The effects of impurities and heat treatment on the internal friction of tungsten at high temperatures

An improved internal friction technique was used to study the relaxation processes in pure tungsten and the following alloys: commercially doped tungsten (218W), W-1 pct ThO₂, 218W-3 pct Re, and 218W-20 pct Re. Internal friction experiments were performed on worked and recrystallized specimens in the temperature range of 300° to 3000°K. The effects of impurities and alloying additions on the damping and recrystallization behavior of tungsten are demonstrated. It is further shown that the internal friction curves can conveniently be used to determine the temperature of primary and secondary recrystallization. The important effect of impurities on the microstructure and high temperature strength is also revealed by the temperature dependence of shear modulus.     

Ni53Mn22Ga25磁性形状记忆合金的相变内耗

研究了Ni53Mn22Ga25磁性记忆合金在190～390K温度范围的内耗行为。结果表明．当合金发生正、逆马氏体相变时出现明显的内耗峰,并伴随模量变化出现极小值。马氏体相变后继续降温。在215K附近出现新的内耗峰,模量明显增高。合金在居里温度发生顺磁及铁磁正反转变时．内耗及模量均不发生变化。对上述现象进行了分析和讨论。     

A study of B2↔B19↔B19′ two-stage martensitic transformation in a Ti50Ni40Cu10 alloy

The B2↔B19↔B19′ two-stage martensitic transformation in a Ti₅₀Ni₄₀Cu₁₀ alloy has been investigated by electrical resistivity, DSC, X-ray diffraction and internal friction measurements. The shear modulus of B19 martensite has an unusually low value over a broad temperature range between the two shear modulus minima. The B2↔B19 transformation is thus proposed to proceed under the condition of deep shear modulus softening. X-ray diffraction results show that the B19↔B19′ is an incomplete transformation and that the monoclinic angle β of B19′ martensite will increasing with decreasing temperature. This indicates that the B19↔B19′ transformation has the characteristic of the continuously monoclinic distortion of B19′ martensite, which is similar to that of the continuously rhombohedral distortion of R-phase. The opposite behavior observed in electrical resistivity and DSC measurements for B2↔B19 and B19↔B19′ transformations is also discussed.     

Processing and damping behaviour of porous copper

Abstract

The well distributed open cellular porous copper was fabricated by present powder metallurgy technique based on space holder method, Depending on the volume fraction and size of the space holding particle, the porosity can be varied in the wide range of 30–85% and pore size from micron to millimetre in magnitude of order respectively. The damping behaviour and related relative dynamic modulus of the porous copper were investigated by a multifunction internal friction apparatus as a function of temperature from room temperature to 600°C. The results of investigation disclose that the porous copper can obtain a higher damping capacity than that of bulk one. In addition to this, an internal friction peak was found in the spectra of internal friction against temperature for the porous copper, it was proposed that the viscous sliding of the grain boundaries should be responsible for the appearance of the peak, and the dependence of the peak on porosity can be understood in terms of the anelastic relaxation mode of grain boundary.     

Effect of equal-channel angular pressing on the fatigue strength of titanium and a zirconium alloy

The static and fatigue strength of commercial-purity VT1-00 titanium and a Zr-2.5% Nb alloy subjected to equal-channel angular pressing (ECAP) are studied. The formation of a submicrocrystalline structure after ECAP is shown to result in significant hardening, an increase in the fatigue life at high stress amplitudes, and an increase in the fatigue limit as compared to the annealed state. The mechanisms of fatigue fracture of the materials in various structural states are investigated.     

Achieving enhanced ductility in a dilute magnesium alloy through severe plastic deformation

Experiments were conducted to evaluate the utility of a new processing procedure developed for Mg-based alloys in which samples are subjected to a two-step processing route of extrusion followed by equal-channel angular pressing (designated as EX-ECAP). The experiments were conducted using a Mg-0.6 wt pct Zr alloy and, for comparison purposes, samples of pure Mg. It is shown that the potential for successfully using ECAP increases in both materials when adopting the EX-ECAP procedure. For the Mg-Zr alloy, the use of EX-ECAP produces a grain size of ∼1.4 μm when the pressing is undertaken at 573 K. By contrast, using EX-ECAP with pure Mg at 573 K produces a grain size of ∼26 μm. Tensile testing of the Mg-Zr alloy at 523 and 573 K after processing by EX-ECAP revealed the occurrence of significantly enhanced ductilities with maximum elongations of ∼300 to 400 pct.     

The Microstructural, Textural, and Mechanical Properties of Extruded and Equal Channel Angularly Pressed Mg-Li-Zn Alloys

The microstructural and textural evolution of the Mg-6Li-1Zn (LZ61), Mg-8Li-1Zn (LZ81), and Mg-12Li-1Zn (LZ121) alloys were investigated in the as-extruded condition and after being equal channel angularly pressed (ECAPed) for one, two, and four passes. The shear punch testing technique was employed to evaluate the room-temperature mechanical properties of the extruded and ECAPed materials. Microstructural analysis revealed that the grain refinement in both LZ61 and LZ121 alloys could be achieved after multipass ECAP through the continuous dynamic recovery and recrystallization process. For the LZ81 alloy, however, the occurrence of Li loss in the four passes of ECAP condition partly offsets the grain refining effect of the ECAP process by increasing grain size and volume fraction of the α phase. Textural studies in both LZ61 and LZ81 alloys indicated that the developed fiber texture after extrusion could be replaced by a typical ECAP texture, where the basal planes are mainly inclined about 45 deg to the extrusion axis. The increased volume fraction of the β phase in LZ81 significantly affected the α-phase texture by decreasing the intensity of the maximum orientations of the basal and prismatic planes in all deformation conditions, compared with the LZ61 alloy. It was also observed that the abnormal grain growth might be promoted by the strong texture developed in the extruded LZ121 alloy. This texture became more randomized when the number of ECAP passes increased. The SPT results showed that the shear yield stress, ultimate shear strength and normalized displacement in all studied alloys were improved through the grain refinement strengthening caused by ECAP. It was also established that increasing Li content decreased the shear strength and enhanced the shear elongation in all deformation conditions.     

Effect of equal-channel angular pressing and annealing conditions on the texture,microstructure, and deformability of an MA2-1 alloy

Equal-channel angular pressing (ECAP) of am MA2-1 alloy according to routes A and Bc is used to study the possibility of increasing the low-temperature deformability of the alloy due to grain refinement and a change in its texture. To separate the grain refinement effect from the effect of texture on the deformability of the alloy, samples after ECAP are subjected to recrystallization annealing that provides grain growth to the grain size characteristic of the initial state (IS) of the alloy. Upon ECAP, the average grain size is found to decrease to 2–2.4 μm and the initial sharp axial texture changes substantially (it decomposes into several scattered orientations). The type of orientations and the degree of their scattering depend on the type of ECAP routes. The detected change in the texture is accompanied by an increase in the deformability parameters (normal plastic anisotropy coefficient R, strain-hardening exponent n, relative uniform elongation δ_u) determined upon tensile tests at 20°C for the states of the alloy formed in the IS-4A-4Bc and IS-4Ao-4B_cO sequences. The experimental values of R agree with the values calculated in terms of the Taylor model of plastic deformation in the Bishop-Hill approximation using quantitative texture data in the form of orientation distribution function coefficients with allowance for the activation of prismatic slip, especially for ECAP routes 4B_c and 4B_cO. When the simulation results, the Hall-Petch relation, and the generalized Schmid factors are taken into account, a correlation is detected between the deformability parameter, the Hall-Petch coefficient, and the ratio of the critical shear stresses on prismatic and basal planes.     

铜铁预合金粉末对铜基摩擦材料摩擦磨损性能的影响

利用粉末冶金方法制备了含不同质量分数铜铁预合金粉末的铜基摩擦材料,并在不同温度下对材料摩擦性能进行测试。结果表明:铜铁预合金粉末的引入使得铁元素在烧结后铜基体中及铜基体与其他组元界面处析出,阻碍了烧结,导致材料密度下降。存在于界面处的铁以及反应生成的珠光体成为硬质强化相,使得材料的磨损机理从纯铜基体时的黏着磨损向添加铜铁预合金粉末之后的磨粒磨损转变,导致摩擦系数先下降后上升。200～250 ℃为摩擦系数保持稳定的临界温度。当超过临界温度时,摩擦表面铜软化,其自润滑作用使得摩擦系数下降。含30%铜铁预合金粉末的铜基摩擦材料（质量分数）的摩擦磨损性能最佳,这是由于此时摩擦材料兼具铜良好的塑性以及生成的适量硬质相能够强化摩擦表面。     

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy have been studied by internal friction and shear modulus measurements, hardness test and TEM observation. The martensite stabilization can be induced by cold rolling at room temperature. A variety of deformed martensite structures has been observed. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. The hardness test results also support this viewpoint. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. Experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy.     

Experimental and theoretical aspects of internal friction associated with the melting of embedded particles

Internal friction associated with the volume change during melting was studied in an aluminum-16 wt% indium alloy. This alloy was processed to obtain microstructures consisting of nominally pure indium inclusions embedded in an aluminum matrix. Two sharp internal friction peaks were observed near the indium melting temperature of 156°C and both were associated with the formation and growth of liquid nuclei. A general theoretical model for the internal friction was developed which incorporates both the rates of phase deformation and the relaxation of the surrounding matrix. The proposed model considers the effect of the hydrostatic stress generated during melting that initially opposes the transformation. Matrix relaxation around the transforming particles resulting from vacancy flux and dislocation climb contributes an additional frequency dependence to the internal friction. Both the matrix relaxation and the internal friction peak height were found to be dependent upon thermomechanical processing.     

Solid Solution Strengthening for Mg-3.0 Mass?Pct (2.71 At.?Pct)Al and Mg-0.06 Mass?Pct (0.036 At.?Pct)Ca Alloys

Tensile tests were carried out at 123 K to 373 K (–150 °C to 100 °C) on pure Mg, Mg-3.0 mass pct (2.71 at. pct) Al alloy, and Mg-0.06 mass pct (0.036 at. pct) Ca alloy. Little decrease occurred in the yield stress of the pure Mg and the Mg-Ca alloy with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). For the Mg-Al alloy, however, its yield stress decreased with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). Analyses based on the existing solid-solution strengthening theories, focusing on the athermal component of stress, revealed that the dominant strengthening mechanism is the shear modulus effect for the Mg-Ca alloy and the chemical interaction for the Mg-Al alloy. It is suggested that the shear modulus effect is dominant at a low concentration and the chemical interaction is dominant at a high concentration for Mg alloys.     

Pseudoelastic and anelastic effects due to the motion of twin boundaries in copper-based alloys

Copper alloys, e.g. with a few at.% of Ge, are known to deform by slip and twinning when subjected to a tensile test at room temperature. Such alloys exhibit a large hysteresis loop or pseudoelasticity in a loading-unloading cycle, which is believed to be due to reversible motion of twin boundaries. The mobility and the annealing behaviour of twinning dislocations in several copper alloys with Ge, Si or Al have been studied by low-frequency internal friction measurements, electrical resistivity measurements and electron microscopy. Deformed samples show a characteristic anelastic effect; internal friction increases linearly with temperature in the range from 80 to 280 K and falls down rapidly above room temperature. The magnitude of the anelastic effect depends on the solute concentration and on the degree of deformation similarly to that of the pseudoelastic effect. The internal friction is tentatively explained as arising either from l ocal motion of twinning dislocations or from rearrangement of metastable stacking sequences.     

GH4720Li合金中温区低周疲劳行为研究

奥氏体沉淀强化GH4720Li合金用于制备航空发动机涡轮盘,其服役条件下的疲劳性能是当前研究的热点.研究了GH4720Li合金在中温区(450和550 ℃)不同应变条件下的低周疲劳行为.结果表明,GH4720Li合金的剪切模量和杨氏模量随着温度的升高而不断降低,但是中温区的抗拉强度和屈服强度不随温度而剧烈下降.GH47...     

Texture of the Mg-0.49% Al-0.47% Ca alloy after equal-channel angular pressing

Equal-channel angular pressing (ECAP) is used to refine grains and to change the texture of the initial pressed Mg-0.49% Al-0.47% Ca alloy rod in order to study the possibility of increasing the low-temperature ductility of the alloy. ECAP is performed at 300°C in six passes at a total true logarithmic strain ε = 6.8 according to route B _C . As a result, an ultrafine-grained structure with a grain size of 2–5 μm forms. The initial texture of the pressed rod is characterized by the [12 11] axial orientation parallel to the pressing direction. After ECAP, the texture changes its type and is characterized by a set of preferred orientations that represent basal planes located at an angle of 40°–50° with respect to the pressing direction. An analysis of the generalized Schmid factors, which were calculated for the main operating deformation systems with allowance for the critical shear stresses in them and the volume fractions of the preferred orientations, indicates that the texture caused by ECAP affects the decrease in the strength properties of the alloy measured at room temperature and the increase in the low-temperature ductility of the alloy.     

纯钼骨架熔渗Mo-Cu合金工艺研究

研究了利用纯钼骨架熔渗法制备Mo70Cu30、Mo60Cu40(质量分数)合金的相关工艺。结果表明:试验原料可选取经高温预处理及筛分后费氏粒度为5.2μm,粒度分布范围较窄及C、O含量低的纯钼粉,不添加诱导Cu粉。利用限位法压制成型的钼素坯经H2气氛在1200～1300℃预烧结出孔隙率ε(%)为33.5%、45.2%的纯钼骨架,在H2气氛熔融态Cu中经1 200～1 350℃熔渗60～120 min可制得Cu含量为31.3%、40.7%的Mo70Cu30、Mo60Cu40合金。利用纯钼骨架熔渗法制备的Mo-Cu合金相对密度可达到98%以上,微观形貌可见Mo、Cu两相均匀分布。     

Densification of Al-2024 and Al-2024/Al2O3 Powders by Conventional P/M Route and ECAP: A Comparative Study

In this study, mechanically alloyed Al-2024 and Al-2024/Al₂O₃ powders are densified by conventional sintering and by equal channel angular pressing (ECAP) with and without back pressure. The powder was encapsulated in an aluminium can for consolidation through ECAP. The properties obtained in the compacts by conventional sintering route and by ECAP are compared. The effect of conventional sintering and ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. Room temperature back pressure aided ECAP results in nearly full denser (97?% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the consolidated compacts.     

Anelastic behavior of barium-titanate-based ceramic materials

The internal friction (Qsu−1) and Young’s modulus (E) of BaTiO₃-based ceramics were measured vs temperature from −100 °C to 150 °C. Rectangular bars of high-density (96 to 99 pct) ma-terials were driven electrostatically in flexural vibration at a resonance frequency of about 3 kHz, at maximum strain levels of about 10⁻⁶. The curves ofQ ⁻¹(T) andE(T) allow the study of the following three phase transformations: tetragonal to cubic (about 130 °C in pure material), orthorhombic to tetragonal (about 0 °C in pure material), and rhombohedral to orthorhombic (about −80 °C in pure material). Internal friction and modulus data were obtained on pure material and on materials doped with niobium and cobalt to give semiconducting and insulating X7R behavior. Permittivity, dielectric loss, and microstructure data are given and used to aid interpretation of the mechanical measurement data. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee     

Unidirectional Solidification of Eutectic Cast Iron

Abstract

High intrinsic damping in alloys finds many applications in the elimination of unwanted vibrations and reduction of acoustic noise. The increasing interest in these materials has resulted in the development of a new class of engineering alloys, the HIDAMETS (high-damping metals), specifically tailored for use in vibration-resistant structures. Numerous different physical mechanisms (usually controlled by the alloy microstructure) can be responsible for the level of internal friction or damping in a candidate alloy for a particular application. Consequently, alloy selection depends mainly upon the conditions to be encountered in service, i.e. temperature, frequency of vibration, amplitude of vibration, magnetic field, etc. Characterization of the internal friction in high damping alloys is described and some of the mechanisms involved are discussed. Results obtained for two different alloys are presented. The first is a commercial Mn-Cu alloy used for marine propellers and the second is a die-cast Zn-Al alloy with possible application in the automotive industry.     

Mechanisms of slip mode modification in F.C.C. solid solutions

While it is widely recognized that alloy factors other than stacking fault energy play a role in promoting planarity of slip, no detailed model has been advanced to explain the mechanism of planar vs wavy slip mode. Therefore, friction stress effects of alloying on disolcation behavior are reviewed, as well as the role of stacking fault structure in inhibiting the clustering of dislocations in planar slip metal. A model of cross-slip inhibition (and thus planar slip behavior) is developed from the idea that the joining of partials is resisted by frictional effects. Planarity of slip is promoted not only by low stacking fault energy but by increase in shear modulus, atomic size misfit and solute content. A critical solute concentration is predicted by the model for the transition from wavy slip to planar slip and this is shown to be in good agreement with observations for copper base solid solutions and other alloy systems.