Effects of pressing temperature on the consolidation of titanium nickelide powders

The plastic aftereffect and the shape memory effect hinder the manufacture of high-density materials based on titanium nickelide by traditional powder metallurgy methods. Studies have been made on the effects of temperature and pressing pressure for powders containing titanium nickelide on the elastic aftereffect, crystal growth during the initial stage of sintering, and density of the sintered material. If the pressing is at a temperature exceeding the temperature for the start of the martensite transformation, one can increase the density of the material substantially by comparison with pressing at room temperature. For that purpose, the pressing temperature needs to be raised only to 200°C.Tomsk Polytechnical Institute. Translated from Poroshkovaya Metallurgiya, No.3 (363), pp. 19–22, March, 1993.     

Effect of the quenching temperature on the structure and deformation mechanism of a VT22I alloy

The effect of the temperature of heating for quenching on the phase composition, structure, and deformability of a high-strength VT22I titanium alloy is studied. After quenching from a critical temperature or above, an unstable β phase forms in the structure of the alloy; upon further deformation up to 3%, this phase transforms into the α” martensite. This process occurs at low stresses (150–200 MPa) and is characterized by an increase in the internal friction of the material, which is related to the high mobility of interfacial and twin boundaries of β-phase and martensite crystals. In VT22I alloy samples quenched from below the critical temperature, a mechanically stable β phase forms; it is deformed according to a slip mechanism at sufficiently high stresses (550 MPa).     

Effect of cyclic pressure consolidation on the uniformity of metal matrix composites

The cold compaction of ceramic-metal powder blends by the cyclic application of pressure improves compacted density. This enhanced densification has been attributed to a repeated generation of mismatch strain between the matrix and reinforcement phases, in a process similar to that seen in the thermal cycling of fully dense composites. It was postulated that cyclic compression may also reduce density gradients and improve overall uniformity. To test this hypothesis, aluminum and alumina powders were mixed and consolidated under both monotonic and cyclic loading at room temperature. Three aspect ratios were used and double-action compaction was included. X-ray computed tomography (CT) shows that the spatial density of uniformity improved during cyclic compaction. The distribution of ceramic particles was found to be more uniform following pressure cycling, especially for compacts having lower aspect ratios. A composite with 40 pct reinforcement achieved nearly uniform full density following double-action cyclic compaction. Cyclic compaction improves overall density as well as density uniformity and reinforcement distribution uniformity.     

Effect of particle composition on consolidation of hot briquetted iron

Abstract

The influence of the carbon concentration of directly reduced iron (DRI) powders on the compressibility and fracture strength of hot briquetted iron (HBI) has been studied. Industrially produced DRI, pure iron powder and Fe–C alloy powders (synthetic DRI) were used in the study. It was found that the mechanism of compaction could be attributed to pure yielding. The pressure required to attain a given density increased proportionally with the carbon content. The morphology and phases present in DRI powder had a significant influence on the compressibility. The fracture strength of the compacts increased with increasing carbon content of the DRI powder. These observations are discussed with reference to the current understanding of the mechanisms of compaction and fracture of compacted particulate materials.     

Al-Ni-Y合金雾化粉末的超高压固结成形及其机理研究

采用紧耦合惰性气体雾化技术制备具有非晶和纳米晶结构的Al82Ni10Y8(%)合金粉末, 粒径小于26μm粉末为非晶和纳米晶结构, 而粒径小于15μm粉末为非晶单相结构。在250, 310, 360, 400, 450℃5个不同温度下对粉末进行了超高压固结成形, 并对致密化后合金的显微结构进行了分析。致密化后粉末颗粒基本保持原有粉末形貌, 但随着致密化温度的升高, 晶化和晶化相长大愈发明显, 合金密度呈增高趋势。对粉末致密化的机理进行了讨论, 认为非晶相的粘滞流动是低温下获得高致密度的主要原因。     

温度场对热力模拟压缩试验变形行为影响的数值模拟研究

针对热力模拟压缩变形试验中由于模具与材料之间的隔热效果不理想导致的试样端部温度下降造成的试样温度场不均匀情况,对热力模拟压缩试验的影响使用数值模拟方法进行了研究,考察了不同温度场分布对于压缩试验中的变形抗力、变形后试样应变分布和应变速率的影响.结果表明,温度场的分布对载荷力、应变和应变速率的分布都有较大影响,会影响热力模拟压缩试验结果的正确性.     

Effect of fiber strength on the room temperature

To increase understanding of what controls SCS-6 SiC/Ti-24Al-11Nb (at. pct) composite strength, fibers of known strength were incorporated into composites and the effect of fiber strength variability on room temperature composite strength was investigated. Fiber was also etched out of a composite fabricated by the powder cloth technique, and the effect of the fabrication process on fiber strength was assessed. The strength of the composite was directly correlated with the strength of the as-received fiber. Fabrication by the powder cloth technique resulted in only a slight degradation of fiber strength. Examination of failed tensile specimens revealed periodic fiber cracks, and the failure mode was concluded to be cumulative.     

The effect of temperature and extrusion speed on the consolidation of zirconium-based metallic glass powder using equal-channel angular extrusion

In this study, gas-atomized amorphous Zr_58.5Nb_2.8Cu_15.6Ni_12.8Al_10.3 (Vitreloy 106a) containing 1280 ppmw oxygen was consolidated by equal-channel angular extrusion (ECAE). The powder was vacuum encapsulated in copper cans and subjected to one extrusion pass in the temperature region above the glass transition temperature (T _g) and below the crystallization temperature (T _x). The effects of extrusion temperature and the extrusion rate on microstructure, thermal stability, hardness, and compressive strength are investigated. Compression fracture surfaces were examined to determine the deformation mechanisms. The consolidates in which the time-temperature-transformation (TTT) boundary was not crossed during processing exhibit differential scanning calorimetry (DSC) patterns similar to the initial powder, with a slight decrease in T _x. Compressive strengths of about 1.6 GPa are recorded in the consolidates processed at 30 °C and 40 °C below T _x, which is close to what is observed in cast counterparts. The fracture surfaces exhibit vein patterns covering up to 90 pct of the surface area in some samples, which are characteristic of glassy material fracture. The slight decrease in T _x after consolidation is attributed to thermal-history-dependent short-range order and formation of nanocrystalline islands. The present results show that ECAE is successful in consolidation of metallic glass powder. This processing avenue opens a new opportunity to fabricate bulk metallic glasses (BMGs) with dimensions that may be impossible to achieve by casting methods.     

Effect of low-temperature shock compression on the microstructure and strength of copper

Copper with two purities (99.8 and 99.995 pct) was subjected to shock compression from an initial temperature of 90 K. Shock compression was carried out by explosively accelerating flyer plates at velocities generating pressures between 27 and 77 GPa. The residual microstructure evolved from loose dislocation cells to mechanical twins and, at the 57 and 77 GPa pressures, to complete recrystallization, with a grain size larger than the initial one. The shock-compressed copper was mechanically tested in compression at a strain rate of 10⁻³ s⁻¹ and temperature of 300 K; the conditions subjected to lower pressures (27 and 30 GPa) exhibited work softening, in contrast to the conventional work-hardening response. This work softening is due to the uniformly distributed dislocations and the formation of loose cells, evolving, upon plastic deformation at low strain rates, into well-defined cells, with a size of approximately 1 μm. The 99.995 pct copper subjected to the higher shock-compression pressures (57 and 77 GPa) exhibited a stress-strain response almost identical to the unshocked condition. This indicates that the residual temperature rise was sufficient to completely recrystallize the structure and eliminate the hardening due to shock compression. Thermodynamic calculations using the Hugoniot-Rankine conservation equations predict residual temperatures of 570 and 1000 K for the 57 and 77 GPa peak pressures, respectively. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

Effect of growth temperature on the cryopreservation of prototheca

The temperature at which Prototheca spp. were grown determined their response to freezing to -196 degrees C and subsequent thawing. Cells cultured at 35 degrees C were the most sensitive to freezing injury; at lower growth temperatures, resistance to freezing damage was seen. At all culture temperatures examined, the freezing tolerance varied with the age of the culture.     

Effect of temperature on the structure of liquid iron

Review of published data on the properties of liquid iron at different temperatures. Conclusion supporting a proposal that a structural change takes place in the melt.     

Effect of high-speed loading conditions on the pressing mechanism

Conclusions In the high-speed pressing of powders it is possible to distinguish several processes with differnt operative mechanisms of powder particle deformation. Deformation mechanisms are determined by a complex criterion whose value depends on the mass velocity, since, thermal diffusivity, density, and hardness of particles. An understanding of the particle deformation mechanisms is of practical value. In the pressing of fine powders it is best to employ a quasistatic process represented by field II. To obtain compacts of high density and strength, use should be made of coarse powder fractions or pregranulated fine fractions in order to create conditions corresponding to field III. Incidentally, granulation not only increases the size of particles but also lowers their hardness, which is desirable in the case of difficult-to-press materials. Of particular interest is the pressing of rapidly quenched powders, which do not readily lend themselves to heat treatment. In such a case parameters corresponding to field IV should be chosen, and strength will then be imparted to compacts by melting and rapid solidification of the particle surfaces.Translated from Poroshkovaya Metallurgiya, No. 11(323), pp. 14–19, November, 1989.     

温度对镍钨合金镀层电镀质量的影响

Ni-W合金镀层具有优良的耐蚀性、耐热性和耐磨性,良好的Ni-w合金镀层可以用以取代硬铬镀层从而减少对环境的污染.应用电子探针和x射线衍射(XRD)等现代材料测试方法研究了镀层的组成和组织结构,分析了镀液温度对镀层组成的影响以及镀层中钨含量对镀层组织结构的影响.     

介质温度对耐高温铁素体不锈钢点蚀行为的影响

通过阳极动电位极化测试、电化学阻抗谱(EIS)测试及钝化膜电容测试等电化学腐蚀方法,研究了在不同温度的典型介质(NaCl质量分数为3.5％的溶液)中444铁素体不锈钢的耐点蚀行为及其影响规律.结果表明,随着介质温度的升高,444铁素体不锈钢的自腐蚀电位Ecorr和点蚀电位Ep均呈现降低趋势,而自腐蚀电流密度icorr增...     

Effect of storage temperature on sperm cryopreservation

OBJECTIVE: To evaluate the influence of cryopreservation temperature on human sperm motility and morphology. DESIGN: Controlled study, investigator was blinded to the type of cryopreservation. SETTING: University-based andrology laboratory. PATIENT(S): Sixteen semen samples with normal motility and sperm count from men after a fertility work up. INTERVENTION(S): Semen aliquots were either stored in a mechanical freezer at -70 degrees C or in liquid nitrogen at -196 degrees C for 7 days or 3 months. Test yolk buffer was used as a cryoprotectant. With use of a programmable freezing unit, all samples were cooled at a controlled rate. MAIN OUTCOME MEASURE(S): Sperm motility and morphology. RESULT(S): After 7 days of cryopreservation, there was a greater decrease in sperm motility among specimens maintained at -70 degrees C than among those maintained at -196 degrees C (47% versus 39% decrease). The difference in sperm motility was even greater after 3 months of cryopreservation (72% versus 39% decrease). No difference in postthaw sperm morphology was detected among sperm preserved at -70 degrees C versus -196 degrees C. CONCLUSION(S): Sperm cryopreservation at -196 degrees C is superior to cryopreservation at -70 degrees C. Sperm can be stored at -70 degrees C for a short period of time with a relatively modest loss of motility.