800 °C, 1000 °C, and 1200 °C. Cold rolling resulted in extensive grain elongation, formation of deformation bands within the grains, and development of crystallographic textures that depended on the rolling reduction. The 86.4% cold-rolled sheet had true tensile stress of 1295 MPa and tensile ductility of 4.7%. After annealing at 1000 °C and 1200 °C, complete recrystallization of the cold-rolled sheet occurred. After annealing at 1000 °C, the true tensile stress and ductility of the sheet were 1262 MPa and 9.7%, respectively.

Effect of Al addition on mechanical properties and microstructure of refractory AlxHfNbTaTiZr alloys

This study developed a series of refractory AlxHfNbTaTiZr high-entropy alloys (HEAs) with an aim to improve strength, and reduce density of the very ductile base alloy HfNbTaTiZr. Despite the diversity of crystal structures among the constituent elements, all the HEAs are single solid solution phase with body-centered cubic (BCC) structure. The addition of Al significantly improves the strength but reduces the ductility due to large solution hardening. The linear relation between yield strength and atomic percentage of Al suggests that the strengthening effect of a certain element in a single-phase HEA alloy can be explained based on quasi-binary alloy concept. Crack formations in deformed AlHfNbTaTiZr alloy with the lowest fracture strain are mainly along the boundaries between dendrite and interdendrite. This agrees with its large deviation of Al content and thus strength between dendrite and interdendrite.