Ductilisation of tungsten (W): On the increase of strength AND room-temperature tensile ductility through cold-rolling

Here we show that cold-rolling is a method to achieve room-temperature ductility in commercial purity, monolithic tungsten (W). Furthermore, we show that a decrease in rolling temperature concomitantly increases the strength and ductility of tungsten. So cold-rolling is a way to overcome the strength–ductility trade-off.In this work, we assess three different cold-rolled microstructures obtained from rolling at (i) 1000 °C (1273 K), (ii) 800 °C (1073 K), and (iii) 600 °C (873 K). Benchmark experiments were performed on a sintered ingot as well as on a hot-rolled plate. From these plates tensile test specimens were cut by spark erosion and tested at room temperature. The results show an increase of total uniform elongation, A_ut, ranging from 1.38% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 1.47% (cold-rolled at 600 °C (873 K)) and an increase of the total elongation to fracture, A_t, ranging from approximately 3% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 4.19% (cold-rolled at 600 °C (873 K)) with decreasing rolling temperature.The microstructure of the plates is analysed by means of scanning electron microscopy (SEM) (grain size, subgrains, crystallographic texture) and transmission electron microscopy (TEM) (bright field imaging, scanning TEM). Furthermore, strain-rate jump tests have been performed at 400 °C (673 K) to determine the strain-rate sensitivity, m, (sintered ingot m = 0.088, cold-rolled at 600 °C (873 K) m = 0.011) and the activation volume, V, (hot-rolled W plate V = 191 b³, cold-rolled at 600 °C (873 K) V = 111 b³) of the tungsten sheets.The question of why cold-rolling increases both strength and ductility is discussed against the background of cold-rolling-induced lattice defects. We speculate that the increase of ductility is caused by the ordered glide of screw dislocations, that move with low deformation incompatibility along the high-angle grain boundary (HAGB) channels (confined plastic slip).     

Ductilisation of tungsten (W): Tungsten laminated composites

Here we elucidate the mechanisms of plastic deformation and fracture of tungsten laminated composites. Our results suggest that the mechanical response of the laminates is governed by the plastic deformation of the tungsten plies. In most cases, the impact of the interlayer is of secondary importance.Severely cold-rolled ultrafine-grained tungsten foils possess exceptional properties in terms of brittle-to-ductile transition (BDT), toughness, and tensile ductility. The motivation for investigating laminated composites is to determine whether a bulk material can be made that retains the ductility of the thin tungsten foils.In this paper we analyse W-AgCu, W-Cu, W-V, and W-Pd laminates in their as-produced and annealed conditions (e.g. 10, 100 and 1000 h at 1000 °C (1273 K) in vacuum). The analyses comprise (i) the mechanical characterisation by means of three-point bending (damage tolerance), Charpy impact (BDT), and tensile tests (total elongation to fracture) as well as (ii) the in-depth analyses of the microstructure by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES).W-Cu laminates (60 vol% W) show 15.5% total elongation to fracture in a tensile test at room temperature. Furthermore, the BDT of tungsten laminated composites occurs at a temperature that is several hundreds of Kelvin lower than the BDT temperature of the pure tungsten bulk counterparts.Finally, we present the successful fabrication of a 1000 mm long W-Cu laminated pipe and show its high heat flux performance. Fabrication studies of high heat flux components made of tungsten laminates, in which the laminates are used either as heat spreaders or structural pipes, are presented.     

Ductilisation of tungsten (W): On the shift of the brittle-to-ductile transition (BDT) to lower temperatures through cold rolling

Here we show that cold rolling decreased the brittle-to-ductile transitions (BDT) temperature of tungsten (W). Furthermore, we show that the BDT temperature correlates with the grain size (the smaller the grain size, the lower the BDT temperature) following a Hall–Petch-like equation. This relation between the grain size and the BDT temperature is well known from ferrous materials and is generally accepted in the steel community.Our ductilisation approach is the modification of the microstructure through cold rolling. In this work, we assess three different microstructures obtained from (i) hot-rolled, (ii) cold-rolled, and (iii) hot-rolled and annealed (1 h/2000 °C, annealed in H₂) tungsten plates. From these plates, Charpy impact test samples with dimensions of 1 × 3 × 27 mm³, without notch, were cut and tested in the L-S and T-S directions. The results show the following BDT temperatures: 675 °C/948 K (L-S, “annealed”), 375 °C/648 K (L-S, “hot-rolled”) and 125 °C/398 K (L-S, “cold-rolled”). The microstructure of the plates is analysed by means of SEM (EBSD: grain size, subgrains, texture, KAM), FIB (channelling contrast) and TEM analyses (bright field imaging).The question of how cold rolling decreases the BDT temperature is discussed against the background of (i) microcracking, crack branching, and crack bridging effects; (ii) texture effects; (iii) the role of dislocations; and (iv) the impact of impurities, micropores, and sinter pores. Our results suggest that the availability of dislocation sources (dislocation boundaries, grain boundaries; in particular, IDBs and HAGBs) is the most important parameter responsible for the increase of the cleavage resistance stress, σ_F, or the decrease of the BDT temperature, respectively.     

Modelling of R-curve behaviour in ceramic/metal composites

The incorporation of metals into brittle solids is a successful way to improve their mechanical properties. In the present study, emphasis is placed on the toughness increase of such composites, shown by their R-curve behaviour. Although the increased toughness has been experimentally verified in many cases, the theoretical prediction is still a question of concern. The material specific property which controls the toughening effect is the bridging stress relation of the metal reinforcements. In a first part of this paper, a theoretical calculation of this bridging relation from crack opening displacement measurements will be presented. In a second part, a variety of bridging relations will be used to demonstrate their effect on the R-curve behaviour of a composite. Both calculations will be verified with experimental results of an Al/Al₂O₃ model composite material, where the bridging relation was also measured experimentally.

Abstract

Die mechanischen Eigenschaften spröder Werkstoffe können durch die Einlagerung metallischer Phasen entscheidend verbessert werden. In der vorliegenden Arbeit liegt der Schwerpunkt auf der Beschreibung der Zähigkeitsverbesserung solcher Werkstoffe, welche durch das R-Kurven Verhalten beschrieben wird. Obwohl eine Verbesserung der Zähigkeit in vielen Fällen experimentell nachgewiesen wurde, ist ihre theoretische Vorhersage nach wie vor Gegenstand der Forschung. Die Brückenspannungsfunktion der verstärkenden, metallischen Ligamente stellt hierbei die materialspezifische Größe dar, die das Zähigkeitsverhalten determiniert. Im ersten Teil dieser Arbeit steht die theoretische Berechnung dieser Brückenspannungsfunktion aus gemessenen Rißöffnungsprofilen im Vordergrund. Im zweiten Teil wird eine Reihe unterschiedlicher Brückenspannungsfunktionen benutzt und ihr Einfluß auf das R-Kurven Verhalten eines Verbundwerkstoffes demonstriert. Beide Berechnungen werden durch experimentelle Ergebnisse an einem Al/Al₂O₃ Modellverbundwerkstoff verifiziert, bei dem auch die charakteristische Brückenspannungsfunktion experimentell gemessen wurde.     

四方钨青铜(TTB)相Nb18W16O93材料的锂离子扩散路径(英文)

使用改进的固态烧结方法(1000℃,36h)成功合成四方钨青铜(TTB)相Nb₁₈W₁₆O₉₃,并通过XRD,SEM和XPS对其进行表征与分析。GITT结果表明,Nb₁₈W₁₆O₉₃(10^-12cm²/s)的锂离子扩散系数高于传统的Ti基负极。使用密度泛函理论模拟计算揭示锂离子的扩散机制。TTB相有3种扩散路径,其中扩散能垒最小的层间扩散(0.46 eV)比其他典型负极(例如,石墨0.56 eV)更具有优势,使TTB相Nb₁₈W₁₆O₉₃成为潜在的高特定功率阳极材料。     

90W和93W钨合金动加载下微(细)观响应分析

通过实验分析了90W和93W钨合金在动加载时其断裂机理及特性以及侵彻时其微观结构响应的异同点.通过对比发现,二者的断裂机理基本相似,但侵彻时,由于90W钨合金中存在较多的粘结相而使得其蘑菇头较小,其中的钨颗粒变形也较小;只在93W钨合金中发现了绝热剪切现象的征候.     

On the electrochemical behaviour of tungsten: the formation and dissolution of tungsten oxide in sulphuric acid solutions

The formation and dissolution of the anodic oxide film of tungsten were investigated in sulphuric acid solution of different concentrations. The dissolution process was found to follow a first order mechanism under different conditions. It seemed that, in highly diluted solutions, the hydrate formation is favoured, whereas in concentrated solutions, the process takes place via the formation of WO₂²⁺. The rate of dissolution was found to increase by increasing the initial current density of formation.     

Recrystallisation towards a single texture component in heavily cold rolled tungsten (W) sheets and its impact on micromechanics

In this study, we present the recrystallisation behaviour of heavily cold rolled tungsten sheets and show that the material recrystallises towards a single texture component, i.e. the rotated cube component {001}<110>. This result is remarkable as it distinguishes from the classical concept of recrystallisation and is likely to have a strong impact in various fields in science and technology.The thermal stability of as-rolled tungsten sheets featuring a thickness reduction of 98% (i.e. 4.1 in the logarithmic notation) was investigated. Annealings were performed in a vacuum and in the temperature range of 400 °C (673 K) to 2000 °C (2273 K) for a time period of 6 min up to 500 h. The annealing-induced evolution of the microstructure is displayed by hardness (HV0.1) and electron backscatter diffraction (EBSD) measurements. The results show that after an annealing of 6 min at 800 °C (1073 K), the hardness drops from 666 ± 6.3 to 625 ± 7.9 HV0.1.The evolution of the hardness is fitted by classical kinetic models for recovery (logarithmic kinetics) and recrystallisation. The apparent activation energy of half of the hardness loss, E_ΔHV/2, was found to be 356 kJ/mol (3.69 eV), which can be associated with the activation energy of grain boundary diffusion in tungsten.Furthermore, the hardness data allows for the distinction between two stages. In stage I, the material rapidly develops an equiaxed grain structure by the break-up and spheroidisation of the high aspect ratio grains. We do not observe the formation of nuclei in the classical sense. Moreover, the recrystallisation processes can be described as grain growth. We suggest the dominant driving force to be the reduction of the internal stored energy. Finally, the transition from stage I to stage II is caused by the onset of abnormal grain growth.     

Synthesis and characteristics of 80 vol.% tungsten (W) fibre/Zr based metallic glass composite

Based on full understanding of wettability between Zr based bulk metallic glass (BMG) matrix and tungsten (W) fibre, and liquid/solid interfacial atomic interaction, the high density and high strength 80 vol.% W fibre/Zr based BMG composite, with optimum interfacial layer, was successfully developed by strictly controlling the infiltration and solidification process. Its density was 17 g/cm³. The average value of tensile fracture strength was 1685 MPa. Fracture mode represented instant rupture which was vertical to the axial direction. The average value of compressive fracture strength and strain were 2550 MPa and 23% respectively. Its fracture mode was complex splitting along loading direction. The initiation of shear bands occurred when the compressive specimen reached plastic strength. Shear bands' continuous expanding, branching, propagation and build up responded the improvement of plasticity under the continuous loading. Shear bands distributed on the fracture surface in two ways. One was consistent with the maximum shear force direction and at 45° angle with the loading direction. The other was nearly horizontal and vertical to the loading direction. The characteristics of plastic deformation and fracture were also investigated and discussed in detail.     

Cracking behaviour of PVD tungsten coatings deposited on steel substrates

Tungsten coatings have been deposited on steel substrates by magnetron sputtering. For the same processing conditions, the increase of the coating thickness enhances the (111) component of the crystallographic texture whereas the residual stress level decreases. Tensile and four-point bending tests, associated with an acoustic emission analysis, have been performed inside a SEM chamber in order to study the cracking mechanisms. When the residual stresses are taken into account, an intrinsic critical cracking stress and the associated energy release rate can be determined; the obtained values suggest an intergranular cracking mechanism. No debonding has been observed at the interface despite the large plastic deformation of the substrate at the crack tips. The observed strain localisation modes in the substrate near the interface have been shown to have a major influence on the limit crack density.     

Effects of tungsten on the hydrogen embrittlement behaviour of microalloyed steels

The effects of tungsten (W) additions (0, 0.1, 0.5 and 1 wt.%) on the hydrogen embrittlement behaviour of microalloyed steels were systematically investigated by means of slow strain rate tests on circumferentially notched cylindrical specimens, and the mechanism of hydrogen-induced embrittlement was discussed. W addition is found to increase the activation energy of hydrogen desorption. Microstructural features affect the hydrogen embrittlement behaviour and fracture modes of microalloyed steels. It is suggested that the hydrogen-induced embrittlement in the studied microalloyed steels with different W additions is caused by the combined effects of decohesion and internal pressure in the presence of hydrogen.     

薄W(111) 和W(110) 晶面熔化的原子模拟

根据改进分析型嵌入原子法(MAEAM),利用分子动力学(MD)模拟研究含表面薄W(111)和W(110)晶面熔化机制。首先研究温度对晶面表层原子均方位移(MSD)的影响,表明表面和非谐效应对晶面结构和性质影响明显,且原子振动呈现各向异性的特点。在此基础上进一步分析其原子密度函数(ADF)和结构因子(SF)随温度变化,发现薄W(111)和W(110)晶面熔化机制不同,对于W(111)晶面,表面在其熔化转变过程中起主要作用,在3700K完全熔化,拟合无序原子分数随温度变化表明W(111)晶面预熔-熔化转变温度为3550K;而W(110)晶面则出现过热,直到4000K完全熔化,非谐效应在其熔化转变中起主要作用。     

Transformation behaviors and superelasticity of Ti_(50)Ni_(48)Fe_2 shape memory alloy subjected to cold-rolling and subsequent annealing

The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2→R→B19'two-stage martensitic transformation upon cooling and a B19'→B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress–strain curves show the critical stress for stress-induced martensite(rSIM)of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the rSIM decreases. Upon a cold-rolling and annealing at 623 K for30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.     

高温高强钨铼合金Gleeble热力模拟试验方法探讨

钨铼合金具有高熔点、高强度、高硬度、高再结晶温度等特点,作为结构材料广泛应用在特殊高温领域。针对采用Gleeble热力模拟试验机研究新型钨铼合金材料的高温热变形行为,存在热电偶焊接困难、钨铼合金强度高于碳化钨压头等问题,通过埋线法和耐高温无机胶固定热电偶,采用粉末热压烧结工艺制备碳化锆颗粒增强钨基复合材料作为高温高强压头进行解决,获得了钨铼合金在1 500 ℃、应变速率1s-1时压缩真应力 真应变曲线。该方法可用于测量钨铼合金真实高温强度,为钨铼合金工业生产过程中的热变形工艺参数确定提供准确数据。     

Morphological characterisation and spectroscopic studies of the corrosion behaviour of tungsten heavy alloys

Tungsten-based alloys have been used in a wide variety of industrial and military applications. These alloys are composed mainly of tungsten (88-95%) with various combinations of nickel, cobalt, iron and copper usually making up the remaining fraction.The corrosion behaviours of five munitions grade tungsten alloys of interest have been examined using immersion tests and wet-dry cycle tests to determine the mechanisms involved in the release of the metallic components. Analyses carried out using SEM, EDS and grazing incidence XRD techniques, show the release of tungsten as well as alloying elements due to galvanic corrosion resulting from the difference in electrode potential between the tungsten phase and the binder phase in all cases studied. The extent of corrosion was directly related with the dissolution of tungsten in the binder phase during the sintering stage of manufacture. In W-Ni-Co-Fe alloys binder phase corrosion was observed while the relatively noble tungsten phase was less affected. The reverse was observed for a W-Cu alloy.     

Performance of tungsten fibers for Wf/W composites under cyclic tensile load

Toughness improved tungsten-based composites are one of the currently considered material option for future fusion reactors capable to withstand both high heat flux and irradiation induced embrittlement. Today, fiber-reinforced composites (W_f/W) are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to neutron irradiation embrittlement especially below 800 °C. Operation of a material as an element of a plasma facing component (i.e. divertor monoblock or first wall armour) implies not only high heat flux exposure but also thermal cyclic fatigue caused by repetitive oscillations of the heat loads due to the nature of the plasma and the limitations on the capacity of its confinement. In this work, we assessed the performance of potassium doped tungsten fibers under cyclic loading applied in tensile mode. Stress-controlled fatigue tests were performed at room temperature, 300 °C and 500 °C increasing the load from 50% of the yield strength up to the ultimate tensile strength of the studied fibers. It is revealed that significant cyclic hardening emerges as the fatigue stress limit exceeds the yield strength already within a few cycles. Despite the noticed cyclic hardening, the wire can sustain few hundreds of cycles without any detectable damage unless the cycle stress is increased to reach the value above the mean ultimate tensile strength. Given this observation, we have studied the impact of the cyclic stress (σ_C) on the rupture strength and total elongation of the wires exposed to twenty loading cycles varying test temperature in the range 23–500 °C. At room temperature, the rupture stress after cyclic deformation progressively increases with σ_C and saturates at 2.7 GPa with a moderate reduction of the total elongation, while the nominal ultimate tensile strength of the wire is 2.5 GPa. Thus, the strength of the wire is increased by 200 MPa, on average. At elevated temperature, the rupture stress after the cyclic deformation increases by more than 300 MPa.     

(W,Mo)C粉末及(W,Mo)C-Co基硬质合金制备及性能

以WC-Mo_2C-C粉末混合物为原料,通过热压反应的方法在2 000℃的烧结温度下成功制备得到了"MoC"质量分数为5%和15%的(Mo,W)C粉末。并利用这些粉末分别在Ar和N_2气氛中烧结制备了(Mo,W)C-10%Co-VC-Cr_3C_2硬质合金。检测分析了合金的硬度HV30、断裂韧性K_(1C)、密度和磁性;并通过DTA分析测定了硬质合金的液相形成温度。结果表明,使用热压法获得的(Mo,W)C粉末制备(Mo,W)C-10%Co-VC-Cr_3C_2硬质合金,可以获得"MoC"质量分数为15%的具有低孔隙率的合金产品。当"MoC"质量分数为5%时,可以制备得到完全致密的硬质合金,其硬度HV30和断裂韧性K_(1C)均优于同粘结相含量的WC-Co硬质合金。在(Mo,W)C基硬质合金的微观结构中,发现了类似于金属陶瓷中的芯-环结构的硬质相颗粒。根据DTA曲线可以发现,与WC-Co硬质合金相比,(Mo,W)C基硬质合金具有较低的液相形成温度。     

Non-wetting behaviour of tungsten carbide powders in nickel weld pool: new loss mechanism in GMAW overlays

This paper presents a new mechanism, observed directly for the first time, to explain low carbide fractions in Ni–WC overlays produced with GMAW. In this loss mechanism, a significant amount of powder loss is a consequence of the non-wetting behaviour of tungsten carbide. High speed videography and quantitative metallography of weld deposits are used to identify this mechanism. The non-wetting mechanism found acts simultaneously with the carbide dissolution mechanism, which until now was the only suggested cause of low carbide fraction in GMAW Ni–WC overlays. The non-wetting behaviour is observed in both short circuit and free flight metal transfer, accounting for carbide losses between 20 and 70% in the experiments performed. Low carbide fraction has prevented the mainstream use of GMAW for Ni–WC overlays, despite the advantages of simplicity, capability of in situ repair, and low capital costs. The findings presented here have a potential large impact for further consumable and process development.     

Hot-pressed sintering of W/Cu functionally graded materials prepared from copper-coated tungsten powders

The three-layered(W-60 vol%Cu/W-40 vol%Cu/W-20 vol%Cu) W/Cu functionally graded material(FGM) containing a Cu network structure was fabricated at different temperatures by hot-pressed sintering produced from copper-coated tungsten powders.The effects of various sintering temperatures on relative density,microstructure,thermal conductivity,hardness and flexural strength were investigated.Scanning electron microscopy(SEM) and X-ray diffraction(XRD) analysis show that a Cu network extends throughou...