Microstructure and properties of tungsten heavy alloys fabricated by laser direct deposition

In this paper, tungsten heavy alloys (THAs) were fabricated by laser direct deposition technology. The phase composition, microstructure and the influences of Fe and Co content on the properties of THAs were investigated. The results showed that the laser deposition layers were composed by W, Ni₄W and Ni, fine and dense columnar crystals were distributed between non-melting W particles. The direction of crystals growth nearly paralleled the deposition direction at the bottom of deposition layers and nearly paralleled the scanning direction at the top. Both Fe and Co could increase the tensile strength of THAs and the elongation of W–Ni–Co alloys was higher than W–Ni–Fe alloys.     

Dynamic fracture of tungsten base heavy alloys

A recently developed short beam experimental technique has been applied to the characterization of the mode I dynamic fracture toughness (KId) of a commercial tungsten base heavy alloy (w/o-90W-7Ni-3Fe). The specimens were taken from a cylindrical swaged alloy bar and tested at a typical loading rate of the order of 10⁶ MPa\sqrtm/s. Three different crack orientations (one longitudinal and two radial) were investigated. The KIdvalues obtained for the three crack orientations are compared with the corresponding values obtained under quasi-static loading conditions (KIc). Our results show that the dynamic fracture of heavy alloys is both anisotropic and rate sensitive. For specimens containing radial cracks (LR, RR), the dynamic fracture toughness is higherthat its static counterpart. By contrast, for longitudinal cracks (RL), the dynamic fracture toughness is lowerthan the static one. Also, for radial cracks, both the (average) static and the dynamic fracture toughness are higher than in the longitudinal orientation. These new results about the anisotropy of the dynamic fracture toughness of the heavy alloys are reported and correlated with metallographic and fractographic examinations.     

Influence of stress condition on adiabatic shear localization of tungsten heavy alloys

Dynamic deformation and failure behavior of a tungsten heavy alloy (93W) under complex stress condition are studied using a split Hopkinson pressure bar (SHPB) apparatus. Cylindrical, step-cylindrical and truncated-conic specimens are used to generate different stress condition in an attempt to induce strain localization in the alloy. The microstructure of the specimens after tests is examined by optical microscopy and scanning electronic microscopy (SEM). It is found that in all the specimens, except the cylindrical ones, intense strain localization in the form of shear bands is initiated at stress concentration sites. In order to analyze the stress condition of different specimen geometry, finite element simulations are also presented. The Johnson-Cook model is employed to simulate the thermo-viscoplastic response of the material. It is found that dynamic deformation and failure modes are strongly dependent on the geometry of the specimens. The stress condition controlled by specimen geometry has significant influence on the tendency for shear band formation. The adiabatic shear band has general trends to initiate and propagate along the direction of maximum shear stress. It is suggested that further studies on the control of the stress condition to promote shear band formation be conducted in order to improve the penetration performance of the tungsten heavy alloy.     

Experimental studies of oxygen and carbon segregation at the interfacial boundaries of a 90W–7Ni–3Fe tungsten heavy alloy

The paper presents the results of a study of the heat treatment effect on the segregation in a 90%W–7%Ni–3%Fe alloy by an X-ray microanalysis and nanoSIMS technique. The measurements revealed oxygen and carbon segregations as well as enrichment of interfacial boundaries with nickel and iron.     

Mixed mode I/III fracture toughness in extruded magnesium alloys

Fracture toughness under mode I and mixed mode I/III loading were determined for magnesium (Mg) as well as binary Mg-Al and Mg-Zn alloys in as-extruded condition. It was found that in Mg and in Mg-1.25Zn alloy the fracture toughness under mixed mode I/III loading was higher than that under mode I loading whereas in binary Mg-1Al and Mg-3Al alloys it was lower than that under mode I loading. The results have been explained on the basis of the fracture mechanism and the nature of the stress fields ahead of the crack tip under mixed mode I/III loading.     

纳米晶钨粉对液相烧结93W合金组织性能影响

采用高能机械球磨方法制备了超细钨粉,经冷等静压和1465℃分解氨气氛中液相烧结制得高密度钨合金.研究了纳米晶亚微米颗粒钨粉对烧结态93W-4.9Ni-2.1Fe高密度钨合金微观组织及性能的影响.研究表明:采用超细钨粉与低温液相烧结技术,获得了高相对密度(大于99.7%)的烧结态高密度钨合金,且细钨颗粒组织均匀分布于粘结相中;与采用亚微米颗粒钨粉的烧结态钨合金相比较,不仅微观组织弥散分布,而且具有较高的力学性能;液相烧结态钨合金的力学性能主要与原始钨粉粒度及烧结温度有关.     

Fracture toughness of polycrystalline tungsten under mode I and mixed mode I/II loading

The fracture toughness of swaged polycrystalline tungsten was tested parallel and perpendicular to the swaging direction and under mixed mode I/mode II loading. The fracture mode is dominated by the microstructure and changed from all-transgranular cleavage in mode I to almost all-intergranular fracture in mode II. The mixed mode results can be related to two common failure criteria, the maximum tensile stress criterion (Maximum σ) and the maximum energy release rate criterion (Maximum G), but the large scatter in the data prohibits a clear distinction between the two criteria. Tests at 77 K show that the polycrystal is significantly tougher than the single crystal at this temperature. This is a consequence of the deflection of the crack into the grain boundaries and the imperfect texture (as compared to a single crystal) of the polycrystalline material.     

Dynamic fracture toughness of heavy section, narrow gap, gas tungsten arc weldments

Dynamic fracture toughness tests were performed on three, ASME SA533 Gr A Cl 2 narrow gap, gas tungsten arc weldments (minimum yield strength equals 70 ksi, 485 MPa). Linear elastic K_Id results were obtained at low temperatures while J-integral techniques were utilized to evaluate dynamic fracture toughness over the transition and upper shelf temperature ranges. Loading rates in terms of K averaged 4.41 × 10⁴ksi√(in.)/sec (4.88 × 10⁴MPa√(m)/sec). Tensile, Charpy impact and drop weight nil ductility transition (NDT) tests were also performed. The dynamic fracture toughness of both stress relieved (24 hr at 1125°F, 607°C) plus quenched and tempered SA533 Gr A Cl 2 narrow gap, gas tungsten arc weldments: (a) easily transcended the ASME specified minimum reference toughness K_IR curve, and (b) significantly exceeded the fracture toughness demonstrated by lower strength, stress relieved (3/3.5 hr at 1125°F, 607°F) SA533 Gr A Cl 2 automatic submerged arc weldments.     

Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion

Minor alloying additions such as molybdenum (Mo) have major effects on the localized corrosion resistance of corrosion resistant alloys containing chromium. However, progress in alloy development is mostly based upon empirical observations, where any mechanistic insights are largely relegated to the latter stages of localized corrosion (i.e., stabilization and propagation) that are more readily accessible experimentally. For instance, it is well understood that Mo and tungsten (W) affect repassivation of local active, as well as widespread transpassive, corrosion sites and Mo surface enrichment during corrosion is well-documented. In this paper, a comprehensive examination of the functions and mechanism by which selected Mo and W operate to improve the passivity and resistance to breakdown during the initial stages of localized corrosion of the most common Ni-based solid solution alloys is presented. It is shown that Mo and W exert considerable influence on many stages of corrosion, including both passivation and film breakdown, re-enforcing old and introducing more recent ideas in this comprehensive review of the current state of corrosion research on Ni-Cr-(Mo + W) alloys.     

A micromechanical model for the prediction of the temperature fracture behaviour dependence in metallic alloys

In the present paper, a micro-mechanical model based on energetic considerations is developed to simulate the effect of environmental temperature on the fracture toughness of metallic alloys. By considering a reference elementary volume (REV) with the same composition of the real material, the stress-strain field inside such a volume and the corresponding strain energy due to a temperature variation is determined. The energy balance to determine the material fracture toughness is generalised in order to take into account the temperature effects. The proposed micro-mechanical model is governed by few parameters which can be simply estimated, and allows us to determine the fracture toughness for any temperature below the room temperature. Such a model is applied to three metallic alloys which show a ductile-brittle transition temperature: ASTM A471, Carbon Steel D6ac, Steel S275 J2. From the comparison of theoretical results with experimental data, it can be concluded that the model seems to be able to correctly predict the fracture toughness at low temperatures.     

Densification behavior,mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1 wt.%Nb–0.1 wt.%Ni material sintered at 1600 °C was obviously greater than 93.7% of theory density for W material sintered at 2000 °C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1 wt.%Nb–0.1 wt.%Ni powder, and the powder milled for 10 h (W10) could be sintered to near full density (99.4% of theory density) at 1600 °C. The ball milling for 15 h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.     

Processing strategy for consolidating tungsten heavy alloys for ordnance applications

The environmental concern over the use of depleted uranium (DU) alloys as kinetic-energy (KE) penetrator for high strain rate applications has focussed the interest in tungsten alloys. However, in general, tungsten-based alloys exhibit about 10% lower performance than DU at high strain rate. This paper provides an update on some of the processing strategies adopted for fulfilling this objective.     

Aging Behaviors of W-Ni-Fe Ternary Alloys with High Ni-to-Fe Ratios

The hardness variation of two kinds of alloys with 36 wt pct W content and 7/3, 9/1 Ni-to-Fe ratios during strain aging at 800℃ was studied. The microstructures of the aged alloys were analyzed by X-ray diffraction and TEM. The results show that the strain aging hardness of W-Ni-Fe ternary alloy with 7/3 Ni-to-Fe ratio decreases monotonically with the increase of aging time. Under the same conditions, the hardness of 9/1 Ni-to-Fe ratio alloy decreases in the initial aging stage, but then increases as aging process goes on. X ray diffraction and TEM analysis show that there is not any precipitation depositing from the alloy with 7/3 Ni-to-Fe ratio during aging. The monotonic decrease in hardness of this alloy during aging process results from the recovery, recrystallization and solid solubility declining. In the alloy of 9/1 Ni-to-Fe ratio, the fine β phase precipitates dispersively during aging which hardens the alloy. The two different kinds of mechanisms (the softening one and the hardening one) decide the hardness variation of the alloy with 9/1 Ni-to-Fe ratio mentioned above.     

Making tungsten targets using tungsten oxide powder

A straightforward method of making tungsten targets from tungsten oxide powder on Cu backing is presented. A ceramic crucible containing WO₃ powder placed on a copper foil is heated in hydrogen atmosphere at temperatures ranging from 650 up to 850 °C. The resulting thickness of tungsten deposited in our trials was in the range of 0.5-3.4 mg/cm².     

Nanoindentation and micro-mechanical fracture toughness of electrodeposited nanocrystalline Ni-W alloy films

Nanocrystalline nickel-tungsten alloys have great potential in the fabrication of components for microelectromechanical systems. Here the fracture toughness of Ni-12.7 at.%W alloy micro-cantilever beams was investigated. Micro-cantilevers were fabricated by UV lithography and electrodeposition and notched by focused ion beam machining. Load was applied using a nanoindenter and fracture toughness was calculated from the fracture load. Fracture toughness of the Ni-12.7 at.%W was in the range of 1.49-5.14 MPa √m. This is higher than the fracture toughness of Si (another important microelectromechanical systems material), but considerably lower than that of electrodeposited nickel and other nickel based alloys.     

注射成形高密度钨合金脱脂工艺的研究

采用石蜡基多组元粘结剂体系,研究了注射成形高密度钨合金喂料的热脱脂工艺参数,包括样品厚度,温度,低温预脱脂和升温速率的。实验表明,当温度小于２５０℃时,脱脂量的对数与厚度的倒数呈比例,在长时间情况下,ＰＷ可以在２５０℃以衣全部脱除。温度,脱脂量和升温速率的撑峭当将导致脱脂缺陷的产生。     

Microstructural study of tungsten influence on Co–Cr alloys

Alloying elements, such as W, Mo, Mn, …, are of a great importance in the preoxidation of dental alloys and, consequently, on the ceramic/metal bond quality. This study deals with the effect of tungsten addition on the microstructural state of Co–Cr dental alloys, before the ceramisation process. These materials were prepared by unidirectional solidification. Their characterization has been carried out, using transmission electron microscopy (TEM) and X-ray diffraction. It shows that the addition of tungsten up to 8 wt.% induces structural transformations, which are believed to be linked to the added amount of tungsten.     

Microstructural dependence of fracture toughness in high-strength 7000 forging alloys

The effects of microstructural parameters associated with the coarse intermetallic phases particles and precipitates on the fracture toughness of the overaged 7000 alloy forgings are investigated. Detailed microstructural and fractographic analysis together with fracture toughness tests are carried out using three alloys with different (Fe + Si) contents. The fracture mechanisms are identified and area fractions of different fracture modes are assessed. The data are then quantitatively correlated to plane-strain fracture toughness, K_Ic, and the bulk microstructural attributes estimated via image analysis. A multiple micromechanism-based model is developed, which accurately describes the dependence of K_Ic on individual microstructural parameters.     

Aligned tungsten oxide nanowires on tungsten (100) substrates

WO₃ nanowires in body center cubic structure were grown on W (100) substrates by heating in an argon atmosphere. Scanning electron microscope and transmission electron microscope characterizations show the WO₃ NWs grew along the [100] crystallographic orientation and were aligned in three directions. The diameter of WO₃ NWs is in the range of several to 20 nm and the length is up to 1 µm. Field emission measurements show that the field emission current density can reach 1.8 mA/cm² under electrical field 10 V/µm and the turn-on field can be as low as 2.6 V/µm.