钼及钼合金烧结技术研究及发展

从粉末冶金烧结理论、烧结设备、烧结工艺参数及新型烧结技术等方面概述了钼及钼合金烧结技术的研究现状及进展。结论指出, 钼及钼合金烧结理论仍集中在传统粉末冶金理论体系; 烧结技术发展方向是获得全致密化、细晶、均质化的烧结坯体; 发展趋势是烧结设备及工装与粉末冶金新技术结合更为紧密, 出现更多交叉研究; 研究热点和难点是大型钼及钼合金坯件烧结致密化、微观组织均质化、细晶化及复杂烧结态异型坯件烧结过程中形状精确控制等。     

纳米晶钨合金粉末常压烧结的致密化和晶粒长大

高比重合金由于具有密度和强度高、延性好等一系列优异的性能,在军工上被用作动能穿甲弹材料.纳米材料被认为是21世纪应用前景最为广阔的新型材料.采用纳米粉末可望大大细化钨合金晶粒,显著提高合金的强度、延性和硬度等力学性能,因而是制备新型高强韧、高比重钨合金的一个很重要的研究方向.作者采用机械合金化(MA)工艺制备了纳米晶钨合金复合粉末,研究了纳米晶钨合金粉末在常压氢气气氛中的烧结致密化和在烧结过程中的钨晶粒长大行为.研究结果表明,MA纳米晶粉末促进了致密化,使致密化温度降低约100～200℃.在一般固相烧结温度时可以得到晶粒尺寸为3～5μm的细晶高强度合金.同时,指出了在液相烧结时存在的问题,即钨晶粒加速重排、产生晶粒聚集与合并,迅速发生钨晶粒长大,在较短时间内液相烧结时,钨晶粒尺寸又长大到接近传统高比重合金水平.     

纯钛粉放电等离子烧结致密化的动力学与组织演变行为

放电等离子烧结(spark plasma sintering, SPS)具有快速致密化的显著特点,然而目前对SPS快速致密化的动力学行为缺少深入理解与认识。考虑到纯钛的优异性质及广泛应用,本文以纯钛粉为典型材料,在压强20 MPa、温度为600~875℃条件下,进行纯钛粉的SPS烧结,获得了其在不同温度下的致密化过程与时间的函数关系,揭示了其快速致密化的动力学行为。并深入探讨烧结温度对其微观组织、孔隙度及力学性能的影响。结果表明：在低温阶段(600~725℃),致密化指数为1.5,扩散与高温蠕变共同作用实现样品的致密化;在温度较高时(800~875℃)致密化指数为2,此时主要为高温蠕变导致的致密,随温度升高,样品的维氏硬度增加,且温度越高增加速率越快,样品的力学性能提高。     

钼、钨及其合金

对钼、钨及其合金的重要物理性能、室温力学性能,钼及钨的合金,钼合金的应用等方面的近期文献资料作了综合评述。     

W-Ni-Fe高比重合金的SPS烧结行为

采用放电等离子烧结(SPS)设备制备了93W-5.6Ni-1.4Fe高比重合金,烧结温度范围为1100~1180℃,保温时间为5min.对不同烧结温度下的样品进行了密度、硬度、抗弯强度等性能测试,采用场发射SEM观察了样品表面形貌及断裂行为.结果表明:采用SPS烧结,可以在较低的温度下实现93W-5.6Ni-1.4Fe高比重合金的固相烧结,使合金致密化,并能有效控制钨晶粒长大,提高材料的硬度、抗弯强度等力学性能.     

钼合金、钨合金圆片

钼合金、钨合金圆片涉及一种电力半导体器件及电真空器件中的基片（俗称圆片）。主要是为解决现有的用钼制作的圆片只能制作普通的电力半导体器件及电真空器件。而用钨制作的圆片加工困难的问题而研制的。该发明的钼合金圆片是用钼钨合金或TZM合金或稀土钼合金制成的：钨合金圆片是用钨钼合金或钨钍合金或稀土钨合金或钨铼合金制成的。优点是加工比较容易，性能比现有的钼圆片优越，可用于制作高档的电力半导体器件及电真空器件。     

碳含量对真空烧结钼合金的影响

通过在真空烧结TZM合金中添加不同比例的碳元素,研究了碳元素在不同温度下真空烧结时与其他元素的作用及其反应机理。结果表明:(1)添加碳元素的量要与钼合金未烧结坯料中的氧元素成一定比例,才能同时保证有效降低氧含量和合金中碳元素处于适中范围。(2)从热力学反应生成自由能计算结果来看,在Mo-Ti-Zr-C四元烧结体系中钛锆优先与碳反应生成(Ti,Zr)C;部分碳元素先与Mo反应生成Mo2C,在有Ti、Zr元素存在时,Mo2C将会与Ti、Zr发生Mo2C+2Ti=2TiC+Mo方式的反应,生成金属Mo和(Ti,Zr)C。     

钨铜复合材料致密化问题和方法

在常规溶渗、烧结条件下金属钨和铜之间互不溶解,浸润特性能差,使两种粉末复合的致密化过程进行相对困难。阐述了如何提高该系统致密化过程和速度,弱化致密化条件,获得均匀弥散且两相分别连续分布的组织结构,并实现对成分和尺寸的准确控制。结合各类应用的性能要求,对钨铜复合材料致密化的各种方法进行分析和讨论,试图为选择高性能钨铜复合材料加工方法提供依据。     

钨铜粉末材料烧结-挤压致密化研究

为探索难熔金属和铜粉末混合坯致密化工艺,提出了钨铜粉末材料液相烧结和热静液挤压致密新工艺,经过实验获得了近致密、组织细小、性能优异的复合材料.结果表明,WCu40混合粉末冷压坯的相对密度约为70%,经过液相烧结和热静液挤压,可以获得相对密度大于99.8%的钨铜(WCu38)材料.致密后材料导电率可达到41～48 m·Ω-1·mm-2,硬度可达到HB173～176.     

纳米晶WC-10Co复合粉末的烧结致密化行为

研究了机械合金化纳米晶WC-10Co复合粉末的真空烧结致密化行为和一般规律。结果表明:提高烧结温度和延长烧结时间有利于样品的烧结致密化过程,在1275-1300℃时致密化速度较快,在1300℃烧结15min后致密化过程基本完成;VC和Cr3C2复合晶粒长大抑制剂含量的增加不利于致密化过程;新型晶粒长大抑制剂A可以更有效地抑制晶粒长大;纳米晶WC-10Co-0.8VC／Cr3C2-0.2A复合粉末压坯在1375℃烧结30min后,所得的烧结密度为14.48g／cm3,晶粒尺寸约为180nm。     

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.     

Sintering of alloys of zirconium diboride with molybdenum disilicide

Summary The sintering of zirconium diboride with molybdenum disilicide is accompanied by the formation of a solid solution based on zirconium diboride, formation of a liquid phase at temperatures above 1800°C, and partial vaporization of silicon in the ZrB₂+15% MoSi₂ alloy. At temperatures up to 1800°C, solidphase sintering takes place; at low temperatures, this is accompanied by specimen growth due to heterodiffusion processes resulting from the difference in the partial diffusion coefficients of the components and to the vaporization of excess silicon in the case of the ZrB₂+15% MoSi₂ alloy.At temperatures above 1800°C, shrinkage is caused by the formation of a liquid phase, which disappears during sintering. Under these conditions, grain recrystallization and growth in the solid solution of Mo and Si in zirconium diboride in the case of 15% MoSi₂ alloys are not completed even after 4-h holding at temperatures of 1800, 1900, and 2000°C.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 11–16, September, 1966.     

Effect of tungsten particle shape on dynamic deformation and fracture behavior of tungsten heavy alloys

The effect of the tungsten particle shape on the dynamic deformation and fracture behavior of tungsten heavy alloys was investigated. Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregularly shaped, cleavage fracture occurred in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

Effect of tungsten particle shape on dynamic deformation and fracture behavior of tungsten heavy alloys

The effect of the tungsten particle shape on the dynamic deformation and fracture behavior of tungsten heavy alloys was investigated. Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregularly shaped, cleavage fracture occurred in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

An in situ field-ion microscope study of the recovery behavior of ion-irradiated tungsten and tungsten alloys

Five grades of tungsten specimens with different purity levels (resistivity ratios R of 5 × 10⁴, 1.5 × 10⁴, 50, 15 and 5) were irradiated in situ with 30 keV W⁺ ions to a dose of typically 5 × 10¹² ion cm⁻² at 18 K. Examination with a low-temperature field-ion microscope (FIM) showed the isochronal-annealing spectra of the specimens to result from a large self-interstitial atom (SIA) flux at ~ 38 K, followed by significant SIA flux from ~ 50 to 80 K and a small amount of additional recovery up to 120 K. The spectra for these five different R value specimens were essentially identical between 18 and 120 K. High-purity W specimens (R = 5 × 10⁴) doped with 5 × 10⁻⁵ to 1 × 10⁻⁴ atom fraction carbon showed only a small reduction in the amount of recovery observed for the long-range migration peak at 38 K. The isochronal recovery spectra for WRe alloy specimens (5 × 10⁻³ and 3 × 10⁻² atom fraction Re) were radically different from the isochronal recovery spectra of pure W specimens. For both alloys the recovery of the Stage I long-range migration peak at 38 K was strongly suppressed; for the 3 × 10⁻² atom fraction alloy, all recovery from 18 to 120 K was virtually eliminated. This result indicated that during the long-range migration substage at 38 K tightly-bound, immobile SIA-Re complexes were formed that suppressed the SIA-SIA reaction. However, this effect was only observed at these high Re atom concentrations. The lack of any significant differences for the annealing spectra of the five purity-levels of undoped tungsten and the appearance of impurity effects only in the extremely concentrated W alloys (i.e. 5 × 10⁻³ to 3 × 10⁻² atom fraction Re) indicated that the early Stage II recovery (45–120 K) observed in the FIM isochronal-annealing spectra of self-ion irradiated high-purity W was intrinsic in nature. Because of the highly inhomogeneous SIA distribution of the W⁺ ion damage, the SIA-SIA interaction during Stage I long-range migration at 38 K appeared to be the dominant trapping mechanism. The early Stage II SIA recovery was therefore attributed to the migration or dissolution of these SIA clusters.     

Sintering time and atmosphere influences on the microstructure and mechanical properties of tungsten heavy alloys

The mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Sintering times greater than 2 hours usually result in pore growth with degraded properties. The development of an optimized sintering atmosphere has allowed exploration of long sintering times without significant property degradation due to pore growth. The optimal cycle was used to sinter two heavy alloy compositions (88 and 95 wt pct W) for times up to 600 minutes at 1480 °C. The 88 pct W samples slumped, but the 95 pct W samples were fully densified and suitable for tensile testing. At long sintering times, the tungsten grains flattened and the tungsten contiguity decreased, indicating a transition to low-energy configurations for the solid-liquid interfaces. The cube of the mean grain size varied linearly with the isothermal sintering time. This allowed determination of grain size effects on mechanical properties, showing a decreasing yield strength with increasing time in agreement with the Hall-Petch behavior. The tensile strength and elongation were highest for sintering times from 30 to 90 minutes, reflecting a minimum in the residual porosity.     

粉末冶金铝合金烧结致密化过程

以纯Al粉为主要原料,添加Cu单质粉末以及Al-Mg、Al-Si中间合金粉,利用粉末冶金压制烧结方法制备出相对密度98%以上的Al-Mg-Si-Cu系铝合金.研究表明,烧结致密化过程主要分为3个阶段:初始阶段(室温~460℃),坯体内首先形成Al-Mg合金液相,液相中的Mg原子分别扩散至Al或Al-Si粉末中,与Al₂O₃反应并破除氧化膜,形成Al-Mg-O等化合物;同时,Al-Cu发生互扩散,形成Al₂Cu等金属间化合物.第二阶段(460~560℃),Al-Cu、Al-Si液相快速填充颗粒缝隙或孔洞,坯体相对密度显著提高;此阶段的致密化机制主要是毛细管力引起的颗粒重排,以及溶解析出导致的晶界平直化.第三阶段(560~600℃),随温度的升高,液相润湿性提高,晶粒快速长大,使得大尺寸孔洞填充,烧结体基本实现全致密,此阶段的致密化主要由填隙机制控制.在铝合金晶界处发现了MgAl₂O₄和MgAlCuO氧化物的存在,推测Al粉表面氧化膜的破除机制与合金成分有关.由于Al-Cu液相在Al表面的润湿速率远高于AlN的生长速率,因为在本体系中未发现AlN的存在.