Effect of nickel on sintering processes in the system Ti-Fe. III. High-temperature radiographic investigation of the sintering process

Conclusions During sintering of compacts of mixtures of powdered titanium and iron, and also of powdered titanium, iron, and nickel, the processes of formation of intermetallic compounds at temperatures below the eutectic temperature proceed as a result of solid-phase interaction between the components. The intensity of this interaction increases in dependence on the increase of the degree of dispersity of the powders that are used. Sintering at temperatures higher than the eutectic temperature is accompanied by the active formation of intermetallic compounds. The temperature ranges of intensive formation of intermetallic compounds in the systems titanium-iron and titanium-iron-nickel correspond to the temperatures of abrupt change of direction and speed of the sintering process on the dilatometric curves and to the appearance of exothermal peaks on the DTA curves.Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 32–39, June, 1988.     

Dilatometric investigation of the sintering of compacts from titanium and iron powder mixtures

Conclusions The sintering of mixtures of titanium and iron powders at temperatures above the eutectic point is accompanied by a strong reaction between the metals in contact, which manifests itself in a stepwise change in the dimensions of the compact being sintered. The rate of the growth in specimen volume due to the formation of an intermetallic compound during sintering at temperatures above the eutectic point increases with increasing fineness of the powders used and with rising sintering temperature.Translated from Poroshkovaya Metallurgiya, No. 7(235), pp. 26–29, July, 1982.     

Effect of nickel on sintering processes in the Ti-Fe system. Part I

Conclusions Diffusional interaction in the Ti-Fe-Ni system is more vigorous than that in the Ti-Fe system. The temperature of the first ternary eutectic point in the Ti-Fe-Ni system, determined by the contact melting method, is 1040°C. During the sintering of compacts from a mixture of Ti, Fe, and Ni powders at temperatures above the eutectic point growth is observed, whose extent depends on the sintering temperature and the particle size of the powder employed: The smaller the particle size and the higher the sintering temperature, the smaller is the growth.Translated from Poroshkovaya Metallurgiya, No. 7(247), pp. 34–39, July, 1983.     

Sintering behavior of compacts from a mixture of titanium and iron powders in hydrogen

Conclusions The compound TiFe can be synthesized by annealing compacts from mixtures of titanium and iron powders in a hydrogen atmosphere. The homogeneity of the sintering products of titanium and iron can be approximately assessed by determining the amount of hydrogen absorbed by them during sintering. The sintering of compacts in unpurified hydrogen is accompanied by the formation of complex oxygen-containing phases based on the intermetallic compounds of the Ti-Fe system.Translated from Poroshkovaya Metallurgiya, No. 4(280), pp. 34–40, April, 1986.     

Processing of iron-titanium powder mixtures by transient liquid phase sintering

Transient liquid phase sintering was examined for Fe-Ti powder mixtures. The experimental plan included the effects of several processing variables, such as green density, particle size, composition, heating rate, sintering temperature, and sintering time. During heating, pores form at the Ti particle sites. At the first eutectic temperature (1085 °C), liquid spreading is inhibited by a surrounding intermetallic envelope, leading to swelling. At the second eutectic temperature (1289 °C), the liquid penetrates along the iron grain boundaries and provides densification. The amount of densification depends on the amount of liquid formed at the second eutectic temperature and its duration as determined by the titanium content and heating rate. Formerly Research Assistant at Rensselaer Polytechnic Institute.     

Effects of nickel on the sintering behavior of Fe-Ni compacts made from composite and elemental powders

Injection-molded Fe-Ni parts made from composite and elemental powders were prepared, and the effect of nickel on the sintering of iron compacts was investigated. Dilatometry analyses showed that the alpha-gamma phase transformation temperature of the Fe-Ni compact changed from a fixed 912 °C for pure iron to a temperature range between 700 °C and 912 °C where two phases coexisted. The microstructure indicated that nickel impeded surface diffusion and slowed down the neck growth rate of iron powder in the early sintering stage. The dual phase and the small neck size at low temperatures suppressed the exaggerated grain growth, which usually occurs on carbonyl iron powders at 912 °C. It was also observed that nickel impeded the grain growth of iron at high temperatures. Thus, by reducing the exaggerated grain growth during phase transformation, impeding the grain growth at high temperatures, and with high diffusion rates of iron in Ni-rich areas, enhanced densification was obtained for Fe-Ni systems, particularly for those systems made from composite powders. However, when coarse nickel powder was added, expansion was observed due to the presence of large pores around nickel powders. These pores were formed because of the particle rearrangement which was caused by the Kirkendall effect.     

OBSERVATIONS ON THE SINTERING OF COMPACTSFROM A MIXTURE OF IRON AND COPPER POWDERS

Abstract

Three grades of iron powder-an atomized steel powder, a sponge iron powder reduced from magnetite with carbon, and a powder reduced from mill scale with hydrogen were mixed with 3% of copper powder and pressed into compacts. The diametral dimensional changes of the compacts during sintering below and above the melting point of copper were measured, their microstructures examined, and both related to the characteristics of the powders, particularly their specific surface. During sintering below the melting point of copper, compacts of all three powders shrank. Micrographic examination showed that the copper is transported by solid-state diffusion along the surfacesand grain boundaries of the iron powder particles. During sintering above the melting point of copper, compacts of the atomized and the MH-100 sponge iron powders grew while those of the hydrogen reduced mill-scale powder shrank. This phenomenon is related to the different mode of penetration of liquid copper in the compacts from the three powders, observed in the microstructures of the compacts.     

Reaction sintering of shock-compressed Ti + C powder mixtures

Shock compression was used to make dense compacts of Ti and C elemental powder mixtures for subsequent reaction sintering in near-net form. The reaction sintering experiments were performed in an induction-heated hot press at temperatures below the melting point of Ti, with hold times of less than a few hours. The unique combination of defect states and packing characteristics introduced during shock compression results in significant enhancement in the solid-state chemical reactivity of the powder mixtures. Consequently, the reaction behavior of the powders is altered, and the reaction mechanism is dominated by solid-state diffusion, resulting in a microstructure reminiscent of solid-state processes. Reaction-sintered TiC_x compacts, with bulk density in the range of 3.9 to 4.2 g/cm³ (80 to 85 pct TMD of TiC), were produced in near-net form. The compacts had a highly refined microstructure (<6-μm grain size) and microhardness in the range of 1360 to 1934 KHN. In this article, reaction sintering mechanisms involving solid-state diffusion in Ti + C powder mixture compacts will be described, along with characteristics of the titanium carbide produced by the com-bined shock modification and reaction sintering approach. HENRY A. GREBE formerly Graduate Research Assistant with the Department of Materials and Metallurgical Engineering, New Mexico Tech, Socorro, NM 87801. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

Volume changes experienced by Al-Zn compacts during liquid-phase sintering

Conclusions The sintering of compacts from aluminum powders with zinc additions in the presence of a liquid phase is accompanied by their volume growth and a corresponding increase in their porosity. The volume growth of compacts from Al-Zn powder mixtures during liquid-phase sintering is mainly due to the Kirkendall effect, which manifests itself during the formation of a solid solution on the aluminum particles as a result of the diffusion of zinc atoms from the melt to the particles preceding their dissolution in the liquid phase. In general, the porosity of sintered compacts is satisfactorily described by Eq. (1). When, however, the zinc content of a compact does not exceed its limit of solid-phase solubility in aluminum at the sintering temperature, the process of dissolution of aluminum in the melt may be ignored. In such a case the end porosity of compacts is described by Eq. (2) with a correction for shrinkage due to a regrouping of particles. The extent to which the volume of compacts from an Al-Zn powder mixture grows during sintering increases with increasing mean aluminum powder particle size.Translated from Poroshkovaya Metallurgiya, No. 10 (238), pp. 11–16, October, 1982.     

THE FORMATION OF URANIUM AND OF BERYLLIUM ALLOYS BY THE SOLID-STATE SINTERING OF MIXED ELEMENTAL POWDERS

Abstract

Large volume expansions accompany the formation of binary alloys of beryllium with uranium, thorium, iron, copper, zirconium, titanium, and vanadium, and of uranium with aluminium, during the sintering of the mixed, cold-compacted elemental powders. No expansion was detected during the sintering of binary mixtures of beryllium with aluminium, silicon, and magnesium, or mixtures of uranium with zirconium, molybdenum, iron, nickel, manganese, and chromium.

When it occurs, expansion is anisotropic, being greatest in the direction of compacting; the degree of anisotropy varies with the constituents and the composition of the alloys. In systems undergoing expansion, the volume expansion/composition graphs exhibit maxima. For a given system the magnitude of the maximum is a function of the shape of compact, the particle size of the powders, and the sintering time and temperature; the composition at which the maximum occurs is sensibly unaffected by these latter variables.

These experimental observations, together with those of other investigators, can be satisfactorily interpreted on the hypothesis that volume expansion is due to the formation of diffusional porosity during sintering.     

Kinetics of pore size increase in the activated sintering of powdered tungsten

Conclusions An increase in the size of pores determined by the Barus-Bechgold method in porous specimens from fine tungsten and tungsten-nickel powders takes place during heating to the isothermal sintering temperature. The addition of nickel to tungsten activates the pore size growth process. The size of the increased pore channels in porous solids from W and W-0.46% Ni powders in the temperature range 1000–1300°C depends on the particle size and sintering temperature. A correlation has been found between the integral shrinkage during isothermal sintering and the capillary stresses acting on the attainment of the isothermal sintering temperature in compacts from W-0.46% Ni powders of various particle sizes. The rates of isothermal shrinkage are the same, being independent of the previous history of the powders.Translated from Poroshkovaya Metallurgiya, No. 9(249), pp. 18–23, September, 1983.     

Sintering mechanism of iron powder with microadditions of boron

The effect of boron on the sintering of iron powder was investigated. Boron (0–400 ppm) was added to high-purity iron powder of the German firm “Mannesmann.” Powder mixtures were pressed to compacts of identical density and sintered at different temperatures in different atmospheres. The results indicated the absence of a liquid phase and no influence of boron in the high-temperature stage of sintering. However, boron additions substantially improved sintering at low temperatures (up to 800°C) due to an effect on the interparticle contacts. With a properly selected sintering regime, microadditions of boron substantially increase the density of sintered ingots.     

Refinement of Master Densification Curves for Sintering of Titanium

An approximate master curve for the densification of cold-pressed titanium powder during vacuum sintering was published previously. It was based on the combined results for three different titanium powders. The master densification curve model incorporates the effects of particle size, compaction pressure, sintering time, and sintering temperature on densification. The collection of a large amount of additional data now allows refinement of the model. Distinct curves are presented for three different titanium powders, prealloyed Ti6Al4V, and Ti-Ni binary alloys. The master densification curve is sigmoidal, but deviates from the ideal form at high sintered density; the relative sintered density saturates at 90 to 100 pct, depending on the particle size of the titanium powder, and to a lesser extent the compaction pressure. The master densification curve below the saturation level is slightly dependent on the compaction pressure.     

Sintering of very fine molybdenum and tungsten powders

Conclusions Vacuum annealing at a temperature above 900°K enables the specific surfaces of very fine loose tungsten and molybdenum powders to be varied in a wide range. The vacuum sintering of compacts pressed from very fine (particle size less than 0.05m tungsten and molybdenum powders is accompanied by severe cracking. In the hot pressing of very fine Mo and W powders produced by the pyrolysis of carbonyls in a stream of high-temperature plasma, a specimen density close to theoretical is reached at 1600°K i.e., at a temperature not less than 400°K lower than the sintering temperatures of powders of particle size more than 1 m. Sintering lowers the amounts of carbon and oxygen in Mo and W by more than half compared with the starting condition.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 47–51, January, 1982.     

Hot pressing and vacuum sintering of fine titanium nitride powder

Conclusions Increasing the specific surface of titanium nitride from 18 to 90 m²/g lowers the initial recrystallization temperature of loosely poured powder from 1300 to 600°K. The temperature at which blanks attain practically 100% density in the hot pressing of finely divided titanium nitride (a starting powder particle size of 0.05–0.07 m) is 1600°K, which is 500–700°K below the temperature level of full sintering of relatively coarsegrained powders (a particle size of about 0.5 m). At hot-pressing temperatures above 1800°K a fall in the density of sintered compacts is observed, which is apparently attributable to the beginning of nitrogen evolution from the nitride and also to the formation of microcracks. In vacuum sintering without a plasticizer, crack formation lowering the density of specimens by 3–4% is characteristic of the whole sintering temperature range. The grain size in hot pressing and vacuum sintering is practically the same, being determined chiefly by the sintering temperature and time. At the maximum specimen density the maximum grain size is 20 m.Translated from Poroshkovaya Metallurgiya, No. 12(204), pp. 27–32, December, 1979.The authors wish to thank V. I. Berestenko, T. N. Miller, and D. I. Medvedev for the provision of titanium nitride specimens.     

氧化铌阴极微观结构对电脱氧制备铌的影响

采用电脱氧法,以Nb2O5烧结片为阴极,石墨棒为阳极,在CaCl2-NaCl混合熔盐中制备金属铌.分别研究了压制压力、烧结温度对阴极片微观结构和电脱氧反应及其产物的影响.实验结果表明,烧结温度和压制压力对Nb2O5烧结片的晶粒尺寸、孔隙度和孔隙尺寸具有明显的影响,从而影响到电脱氧的反应速率和效果.晶粒细、孔隙度高和连通性好的烧结氧化铌阴极能够提高电脱氧的反应速率和效果.研究表明,12MPa压制成型后经1 200℃烧结的阴极片,电脱氧效果最佳.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

Effect of Fe addition on sintering behaviour of titanium powder

Abstract

The effect of iron on the sintering behaviour of titanium powder was investigated from two aspects: (1) diffusional homogenisation of iron; (2) densification of Ti-5Fe alloy. Under the present process conditions (heating rate of 5 K min^-1 and iron content 5 wt-%), iron dissolved into the titanium matrix thoroughly before the first eutectic temperature; potential liquid phase did not appear. The addition of iron enhances the sinterability of titanium alloys because the mobility of titanium atoms is accelerated by the rapid diffusion of iron. Most sintering shrinkage is achieved during the heating stage from 950 to 1250°C. Based on the diffusion creep mechanism of Nabarro-Hering, the result can be explained as a combination of the diffusion coefficient D and inherent local sintering stress σ, and the dissolution of iron in titanium is expected to reduce the creep strength of the Ti matrix at high temperatures due to its very fast diffusion rate. The effect of iron on the microstructure of Ti-5Fe alloy is also discussed. The formation of a Widmanstättenlike microstructure in Ti-5Fe alloy can be attributed to a β stabilising effect and a high diffusion rate of iron during furnace cooling.