Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

Here we show that cold-rolling of tungsten (W) decreases the stable crack growth onset temperature. Furthermore, we show that stable crack growth is accompanied by crack bridging, which in turn is triggered by dislocation activity. The entire stable crack growth regime shows ductile intergranular fracture.Our ductilisation approach is the modification of microstructure through cold-rolling. In this work, we assess two different microstructures obtained from (i) cold-rolled and (ii) severely cold-rolled tungsten plates. From these plates, single-edge cracked-plate tension (SECT) specimens were cut and tested in the L-T direction. Crack growth resistance (R) curves were obtained using the direct-current-potential-drop method (DCPM). The experiments show the following results: cold-rolled plates are brittle at room temperature (RT), but show stable crack growth at 250 °C (523 K) and a fracture toughness, K_IQ, of about 100 MPa(m)^1/2 at a crack extension, Δa, of 0.6 mm. Severely cold-rolled tungsten plates show stable crack growth at RT and a fracture toughness, K_IQ, of 100 MPa(m)^1/2 at a crack extension, Δa, of 0.3 mm. Scanning electron microscopy (SEM) analyses of the stable crack growth region show intergranular fracture with microductile character.The question of why cold-rolling causes the stable crack growth onset temperature to decrease (or in other words, why cold-rolling causes the brittle-to-ductile transition (BDT) temperature to decrease) is discussed against the background of (i) intrinsic and extrinsic size effects, (ii) crystallographic texture, (iii) impurities and (iv) the role of dislocations. Our results suggest that the spacing between the dislocation nucleation sites (high angle grain boundaries (HAGBs) act as dislocation source) is the most important parameter responsible for the decrease of the stable crack growth onset temperature.     

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.     

Effect of cold-rolling on tensile strength of SiCw/Al composite

SiCw/Al composite was fabricated through a squeeze cast route and cold rolled to about 30%, 50% and 70% re-duction in thickness, respectively. The length of whiskers in the composite before and after rolling was examined using SEM. Some of the rolled composites were recrystallization annealed to remove the work hardening of matrix alloy. The tensile strength of the rolled and annealed SiCw/Al composites was examined and then associated with the change of the whisker length and the work hardening of matrix alloy. It was found that the tensile strength is a function of the degree of cold rolling. For the cold rolled composites, with the increase in the degree of cold rolling, the tensile strength increases at first, and decreases when the degree of cold rolling exceeds 50%. For the annealed ones, however, the tensile strength de-creases monotonously with the increase in rolling degree. The different changes in tensile strength between the rolled and annealed composites could be attributed to the result of     

Effects of working on strength and ductility of molybdenum and tungsten alloys

The variations in strength and ductility characteristics during industrial working of dispersion-strengthened molybdenum and tungsten alloys are described. With progressive working the ductile-brittle transition is decreased. Both strength and fracture elongation increase, in contrast to the usually observed inverse relation between these two properties. This is explained by the beneficial effects of dynamic strain aging which operates while the ingot is deformed at certain critical temperatures. However, such a twofold improvement is not observed in all alloys. It is not possible when the dispersates cannot dissociate; then, the alloying elements are not dissolved in the matrix and are not available for initiating dynamic strain aging, nor may the fracture elongation improve when extensive static age hardening is superimposed.     

On the strength of polycrystalline tungsten monocarbide

The paper consists of analytical and applied parts. The mathematical means of mechanics of hexagonal polycrystals are used in the former. Here the correspondence of the published data on single crystal elastic constants of tungsten monocarbide to elasticity moduli of polycrystalline WC has been analyzed. The relationship between ultimate tensile stress and ultimate compressive stress of polycrystalline tungsten monocarbide has been established too. The dependence of ultimate tensile stress and ultimate compressive stress of polycrystalline tungsten monocarbide on mean grain diameter is considered in the applied part. Here the available experimental data are used.     

Small tungsten carbide nanoparticles: Simulation of structure,energetics, and tensile strength

Ab initio methods of the density functional theory and pseudopotentials were used to study the structure, electronic states, total energy and tensile strength of WC nanoparticles. It has been found that the very small particles (having less than 15 WC atomic pairs) have a cube-like NaCl structure. Particles with trigonal and cubic structures have approximately the same energies in the region of 10–20 WC pairs; however, the local atomic structure keeps the NaCl-like alternation of W and C atoms. The WC₁₅ trigonal particle was used as a typical one to study the WC nanoparticle tensile strength. It has been found that W and C vacancies decrease the tensile strength, but not drastically. Electronic structure of nanoparticles looks like that of bulk fcc-WC with a large density of states at the Fermi level.     

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.     

Improvement of strength and ductility of Al-Cu-Li alloy through cryogenic rolling followed by aging

To develop an improved approach in achieving an excellent combination of high strength and ductility, the solutionized Al-Cu-Li plates were subjected to rolling at cryogenic and room temperatures, respectively, to a reduction of 83%, followed by aging treatment at 160 °C. The results indicate that Al-Cu-Li alloys through cryogenic rolling followed by aging treatment possess better mechanical properties. Rolling at cryogenic temperature produces a high density of dislocations because of the suppression of dynamic recovery, which in turn promotes the precipitation of T₁ (Al₂CuLi) precipitates during aging. Such high density of T₁ precipitates enable effective dislocation pinning, leading to an increase in strength and ductility. In contrast, room temperature rolled alloys after aging treatment exhibit lower strength and ductility due to low density of T₁ precipitates in the grain interior and high density of T₁ precipitates around subgrain boundaries.     

On the history of the determination of ductile fracture (ductility) of metals

Some aspects of fracture in metal forming are considered in this paper. (The topic is very extensive, and it is as old as metal forming itself, but the author will limit himself to the consideration of the subject matter, as it is the continuation of the modern theory of plasticity which is no more than a century and a half old.) In addition to the recent work of Oh (this journal, 53 (1995) 582–587), a brief historical survey is made mainly of Russian sources absent from the work of Oh. Reference is also made to works on the criteria of fracture in plastic forming used by Russian authors. (Unfortunately, these works are little known to English-speaking readers.)     

Eight routes to improve the tensile ductility of bulk nanostructured metals and alloys

Nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequat. This article reviews recent progress in tailoring the nanostructure to achieve coexisting high strength and high ductility at room temperature. The focus is on a summary of the strategies currently being pursued as well as the outstanding issues that await future research.     

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。     

纯钨经高压扭转后韧性及热稳定性观察

采用高压扭转法,在440℃对纯钨进行大塑性变形(HPT)。硬度测试表明试样表面纯钨经高压扭转后硬度高达1150HV。差热分析(DSC)结果表明等效应变较小的试样再结晶温度较高(高于1450℃),等效应变较大的试样再结晶温度较低(约800℃)。XRD分析结果表明试样晶格应变达0.35%,晶格常数达0.3177 nm,位错密度达2.4 × 1015 m-2。高压扭转可以使纯钨在低温下实现固结并具有高强度,一定的韧性和热稳定性。     

AZ31B镁合金的脉冲电流气体保护钨极焊过程参数优化(英文)

建立预测脉冲电流气体保护钨极焊AZ31B镁合金接头的拉伸强度经验方程。研究焊接过程参数如峰值电流、基础电流、脉冲频率和脉冲时间对焊接接头的影响。试验设计了一个四因素五水平的正交实验。建立的经验方程能够有效地预测脉冲电流钨电极惰性气体保护焊AZ31B镁合金的焊缝拉伸强度,可信度为95%。结果表明,脉冲频率对拉伸强度的影响最大,其次是峰值电流、脉冲时间和基电流。     

Evaluation of joint performance of 5056 aluminium alloy friction welded joints by heat input. On the tensile strength of welded joints (1st report)

Ferritic stainless steel SUS430 sheets were friction stir welded by using a Ni-base dual two-phase intermetallic alloy tool. After friction stir welding (FSW), the SUS430 work and the tools were evaluated in terms of microstructure and mechanical properties. The tensile specimens cut from the welded joints fractured in the base metal portion and their fracture strength was equal to that of the base metal. The stir zone comprised of recrystallized fine microstructure was observed, and also the thermo-mechanically affected zone was observed in an advanced side. Hardness in the upper one-third layer of the welded cross section was higher than the base metal. The admixture matter from work to tool surface occurred whereas that from tool to work surface did not take place in the scanning electron microscopy-EPMA resolution level. The amount of wear of tool was negligibly small, suggesting that the Ni-base dual two-phase intermetallic alloy is promising as a new type of FSW tool used for high melting materials such as steel.     

地锚拉杆整体抗拉强度损失分析

试验表明冷拔圆钢镶锻及热浸镀锌后整体抗拉强度有所下降，本文对这一现象进行了讨论，认为再结晶退火是其主要原因，在试验验证的基础上，提出了涂油镶锻的生产工艺。