Amorphous Phase Formation Analysis of Rapidly Solidified CoCr Droplets

This paper investigates amorphous phase formation and rapid solidification characteristics of a CoCr alloy. High cooling rate and high undercooling-induced rapid solidification of the alloy was achieved by impulse atomization in helium atmosphere. Two atomization experiments were carried out to generate powders of a wide size range from liquid CoCr at two different temperatures. Amorphous fraction and kinetic crystallization properties of impulse atomized powders were systematically quantified by means of differential scanning calorimetry. In addition, different but complementary characterization tools were used to analyze the powders microstructures. The fraction of amorphous phase within the investigated powders is found to be promoted by high cooling rate or smaller powder size. The critical cooling rate for amorphous phase formation, which is influenced by the oxygen content in the melt, is found to be ~3 × 10⁴ K s^?1 and corresponds to a 160-µm-diameter powder atomized in helium. Hardness of the powders is found to follow a trend that is described by the Hall–Petch relation when a relatively high fraction of crystalline structures is present and decreases with the fraction of amorphous phase.     

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons I. Densification and formation of porous structure in sintering of powders of tungsten-rhenium alloys

The effect of phase composition, fineness, and morphology of W-Re powders with different rhenium content on compaction, and formation of porous structure has been studied. Fine particle powders with a specific surface of 5–8 m²/g, mechanical mixtures of tungsten and rhenium powders, and W-Re alloy powders were used. Local nonuniform compaction is observed during sintering of powder mixtures and alloy powders. It was established that alloy powders and intermetallic powders have lower sintering activity.     

Toughness variation with test temperature and

A two phase heavy alloy composite based on 95 W-3.5 Ni-1.5 Fe (wt pct) was fabricated from elemental powders by liquid phase sintering. Past reports on the heavy alloys indicate considerable disagreement concerning cooling rate effects on toughness. The present experiments determined the effect of both cooling rate and test temperature on the properties of the 95 W heavy alloy. This alloy undergoes a ductile to brittle transition with decreasing test temperature; the transition temperature is close to room temperature. The cooling rate from post-sintering anneals carried out at temperatures greater than 1000 °C has a large influence on toughness; rapid quenching gives superior toughness. These findings support an impurity segregation explanation for embrittlement in the heavy alloys.     

Effect of cerium on wettability of mechanically milled Cu-based brazing alloy powder

CuSn powders and TiH₂ powders were milled using high energy mechanical milling to prepare Cu-based alloy powders for brazing diamond. And Ce was added to the milled Cu-based alloy powders to improve the wettability. It is found that the wetting angle reaches the minimum value 13.2° and the maximum spreading area 178 mm² is achieved when the amount of Ce is 0.75 wt%. And Ce remarkably reduces the surface tension of liquid alloy, which improves the climbing height along the diamond and forms a massive support profile. And the results show that Ce can effectively improve the transverse rupture strength (TRS) due to high wettability. The wear characteristics of the diamonds brazed with Cu-based alloy containing 0.75 wt% Ce mainly consist of integrity, micro-fracture, fracture and rubdown, diamonds pull-out can not easily happen.     

Microstructural change during liquid phase sintering of W−Ni−Fe alloy

The changes of bulk density and microstructures during heating and liquid phase sintering of 98W-1Ni-1Fe compacts prepared from 1 and 5 μm W powders have been observed in order to characterize the densification behavior. The compact prepared from a fine (1 μm) W powder begins to densify rapidly at about 1200°C in the solid state during heating, attaining about 95 pct density upon reaching the liquid phase sintering temperature of 1460°C. The compact prepared from a coarse (5 μm) W powder begins to densify rapidly at about 1400°C in the solid state, attaining about 87 pct density upon reaching the liquid phase sintering temperature. Thus, the skeleton of grains is already formed prior to liquid formation. During the isothermal liquid phase sintering, substantial grain growth occurs, and the liquid flows into both open and closed pores, filling them sequentially from the regions with small cross-sections. The grains subsequently grow, into, the liquid pockets which have been formed at the pore sites. The sequential pore filling by first liquid thus is shown to be the dominant densification process during the liquid phase sintering of this alloy, as has been demonstrated earlier with spherical model pores and as predicted theoretically.     

超音速气雾化高硅铝合金粉末高温加热组织及性能演变

本文利用超音速气体雾化法制备快速凝固高硅铝合金粉末，对合金粉末在不同温度加热处理后合金相组合，显微组织及显微硬度的变化特征进行系统研究。结果表明合金粉末在加热过程中，随着加热温度的升高，其组织中非平衡相逐渐转变为平衡相，同时有新的第二相沉淀析出，合金组织第二相在高温加热过程中有粗化趋势，合金显微硬度在低温加热时有下降趋势,随后升高温度不再下降，而是维持在相对稳定的水平。     

Powder sintering with application of an electric current and periodic mechanical pulses

Electric-discharge sintering of metal powders and powder composites is accompanied by anomalously high rates for compaction, and alloy and phase formation. Application of low-frequency pressure pulses on powders and powder mixtures during the concluding stage of electric-discharge sintering leads to even greater intensification of compaction and alloy formation. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 105–109, March–April, 2000.     

Production and properties of brass-base P/M constructional materials. A review

Conclusions The best processing properties are exhibited by brass powders manufactured by the diffusional impregnation technique, using a zinc powder, brass swarf, or a copper-zinc master alloy as a point source. However, as this is a very labor-intensive process, normally preference should be given to melt atomization as a method of manufacture of brass powders. Brass P/M parts produced by the conventional method consisting of pressing a powder and sintering the resultant compacts have porosities of not less than 7–10%, and consequently this method is not widely used for the production of constructional brass parts. The sintering of compacts from copper and copper-zinc master alloy powders gives more stable zinc contents compared with the sintering of compacts from copper and zinc powders; the greatest stability of chemical composition is exhibited by sintered compacts from a homogenized brass powder. The formation of diffusional porosity accompanying the evaporation of zinc may be prevented by performing sintering in the presence of a liquid phase (which appears in the presence of a phosphorus or lead addition), saturating the sintering atmosphere with zinc vapor, and adding carbonates or halides of alkali and rare-earth metals to starting powders. The mechanical properties of materials can be markedly improved by eliminating their porosity. This may be achieved by subjecting porous preforms to hot forging, which enables brass P/M parts to be obtained whose mechanical properties are comparable to those of cast parts.Translated from Eoroshkovaya Metallurgiya, No. 3(255), pp. 56–64, March, 1984.     

VIGA-CC法制备球形Ti-35.8Al-18.4Nb合金粉末及其性能研究

以真空自耗电弧熔炼的Ti-35.8Al-18.4Nb（质量分数）合金铸锭为原料,采用水冷铜坩埚真空感应熔炼气雾化制粉技术（water-cooled copper crucible vacuum induction melting-gas atomizing,VIGA-CC）制备球形Ti-35.8Al-18.4Nb合金粉末,利用振动筛分法、扫描电子显微镜（scanning electron microscope,SEM）观察、X射线衍射（X-ray diffraction,XRD）分析等手段对所制备的粉末进行性能表征。结果表明,VIGA-CC技术制备的粉末粒度分布较宽,主要分布在45~150 μm之间,呈正态分布,其中粒径不高于45 μm粉末收得率为15.8%,粒径不低于150 μm粉末收得率为12%;粉末流动性为27.2[s·(50 g)-1],粉末中氧质量分数的增量小于0.01×10-6,粉末整体氧质量分数小于0.06×10-6;TiAlNb合金粉末主要以γ（TiAl）相和α₂（Ti₃Al）相为主,随着粉末粒径的减小,冷却速率逐渐提高,γ（TiAl）相逐渐减少,α₂（Ti₃Al）相逐渐增加;大颗粒粉末表面为枝状冷凝组织,小颗粒粉末为光滑表面。     

Sintering of Titanium with Yttrium Oxide Additions for the Scavenging of Chlorine Impurities

Chloride impurities in titanium powders are extremely difficult to remove and present a long-standing problem in titanium powder metallurgy. We show that the detrimental effects of chlorides on the sintering of titanium can be mitigated with trace additions of yttrium oxide, which has a high affinity for the normally volatile species and forms highly stable oxychloride reaction products. Compacts that would otherwise exhibit gross swelling and excessive porosity due to chloride impurities can be now sintered to near full density by liquid phase sintering. The potency of yttrium oxide additions is observable at levels as low as 500?ppm. The scavenging of chlorine by Y₂O₃ appears to be independent of alloy composition and sintering regime. It is effective when used with high-chloride powders such as Kroll sponge fines but ineffective when used with powders containing NaCl impurities or during solid-state sintering. The identification of highly potent chlorine scavengers may enable the future development of chloride-tolerant powder metallurgy (PM) alloys aimed at utilizing low-cost, high-chloride powder feedstocks.     

Phase equilibria in the Fe-Ti-Cl-H-O system

Conclusions In the Fe-Ti-O system at 1373°K only Ti₂O₃ is stable, and iron is reduced from oxides and passes into solution. At practically all compositions of the solid solution preferential chlorination of titanium takes place, together with its transport to iron. During the chlorination of an iron-titanium alloy only gaseous chlorides are formed (T = 1373°K) because under these conditions equilibrium between the solid solution and liquid chlorides is not attained. Chlorination of titania in the presence of hydrogen as a reducing agent is impractical owing to difficulties in ensuring the required composition of the gaseous phase. The results obtained can be utilized for developing technological processes for the surface alloying of iron parts through a chloride phase and producing iron-titanium alloy powders.Translated from Poroshkovaya Metallurgiya, No. 1(265), pp. 52–55, January, 1985.     

机械合金化法制备(Ag-Cu28)-xSn合金的结构和性能研究

利用机械合金化法制备(Ag-Cu28)-xSn系合金粉末,借助差示扫描量热仪、X-射线衍射仪、扫描电镜等,研究了Sn含量对合金熔化温度、球磨时间对合金粉末的物相组成及显微结构的影响。研究表明:Sn对合金的熔化温度有显著影响,随Sn含量增加合金熔化温度下降趋势减缓;当Sn含量为30%时,合金熔化温度最低为539.3℃。球磨40 h时,(Ag-Cu28)-30Sn粉料合金化完全,其物相组成为Ag4Sn、Cu3Sn和Cu6Sn5。球磨初始阶段(Ag-Cu28)-30Sn粉料颗粒异常长大,球磨至40 h时合金化完成,颗粒断裂和焊合达到平衡,合金粉末粒度均匀,平均粒径约为5~10μm。     

Synthesis of nanocrystalline titanium carbide alloy powders by mechanical solid state reaction

A high-energy ball mill operated at room temperature has been used for preparing titanium carbide (TiC) alloy powders, starting from elemental titanium (Ti) and carbon (C) powders. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been used to follow the progress of the mechanical solid state reaction of Ti and C powders. A complete single phase of fcc-Ti₄₄C₅₆ alloy powders is obtained after a very short milling time (20 ks). The lattice parameter (a ₀ ) of the end product of Ti₄₄C₅₆ was calculated to be 0.4326 nm. The presence of excess starting reactant materials (Ti and/or C atoms) in the final product of the alloy powders could not be detected. The end product of Ti₄₄C₅₆ alloy powders possesses homogeneous, smooth spherical shapes with an average particle diameter of less than 0.5 μm. The internal structure of the particles is marked by fine cell-like features of about 3 nm. On the basis of the results of the present study, the mechanical alloying (MA) process appears to provide a powerful tool for the fabrication of Ti₄₄C{im56} alloy powders at room temperature. The mechanism of mechanical solid state reaction for formation of Ti₄₄C₅₆ alloy powders is discussed. Formerly Lecturer of Materials Science, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11884, Cairo-Egypt.     

Cu-Zr and Cu-Zr-Cr alloys produced from rapidly quenched powders

The production of alloys by means of ingot technology leads to micro-and macrosegregation, separation of phases and impurities, often into large, brittle particles, and coarse grain size. Alloy development is frequently restricted because of the coarseness of the structure and the resultant imposed limitations on hot and cold plasticity. One ready means of avoiding these problems is to produce the alloy in powder or pellet form; this permits attainment of high cooling rates in the liquid and solid, minimizes segregation, alters phase separation and distribution advantageously, and results in significantly finer structures. The powders utilized to produce wrought shapes may be much coarser than press-and-sinter powders, leading to important processing economies without sacrifice of structure refinement. The alloy systems Cu-Zr and Cu-Zr-Cr were selected for the present study; these are high conductivity, high strength alloys of commercial interest. Cooling rates for these specific powders using nitrogen atomization varied from about 10³ to 10⁴°CJsec. Powders were cleaned, canned, and hot extruded to produce bar stock; mechanical testing was performed on both as-extruded and thermomechanically-worked samples, with highly beneficial improvements in strength, ductility, and high-temperature structural stability. V. K. SARIN, formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass.     

Hard Alloy WC–24% Ni Obtained in the Solid Phase from Ultrafine WC,NiO, and C Powders. Part 2. Mechanical Properties

Mechanical properties of WC–24 mass% Ni alloy prepared by a combination in single stage of metal phase synthesis and compaction of an ultrafine mixture of WC–Ni powders by high-energy compaction and sintering are studied. Tungsten carbide, nickel oxide, and carbon are selected as the starting powders. After milling the initial powders the average particle size is 200-300 nm. Previously compacted briquettes of WC + NiO + C are heated, sintered, and pressed in the range 950-1300°C at vacuum of 0.133 Pa. Briquettes are also sintered in the liquid phase at 1350°C for comparison. Ultimate strength in bending, fracture toughness, ultimate strength in compression, and Vickers hardness are determined for specimens prepared at different temperatures. The dependence of mechanical properties on specimen consolidation temperature is studied. It is shown that these dependences for pressed specimens have a maximum at 1200-1250°C. The high level of properties (ultimate strength in bending 2500 MPa, ultimate strength in compression 3100 MPa, fracture toughness 19 MPa·m^1/2, and hardness 10.0 GPa) are achieved for a WC + Ni + C powder mixture to which carbon is added in the form of a liquid carbon-containing compound. Introduction into the mixture of commercial carbon grade P803 leads to low specimen mechanical properties. The effect on mechanical properties of porosity and pore size, and also grain boundary quality between particles is studied.     

液相反应烧结制备Al–Si合金半固态坯料

通过Al、Si元素粉末的液相反应烧结制备了用于半固态成形的Al–6%Si（质量分数）合金坯料。研究结果表明,Al–6%Si混合粉末具有较好的冷压成形性能,经500 MPa冷压之后,其相对密度可达到97.6%。混合粉末的冷压压制特性可用黄培云压制方程进行描述。Al、Si元素粉末可以在585 ℃下发生反应生成液相,液相围绕等轴状的固相Al晶粒形成半固态组织。Al晶粒尺寸和液相含量随着反应时间的增加而增加。Al–6%Si元素粉末的反应烧结是一个液相持续存在的反应烧结系统,可以通过控制反应时间来控制半固态坯料的微观组织。     

Current PM Literature for Engineers and Users

Abstract

Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards. A range of metal flowrates from 0·864 to 1·425 kg min^?1 was studied at a melt temperature of 450°C. Powders were sized using dry sieving down to 45 μm and wet sieving for smaller sizes. The Sauter mean diameter of the powders varied from 9·01 to 10·28 μm, depending on the rate of production. The size distribution was bimodal (albeit not very well defined) with the peak separation at ～44 μm. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Comparison of the tin powders with copper powders from another study, AA 2014 aluminium alloy powders, and magnesium and zinc powders from previous work showed that the differences in mean diameter and standard deviation are small among these common metals at a given volumetric production rate. This confirms the overriding importance of liquid metal volume flowrate under fixed gas flow conditions in gas atomisation, while the actual physical properties of the liquid playa secondary role. Although surface tension is secondary to volume flowrate in importance for controlling particle size, the study has shown that a liquid metal with lower surface tension and viscosity than AA 2014 alloy, together with a higher density, yields finer particles. PM/0667A     

Effects of Pr-Cu-Ti intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

By intergranular addition of Pr-Cu-Ti alloy powders in the Nd-Fe-B sintered magnets with the normal B component, we propose an approach to the optimization of grain boundary and local Nd-Fe-B composition system. The coercivity is enhanced from 1.42 to 1.86 T, while further addition leads to a reduction in remanence and coercivity. The analyses of phase composition reveal that Ti mainly exists in the form of metallic Ti alloy, and part of Ti combines with B to form the TiB₂ phase after the liquid phase sintering process. This process results in a consumption of B in the local Nd-Fe-B composition system and a change of the grain boundary component, which contributes to the formation process of the RE₆(Fe,M)₁₄ phase after the annealing process. Therefore, with the modification of grain boundary and composition system, the intergranular addition of Pr-Cu-Ti induces the generation of continuous thin grain boundary phases. It promotes the intergrain exchange decoupling, increasing the coercivity in the annealed magnet. While the excess addition results in the segregation of TiB₂, as well as the precipitation of TiB₂ into the Nd-Fe-B phase, which leads to structural defects. Thus, the further effort for the addition alloy with Ti to reduce the deterioration of the microstructure will lead to further improvement in magnetic properties.     

Deoxidation of titanium aluminide by Ca-Al alloy under controlled aluminum activity

Removal of oxygen in titanium aluminide (TiAl) by chemically active calcium-aluminum (Ca-AI) alloy was carried out around 1373 K with the purpose of obtaining extra-low-oxygen TiAl. The deoxidation experiments were preceded by an investigation of the phase equilibria of the system Ti-Al-Ca at 1273 and 1373 K. The compositions of the Ca-AI alloy deoxidant, which equilibrates with TiAl, and the experimental conditions suitable for the deoxidation were of particular interest. In experiments in which Ti-Al samples were submerged in liquid Ca-AI alloys at 1373 K, the surfaces of the samples severely deteriorated and became nodular. When TiAl powders were mixed with CaO and the deoxidant was supplied in vapor form, powders which initially contained 510, 1100, and 4200 ppm O were deoxidized to about 160, 490, and 670 ppm O after deoxidation at 1373 K in 86.4 ks (1 day). Among many conditions tested, the use of TiAl powders mixed with CaCl₂ was most effective for deoxidation at 1373 K. CaCl₂ was used as a flux to facilitate the deoxidation by decreasing the activity of the deoxidation product CaO. In the case that TiAl powders mixed with CaCl₂ and reacted with Ca-AI vapor at 1373 K for 86.4 ks, the powders initially containing 510, 1100, and 4200 mass ppm O were deoxidized to a level of 62, 140, and 190 mass ppm O, respectively. No significant change in morphology of the particle after deoxidation was observed. The titanium and nitrogen concentrations in the powders remained constant, whereas calcium, which was present only in trace amounts initially, increased up to 160 mass ppm after the deoxidation treatment.