网络首发时间: 2019-12-02 09:46

稀有金属 2021,45(04),507-512 DOI:10.13373/j.cnki.cjrm.xy19080008

粉末特性对纯铼烧结致密度的影响

王广达 熊宁 刘国辉 陈福鸽

安泰科技股份有限公司

安泰天龙钨钼科技有限公司

摘 要：

粉末的颗粒形貌和粒度分布对于坯料的烧结密度起着重要的作用。由于铼的稀少和高价格，在进行粉末冶金方法生产纯铼制品时，需要重视和研究铼粉颗粒特性对致密度的影响。通过3种铼粉，平均粒度（D₅₀）<30μm常规铼粉和平均粒度（D₅₀）<5μm超细铼粉以及平均粒度（D₅₀）<30μm球形铼粉，单独或合批处理后成形和烧结。分别对3种铼粉与合批粉末的形貌和粒度进行测试，发现常规铼粉具有多种形貌和粒度的不规则颗粒聚合体，超细铼粉的颗粒形貌主要以片状为主，球形铼粉中含有部分小尺寸的球形颗粒以及少量的片状颗粒。通过致密度检测和烧结坯料晶粒度的分析以及烧坯断口形貌的观察，发现具有多种颗粒形貌的常规铼粉的烧结密度最高，含有球形铼粉的坯料的烧结密度最低，超细颗粒的加入可以细化晶粒。通过分析纯铼烧坯的室温和高温力学性能和断裂方式，发现高温处理并未明显改变晶粒大小，但晶粒解理现象增多。

关键词：

粉末形貌;粉末粒度;纯铼;致密度;

中图分类号： TF125.243

作者简介：王广达（1979-），男，河北邢台人，博士，高级工程师，研究方向：难熔金属材料，电话：13683193397,E-mail:wangguang-da@atmcn.com;

收稿日期：2019-08-05

基金：国家重点研发计划专项项目（2017YFB305600）资助;

Effect of Powder Characteristics on Sintering Densification of Pure Rhenium

Wang Guangda Xiong Ning Liu Guohui Chen Fuge

Advanced Technology & Materials Co.,Ltd.

ATTL Advanced Materials Co.,Ltd.

Abstract：

Rhenium had been widely used in nuclear energy,aerospace,semiconductor,medical and chemical industries due to its excellent thermal shock resistance and corrosion resistance. Pure rhenium products were usually made by powder metallurgy method,then further deformed to obtain pure rhenium products with required properties and sizes. When the rhenium powder was pressed,the morphology and particle size distribution of rhenium powder would have an important impact on the sintering density of the billet,and then affect the subsequent deformation processing and product performance. Therefore,it was of great significance to analyze and study the effect of powder morphology on the density of pure rhenium billets through the proportion of rhenium powder and particle size distribution and sintering experiments. Three kinds of powders were treated in batch,and five kinds of powder samples were obtained. Pure rhenium powder was pressed into 80 mm × 80 mm slab and sintered at 2350 ℃. The samples were observed by JSM-6380 LV scanning electron microscope(SEM)and electron backscatter diffraction(EBSD)with ZeissEVO-18 electron microscope. D10 was the maximum particle size corresponding to 10% of the particles in rhenium powder,D₅₀ was the maximum particle size corresponding to 50% of the particles,also known as average particle size,and D90 was the maximum particle size corresponding to 90% of the particles. D₅₀ of conventional powder was the largest,the particle size of mixed powder with spherical rhenium powder decreased gradually,and the particle size of ultrafine rhenium powder was the smallest. For rhenium powders with the same morphology,the ratios of D₅₀/D10 and D90/D10 of No.1 and No.2 powders were very similar,and the sintering densities of both powders were more than 92%(> 19 g·cm-3). The particle size ratio of No.4 powder was close to that of No.1 and No.2 powders,but the density value was smaller. It could be seen that the particle morphology played an important role in the sintering process for mixed rhenium powder. The irregularity of powder shape led to the increase of bonding strength between particles. There were some ultrafine particles in No.3 powder,and the fine particles were evenly distributed among the large particles. High density could be obtained by pressing sintering. No. 4 and No. 5 samples contained spherical rhenium powder,the spherical rhenium powder particles formed a skeleton structure when pressed,and large holes were formed among the skeletons,which was easy to produce arch bridge effect. The irregular powder was not easy to flow to fill these holes,so the density after sintering was only 87%,which was lower than 90% of that of No. 1 and No. 2 powders. By observing the fracture surface of rhenium plate,it could be seen that there were obvious cavities. Obvious pits formed by grain boundary and particle shedding could be seen,and there was a small part of grain tearing morphology,which indicated that the fracture of pure rhenium was mainly grain cleavage,supplemented by a small amount of trans granular fracture. The density of sintered billet prepared by conventional powder was 19.5 g·cm-3,and the density was 92.8%. The tensile strength of sintered pure rhenium at room temperature was only628 MPa,far less than 1002 MPa,and the elongation of sintered pure rhenium was only 10%,which was lower than the value in literature. The results showed that the fracture morphology of the sintered billet was similar after drawing at 1200 and 1600 ℃. Round or regular holes could be found,which were closed shrinkage holes in the sintered billet. The results showed that the grain size of rhenium plate did not change after drawing at 1200 ℃,and there were a lot of tearing traces and a small amount of grain cleavage. Further increasing the temperature to 1600 ℃,the grain size did not change obviously,and the grain cleavage phenomenon increased,which also corresponded to the change of mechanical properties. Compared with the better fluidity of tungsten powder and molybdenum powder,rhenium powder showed different characteristics. From the sintering experiment of pure rhenium powder,following conclusions could be obtained:the conventional rhenium powder had a wide range of particle size and perse morphologies,and the sintering density could reach more than 92%;the ultra-fine rhenium powder had a flaky morphology,the sintering density could reach more than90%,but it was lower than that of the conventional rhenium powder,which might relat to the poor particle size distribution and particle mobility;spherical rhenium powder was easy to form framework structure during compaction,and the powder particles with irregular morphology were not easy to move to fill the gaps between the frameworks,which affected the increase of density;increasing sintering temperature did not change the grain size of pure rhenium,but the cleavage phenomenon increased.

Keyword：

powder morphology; powder size; pure rhenium; densification;

Received： 2019-08-05

铼是密度最高（21 g·cm^-3）的难熔金属，熔点（3180℃）仅次于钨，具有优异的室温塑性和抗蠕变性能 ^[1] 。不同于其他难熔金属的体心立方（bcc）晶体结构，铼是密排六方（hcp）晶体结构，没有塑脆转变温度，适用于从高温到低温的工作环境 ^[2] 。铼还具有良好的抗热冲击性、抗腐蚀性等性能，已被应用于核能、航空航天、半导体以及医疗、化工等行业 ^[3,4,5] 。

应用于核能、电子等领域的纯铼金属制品可以通过粉末冶金方法制备，即将高纯铼粉压制成型，然后进行高温烧结 ^[4,6,7] ，得到较致密的坯料，再根据需要进一步进行变形加工，最终得到所需的纯铼制件。同钨粉 ^[8] 和钼粉 ^[9,10] 进行压制遇到的问题类似，将铼粉进行压制时，铼粉的形貌及粒度分布等特性会对纯铼坯料的烧结致密度产生重要的影响，进一步影响后续变形加工和产品性能。本文将通过对市场上常见的3种形貌和粒度分布的铼粉粉末进行配比烧结实验，比较分析粉末形貌对纯铼坯料致密度的影响作用。

1 实验

实验采用市售纯度≥99.99%的高纯铼粉，分别为平均粒度（D₅₀)<30μm常规铼粉，平均粒度（D₅₀)<5μm的超细铼粉和平均粒度（D₅₀)<30μm的球形铼粉。实验对3种粉末进行了合批处理，获得1～5号粉末样品，配比比例如表1所示。纯铼粉末压制成80 mm×80 mm的板坯，经过2350℃的高温烧结 ^[13,14] ，得到纯铼坯料。使用MASTERSIZER-2000粒度分析设备进行粉末粒度测试，采用阿基米德排水法检测铼片的烧结密度，在SANS-CMT-5205电子拉力试验机上进行室温力学性能的测试，在CSS-44050高温电子万能试验机上进行高温力学性能测试，使用JEOL JSM-6380LV型扫描电镜（SEM）观察断口形貌，使用Zeiss EVO-18电镜进行晶粒电子背散射衍射（EBSD）分析。

表1 铼粉粉末配比比例  下载原图

Table 1 Ratios of Re powders

2 结果与讨论

2.1 铼粉形貌

图1是本实验采用的3种颗粒形貌的铼粉。仔细观察图1(a）常规使用的铼粉，粉末中含有从几微米大小的细小颗粒聚合在一起形成的多面体大颗粒，也含有这些大颗粒靠静电作用形成的条状或朵状的团聚体，还可以发现一些不规则片状颗粒的存在。粉末中的粒度范围较大，形貌也多样，这种粉末组成是在铼粉还原制备过程中形成的 ^[15,16] 。图1(b）是超细粒度的铼粉，颗粒形貌基本呈片状，部分颗粒团聚在一起，形成松散的较大尺寸的片状假颗粒。图1(c）是经过处理得到的球形铼粉，从图中可以看出，铼粉的颗粒呈现出标准的球形形貌，但也存在大小差异，以及小部分片状颗粒。

2.2 铼粉粒度分布与烧结密度

按表1中的配比，将常规粒度铼粉与超细铼粉和球形铼粉进行混合。在表1中，并未出现100%球形铼粉的成份，这是由于烧结后球形铼粉的坯料密度值较低，进行密度检测时，有轻微吸水现象，因而未在本实验中考虑。3号粉末为70%的常规铼粉与30%的超细铼粉混合，4号粉末则是70%常规铼粉与30%的球形铼粉的混合，5号粉末混合了3种铼粉颗粒，混合后的粉末形貌见图2所示。

从图2可以看到，3号粉末相对于添加球形铼粉的4号与5号粉末，颗粒形貌更为接近，大小尺寸搭配更为均匀。5种粉末的粒度分布数据如表2所示。

为了检验铼粉粒度对烧结致密度的影响作用，表2列出了在同一烧结制度下，5种铼坯的密度值。其中，D₁₀为铼粉中10%的颗粒对应的最大粒径，D₅₀为铼粉中50%的颗粒对应的最大粒径，也称平均粒度，D₉₀为铼粉中90%的颗粒对应的最大粒径。常规粒度粉末的平均粒度（D₅₀）最大，添加球形铼粉的4号与5号粉末的粒度紧随其后，2号超细铼粉的粒度最小。对于同一粉末形貌的铼粉，1号和2号的D₅₀/D₁₀和D₉₀/D₁₀的比值很相近，两种粉末的烧结致密度也均超过了92%(>19 g·cm^-3）。4号粉末的粒度比值与1号、2号粉末很接近，但是密度值要小，可见对于混合铼粉来说，颗粒形貌在烧结过程中起着重要的作用 ^[10] 。

图1 3种颗粒形貌的铼粉（SEM)

Fig.1 SEM images of rhenium powders with three particle morphologies

(a)Common particles;(b)Ultrafine particles;(c)Spherical particles

图2 铼粉颗粒混合后的SEM照片

Fig.2 SEM images of mixed rhenium powders

(a)Mixed rhenium powders of common and ultrafine particles;(b)Mixed rhenium powders of common and spherical particles;(c)Mixed rhenium powders of three kinds of particles

表2 铼粉的粒度分布与烧结密度  下载原图

Table 2 Size distribution and sintering density of rheni-um powders

2.3 烧结铼板的晶粒

将粉末压坯进行高温烧结后，观测了铼板的晶粒尺寸和断口形貌，铼板晶粒的EBSD图像如图3所示。常规颗粒铼粉烧结后，晶粒大小不均匀，既有尺寸在100μm左右的大晶粒，也有尺寸小于20μm的细颗粒。对比图3(b）的混合有超细颗粒的铼板，晶粒尺寸明显较小，晶粒均匀性也有较大改善，这表明超细颗粒的铼粉对于晶粒度有明显作用。

结合文献 [ 8, 9, 10, 17] ，粉末形状的不规则导致颗粒之间的结合强度增加。在3号粉末中含有部分超细颗粒，细颗粒均匀分布在大颗粒之间，压制烧结可以得到较高的致密度。在含有球形铼粉的4号与5号样品中，球形铼粉颗粒在压制时形成骨架结构，骨架之间形成较大的孔洞，易产生拱桥效应，而不规则形貌的粉末不易流动将这些孔洞填补，因而在烧结后致密度只有87%，低于1号和2号粉末的90%以上的致密度。

图3 烧结铼板的EBSD图像

Fig.3 EBSD images of sintered Re billets

(a)Commonrhenium powders;(b)Commonandultrafinepowders

进一步对铼板的断口进行观测，如图4所示。由于烧坯不是完全致密，可以看见明显的空洞存在。从图4的烧结铼坯的断口形貌中，均可以看到明显的晶界和颗粒脱落形成的凹坑，并存在着少部分晶粒撕裂的形态，表明纯铼烧坯的断裂是以晶粒解理为主，以少量穿晶断裂为辅。

2.4 铼板的力学性能

为了进一步研究烧坯的致密度与力学性能的关系，以常规铼粉烧结制备的铼板为样品，拉伸试样尺寸参照文献 [ 18, 19, 20] ，取样测试了室温和高温力学性能，实验结果如表3。高温拉伸断口的形貌如图5所示。

常规粉末制备的烧坯密度为19.5 g·cm^-3，致密度92.8%。纯铼烧坯在室温下的抗拉强度只有628MPa，远小于1002 MPa，烧坯的延伸率也只有10%，低于文献 ^[14] 中的数值，这应该与铼板的致密度有关。从图5中可以看出，烧坯在1200和1600℃高温拉伸后，断口的形貌也是很相像，可发现圆形或规则形状的孔洞，这是烧坯中闭合的烧结收缩孔。仔细观察断裂形貌，经过1200℃高温拉伸后，铼板晶粒尺寸基本未发生变化，存在大量撕裂状的痕迹以及少量的晶粒解理。进一步提高温度到1600℃，晶粒尺寸未有明显变化，晶粒解理现象增多，这也对应着表3中力学性能的变化。

图4 烧结铼板的断口形貌（SEM图）

Fig.4 SEM images of sintered billets of rhenium

(a)Mixed rhenium powders of common and ultrafine particles;(b)Mixed rhenium powders of common and spherical particles;(c)Mixed rhenium powders of three kinds of particles

表3 纯铼烧坯的力学性能  下载原图

Table 3 Mechanical properties of pure rhenium sintered billet

图5 纯铼烧坯的高温拉伸断口形貌（SEM图）

Fig.5 SEM images of pure rhenium sintered billets under dif-ferent high temperature tensile

(a)1200℃；（b)1600℃

3 结论

粉末颗粒形貌与粒度对于纯铼的烧结密度有着重要的影响，相比于钨粉、钼粉颗粒较好的流动性，铼粉粉末表现出不同的特征。从3种纯铼粉末的烧结实验，可以得出如下结论：

1.常规铼粉的粒度范围较大且形貌多样，可以得到>92%的烧结致密度。

2.超细颗粒的铼粉具有片状形貌，烧结致密度可以达到90%以上，但比常规铼粉要低，这与铼粉粒度分布及颗粒移动填充性较差有关。

3. 球形铼粉在压制成型时易形成骨架结构，不规则形貌的粉末颗粒不易移动填充骨架之间的空隙，影响了致密度的提升。

4. 升高温度并未明显改变纯铼烧坯的晶粒大小，但解理现象增多。

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