Interstitial Impurity Content Research Articles

The origin and control of interstitial impurities in refractory complex concentrated alloys

Interstitial impurities, primarily O and N, inadvertently introduced during the processing of refractory complex concentrated alloys (RCCAs) significantly limit the mechanical properties of the alloys at room temperature. Plasma arc melting (PAM) has facilitated the quest for RCCAs, showcasing remarkable combinations of strength and ductility. In this work, the composition and chemistry of residual gases in the PAM chamber environment during arc melting and the interstitial impurities in elemental feedstocks were analyzed to quantify the origin of O and N during synthesis. Moreover, the thermodynamic mechanisms governing the origin of interstitial impurities in arc-melted MoNbTaW RCCAs were discussed. With an understanding of the mechanisms governing the content of interstitial impurities in RCCAs during PAM, arc-melted MoNbTaW RCCAs with fewer than 100 ppm O were synthesized. The arc-melted MoNbTaW RCCAs were then characterized using electron microscopy and atom probe tomography to establish the compositional architecture of grain boundaries and thereby elucidate the thermodynamic drive for segregation and oxide formation in MoNbTaW. Additionally, density functional theory calculations of a MoNbTaW grain boundary were employed to analyze the effect of local chemical fluctuations on the O segregation and oxide formation at the grain boundary. This detailed understanding of the thermodynamic mechanisms governing the origin of interstitial impurities and their grain boundary segregation in MoNbTaW RCCAs provides greater context surrounding O interactions in RCCAs, which is key for the successful development of refractory alloys with unique combinations of strength and ductility.

Flexible Powder Production for Additive Manufacturing of Refractory Metal-Based Alloys

The quality and properties of metal powders are essential for powder metallurgical (PM) processes in general and for additive manufacturing (AM) processing routes in particular. Thus, a variety of atomization technologies were established meeting the multiple needs of the different processing technologies. However, the production of refractory metal alloy powder remains challenging due to their high liquidus temperatures (>2000 °C), the formation of brittle intermetallic phases, as well as the reactivity with and sensitivity to interstitials of the constituting elements. In this contribution, powders made of Mo-20Si-52.8-Ti (at.%) were produced by a novel ultrasonic atomization (UA) process at laboratory-scale using an industrial electrode induction gas atomization (EIGA) process with a modified electrode concept for the first time. UA allows flexibility in alloy composition due to the arc melting-based principle, while the EIGA electrode is PM manufactured from elemental powders to provide similar flexibility on a larger scale. The powders resulting from these two processes were compared with respect to size distribution, sphericity, microstructure and phase constitution, chemical composition, and interstitial impurity content. In addition, several powder batches were produced with the UA process in order to assess the process reliability and stability. The properties, quality, and quantities of UA powders perfectly meet the requests for alloy development for powder bed fusion AM, while the modified EIGA process allows the upscaling of the alloy powder quantities.

ВЛИЯНИЕ ПРИМЕСЕЙ ВНЕДРЕНИЯ В СПЛАВАХ МИШЕНЕЙ СИСТЕМЫ Zr–Y НА КАЧЕСТВО ТЕПЛОЗАЩИТНОГО КЕРАМИЧЕСКОГО ПОКРЫТИЯ

The influence of interstitial impurities in the Zr–Y target alloys (manufactured by two different techniques) on the quality of thermal barrier ceramic layers of heat resisting coating are studied in this work. The heat resisting ceramic coating manufactured in the UOKS-2 devices by magnetron medium-feculence plasma–chemical deposition on the surface of components that are used at the high temperatures (above 1150 °C). It was found that when the content of interstitial impurities in the target alloy is more than 0,1% (1000 ppm), the rate of the coating process decreases and has to be maintained by increasing the energy of argon ions. This leads to overheating of the target alloy and the surface of the parts (substrate) which impairs the adhesion of the deposited atoms.

Study of Methods for Increasing Ductility and Formability of Cold-Rolled Ti-Stabilized IF Steels

Study of the influence of the chemical composition and parameters of the complex treatment technology on characteristics of the microstructure, phase precipitates, the solid solution state, and mechanical properties was carried out for hot-rolled and cold-rolled Ti-stabilized IF steels. Cold-rolled products were annealed according to the regime of continuous hot-dip galvanizing units. The following methods of investigation were used: light, scanning and transmission electron microscopy, mechanical properties testing, thermodynamic analysis of phase stability conditions, and determination of the interstitial impurities content in a solid solution by internal friction. The possibility and conditions of a significant increase in ductility, formability of cold-rolled steels are established. They are obtained not by the traditional way consisting in reducing the content of interstitial elements and impurities, but by giving them a favorable form of existence. This is achieved by optimizing the chemical composition, first of all, the sulfur content, and parameters of thermo-deformation treatment of steel, which ensure complete carbon binding in Ti4C2S2 as a result of TiS transformation already at the hot-rolling stage. To reduce the yield and tensile strength, it is necessary to decrease the content of interstitial elements and to increase the temperature of recrystallization annealing of cold-rolled steel.

ZONE REFINING OF ZIRCONIUM IN AN ELECTRIC FIELD

The physical grounds and an experimental study of the efficiency of applying the zone recrystallization method in an electric field for zirconium refining from metal and gas-forming impurities are presented. The changes in the elemental composition, microhardness, and structure of the obtained ingots was investigated. It is shown that the application of the method can significantly reduce the content of interstitial impurities. Zirconium samples with a purity of 99.91 wt.% were obtained.

Investigation of Effect of Electron Beam Surface Treatment of Titanium Alloy Ingots on Structure and Properties of Melted Metal

Based on accumulated experience of application of concentrated power sources for elimination of defects of ingot surface layer it is shown that remelting of surface layer of treated ingot provokes evaporation of alloying elements with high vapor pressure form a liquid pool, i.e. change of composition of treated layer in relation to ingot base. This can indicate the possible change of metal structure since it is determined by content of alloying elements.Ingots of wrought titanium alloys VT3-1, VT-6, VT20, VT22 of 400 and 600 mm diameter and up to 2 m length made by EBM were melted by electron beam for investigation of effect of electron beam treatment of a surface layer on composition, structure and properties of metal of the treated layer. The investigations of mass concentration of alloying elements, penetration depth, macro and microstructure of melted metal layer of treated ingots were carried out.The results of carried works showed that electron beam treatment allows efficient elimination of defects of the surface layer of titanium alloy ingots to 12 mm depth, content of alloying elements corresponds to grade composition, reduction is observed in content of alloying elements, in which vapor pressure is higher than in the alloy base, and respectively rise of content of alloying elements with vapor pressure is lower than in the alloy base. There is insignificant increase of content of interstitial impurities, i.e. oxygen and nitrogen, which do not have obvious effect on mechanical properties of treated metal. It is determined that macrostructure of metal in the treated layer of titanium alloy ingots is characterized with absence of pores and discontinuities, crystals close to equiaxial, the melted layer is formed by finer crystals in comparison with the rest part of the ingot, elongated in solidification direction, i.e. to ingot center. Solidification rates of more than 50 mm/min in the melted layer of titanium alloy ingots provoke two times refining of α-colonies and α-lamellas of ingot grains in comparison with the ingot base.

Atom probe study of grain boundary segregation in technically pure molybdenum

Molybdenum, a metal with excellent physical, chemical and high-temperature properties, is an interesting material for applications in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is limited because of the poor oxidation resistance at high temperatures and a brittle-to-ductile transition around room temperature, which is influenced by the grain size and the content of interstitial impurities at the grain boundaries. Due to the progress of the powder metallurgical production during the last decades, the amount of impurities in the current quality of molybdenum has become so small that surface sensitive techniques are not applicable anymore. Therefore, the atom probe, which allows the detection of small amounts of impurities as well as their location, seems to be a more suitable technique. However, a site-specific specimen preparation procedure for grain boundaries in refractory metals with a dual focused ion beam/scanning electron microscope is still required.The present investigation describes the development and successful application of such a site-specific preparation technique for grain boundaries in molybdenum, which is significantly improved by a combination with transmission electron microscopy. This complimentary technique helps to improve the visibility of grain boundaries during the last preparation steps and to evidence the presence of grain and subgrain boundaries without segregants in atom probe specimens. Furthermore, in industrially processed and recrystallized molybdenum sheets grain boundary segregation of oxygen, nitrogen and potassium is successfully detected close to segregated regions which are believed to be former sinter pores.

Effects of solute atoms on evolution of vacancy defects in electron-irradiated Fe–Cr-based alloys

The evolution of vacancy-type defects in Fe–Cr alloys (13–16 at.% Cr) undoped and doped with C, N, Au, or Sb and in conventional ferritic–martensitic steel (∼13% Cr) has been investigated using positron annihilation spectroscopy under electron irradiation at room temperature and subsequent stepwise annealing. Small vacancy clusters are formed in the undoped Fe–16Cr alloy, which anneal out between 320 and 550 K. It is shown that oversized substitutional solute atoms (Sb, Au) in the Fe–Cr alloy interact with vacancies and form complexes, which are stable up to 600 and 420 K, respectively. It is found that the accumulation of vacancy defects considerably increases in the alloys and the steel with an enhanced content of interstitial impurities. It is shown that this effect is related to the formation of vacancy-carbon complexes. It is known that chromium in iron decreases the diffusion mobility of carbon. Therefore, the structure of vacancy-carbon complexes and the kinetics of their annealing in Fe–Cr alloys differ from those in the Fe–C system.

Fracture toughness of sintered Mo–La 2O 3 alloy and the toughening mechanism

The K IC of sintered Mo–La 2O 3 alloy and pure molybdenum was tested and the micro-structure was investigated by SEM, TEM and AES. The results show that the K IC of Mo–La 2O 3 alloy reaches 24.76 MPa m 1/2, which is 2.5 times as much as that of pure Mo. The sintered Mo–La 2O 3 alloy and pure molybdenum have similar equiaxed grain structure. The AES analysis revealed that the same content of interstitial impurities exist on grain boundaries of Mo–La 2O 3 alloy and pure molybdenum. A toughening mechanism was proposed to be that large number of dislocations were pinned around La 2O 3 particles, shortening the efficient slip length and decreasing the dislocation pile-up on grain boundaries of Mo. The improvement of toughness of Mo–La 2O 3 alloy was attributed to the relief of stress concentration at the grain boundaries of Mo–La 2O 3 alloy and the weaker tendency to intergranular fracture on grain boundaries.

Processing maps for hot working of titanium alloys

In recent years, processing maps are being used to design hot working schedules for making near-net shapes in a wide variety of materials. In this paper, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described. In commercial purity α titanium, dynamic recrystallization (DRX) domain occurs at 775°C and 0.001 s −1 with an efficiency of power dissipation [2 m/( m+1) where m is the strain rate sensitivity of flow stress] of 43%. The DRX domain shifts to higher strain rates when the interstitial impurity content is lowered. In the near- α and α- β alloys like IMI 685, Ti–6Al–4V, the preform microstructure has a significant influence on the processing maps. For example, in the transformed β (Widmanstätten) preform microstructures, these alloys exhibit a domain of spheroidization at lower temperature and a domain of β superplasticity at higher temperatures, both occurring at slow strain rates. These domains merge at the β transus because of the occurrence of damage processes which lower the tensile ductility. On the other hand, processing maps on alloys with equiaxed preform microstructure exhibit a clear superplasticity domain in the α- β range and the β phase undergoes DRX with a power dissipation efficiency of ≈45–55%. Titanium materials in general, exhibit wide flow instability regimes due to adiabatic shear bands formation at higher strain rates and hence careful process design has to be adopted for successful forging and microstructural control.

Grain size strengthening in steel and its relationship to grain boundary segregation of carbon

AbstractA field ion atom probe study has been made of a low carbon steel after furnace cooling and rapid quenching from 700°C. Marked segregation of carbon to the boundaries was observed after furnace cooling, but not after rapid quenching. From comparison of previous and present results, it is concluded that the ky value of a steel can be related to the interstitial impurity content (mainly carbon) at the grain boundaries. Heat treatments that encourage grain boundary segregation of interstitials will result in steels having a high ky value.MST/1569

Influence of interstitial impurity on hydrogen solubility in zirconium.

Hydrogen solubility in Zr-N and Zr-C alloys with various compositions has been measured at a temperature of 8731, 048 K at a pressure below 100 Pa. All the solubility data closely obeyed Sieverts' law. The presence of carbon and nitrogen in zirconium increased the hydrogen solubility at a fixed pressure and provided the negative effect on the enthalpy of solution. Partial thermodynamic functions of hydrogen in the Zr-N and Zr-C alloys were obtained by a dilute solution model and compared to those obtained previously for Zr-O alloys. Dependence of partial molar quantities of hydrogen on interstitial impurity content in zirconium was discussed in terms of dilatometric effect, configurational effect and other contributions.

Use of the titanium solid state gettering process for the improvement of the performance of superconducting r.f. cavities

It is well known that the low-temperature thermal conductivity of niobium is mainly influenced by the interstitial impurity content of oxygen, nitrogen, hydrogen and carbon. Solid state gettering by materials with a higher affinity to gaseous impurities at elevated temperatures tends to improve the thermal properties of the material. In this paper, the use of titanium as a getter material is described and the effects of the treatment at different temperatures and for varying durations of time on the thermal conductivity of niobium are presented. Improvements of the thermal conductivity at 4.2 K by a factor of 15 corresponding to a value of 160 W m −1 K −1 have been obtained. Measurements on single-cell and multi-cell niobium r.f. cavities at 1500 MHz before and after treatment with titanium are reported. These measurements clearly show an improved performance of these cavities. Residual Q values as high as 1.7 × 10 11 and accelerating gradients of E acc = 11.9 MV m −1 have been measured.

Advances in production of high purity Nb for RF superconductivity

In the last three years, the purity of commercially available Nb has improved a factor of 10 in response to the needs of rf superconductivity for application to electron-positron storage rings for high energy physics as well as to electron linacs for nuclear physics and free electron lasers. Yttrification of this premium quality Nb enhances the purity by yet another factor of 2-3. Superconducting cavities built with high purity Nb provide higher accelerating gradients, increasing roughly as the square root of the purity enhancement. Results are presented on analysis of the interstitial impurity content on the ingot and final sheet product as well as on the sheet properties of the final product that are important to cavity manufacturers, such as residual resistivity ratio, yield strength, grain size, etc. Prospects for further advances are discussed.

Plastic deformation and dislocation arrangement of Nb[sbnd]34 at.% Ta alloy single crystals

Abstract Nb66 Ta34 alloy single crystals having orientations in the central part of the stereographic triangle and an interstitial impurity content of less than 10p.p.m. (C + N + O) have been deformed in tension at 77, 195, 295, 330 or 470 K. The dislocation behaviour and arrangement have been studied by slip-line observations, Laue X-ray back-reflection, X-ray topography and transmission electron microscopy (TEM). The slip-line investigations indicate that at each deformation temperature the plane that carries the maximum resolved shear stress (MRSS) is the active macroscopic slip plane. At low deformation temperatures (i.e. 77 and 195 K) the work-hardening curves are parabolic and the dislocation arrangement consists of primary and secondary screw dislocations and of tangles of non-screw dislocations. At intermediate temperatures (i.e. 295, 330 and 470 K) the work-hardening curves resemble that of f.c.c. crystals with an extended stage I. However, the experimental findings suggest that a gradual transit...

Effect of interstitial impurities on the mechanical properties of niobium and its alloys

1. The mechanical properties of niobium and its alloys depenet to a considerable extent on their content of interstitial impurities at both low and high temperatures; this opens up the possibility of alloying with such impurities finished components of niobium and its alloys in order to obtain the best combination of strength and ductility in each particular case. 2. The results of long-term strength tests on alloy 5VMTsU at 1100°C in vacum of 10−5 and 10−8 mm Hg have shown that the strength of this alloy on a basis of 800 h falls by 49% in a vacuum of 10−8 mm Hg.

Effect of impurities and strain amplitude on slip in fatigue of niobium single crystals

The type of slip deformation and the fatigue life in torsional fatigue of niobium single crystals were found to depend on both the cyclic strain amplitude and the interstitial impurity content. Two different slip-type populations were observed. At low strain amplitude, one population showed persistent slip bands, but the other, fatigued under identical conditions, showed asymmetric slip and asymmetric deformation. Differences in the two populations were related to interstitial impurity content. It is suggested that interstitial impurities affect the slip type by inhibiting cross-slip of dislocations.

A re-assessment of the monotectoid loop (β-Nb+β-Zr) in the niobium–zirconium system

The shape of the two phase, β-Nb + β-Zr, monotectoid loop in the niobium–zirconium phase diagram has been re-investigated by X-ray analysis for alloys with controlled content of interstitial impurities.

Electrotransport purification and some characterization studies of vanadium metal

High-purity vanadium metal having an interstitial impurity content of <5 wt ppm and a resistance ratio,R 300 K/R 4.2 K, of >1880 has been prepared by an electrotransport technique. The lattice parameter of vanadium of this purity is 3.0269 +- 3A. An extrapolated value for the room temperature resolved flow stress of vanadium free of C, O, and N was determined to be 0.41 kg/mm2 on this material.

High-temperature creep of polycrystalline chromium

A study has been conducted to determine the high-temperature creep properties of high-purity, polycrystalline chromium. Iodide chromium was consolidated into 100 g buttons by arc melting and fabricated to sheet by rolling. Test specimens machined from the wrought sheet were annealed in palladium-purified hydrogen in order to produce stable grain structures for testing. Chemical analysis indicated a total interstitial impurity content of < 100 ppm. Step load creep tests were conducted over the temperature range 0.51–0.78 T m . Results showed that creep rates can be represented by the general creep equation g3 D = k σ n E where n, for chromium, is equal to 4.3. This correlation, which uses the previously determined activation energy of 73.2 kcal/mol for self diffusion in chromium, and observations of surface slip lines and dislocation substructures strongly suggest that a diffusion controlled, dislocation climb mechanism is operative in the creep of chromium over the temperature range investigated.