Trans. Nonferrous Met. Soc. China 30(2020) 3296-3306

Effects of doping route on microstructure and mechanical properties of W-1.0wt.%La₂O₃ alloys

Jun-jun YANG^1*, Gang CHEN^1*, Zheng CHEN¹, Xiao-dong MU¹, Ying YU^1,2, Lin ZHANG¹, Xing-yu LI¹, Xuan-hui QU¹, Ming-li QIN¹

1. Beijing Advanced Innovation Center for Materials Genome Engineering, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China;

2. Grinm Group Co., Ltd., Beijing 100088, China

Received 15 February 2020; accepted 19 August 2020

Abstract: A comparative study was conducted by using solution combustion synthesis with three different doping routes (liquid-liquid (WL10), liquid-solid (WLNO) and solid-solid (WLO)) to produce nanoscale powders and further fabricate the ultrafine-grained W-1.0wt.%La₂O₃ alloys by pressureless sintering. Compared with pure tungsten, W-1.0wt.%La₂O₃ alloys exhibit ultrafine grains and excellent mechanical properties. After sintering, the average grain size of the WLO sample is larger than that of WL10 and WLNO samples; the microhardness values of WL10 and WLNO samples are similar but larger than the value of WLO sample. The optimized La₂O₃ particles are obtained in the WL10 sample after sintering at 1500 °C with the minimum mean size by comparing with WLNO and WLO samples, which are uniformly distributed either at grain boundaries or in the grain interior with the sizes of (57±29.7) and (27±13.1) nm, respectively. This study exhibits ultrafine microstructure and outperforming mechanical properties of the W-1.0wt.%La₂O₃ alloy via the liquid-liquid doping route, as compared with conventionally-manufactured tungsten materials.

Key words: tungsten alloy; solution combustion synthesis; doping route; ultrafine grain; microhardness

1 Introduction

Tungsten-based materials are very attractive for many engineering applications, such as military, aerospace and nuclear industries owing to their high melting point, high thermal conductivity, high strength at elevated temperatures and low thermal expansion [1-5]. However, their applications are limited due to the well-known embrittlement problems [6-8]. Therefore, it is of great importance to improve their mechanical properties, which are strongly related to the concentration of unavoidable solutes e.g. O, C, P and N [9-12]. However, these impurities are mainly located at grain boundaries (GBs) [13], which weaken the GBs and ultimately lead to brittle failure and deterioration of strength. To date, this problem can be mitigated by modifying grain boundaries and impurity distribution of the tungsten-based alloys. In such a case, the most widely-used pathway is to refine their microstructure [14-16], since it not only increases the final strength but creates abundant grain boundaries that can effectively promote the uniformity of impurity distribution and thus the ductility [13]. Therefore, nano-structured tungsten- based materials have been intensively studied in recent years, which exhibit promising results.

The availability of nanoscale tungsten powders is the key point to prepare ultrafine-grained tungsten alloys. There are several effective routes to fabricate nanoscale powders, including hydrothermal synthesis [17,18], co-precipitation processing [19], sol-gel method [20], mechanical alloying [21] and azeotropic distillation [22-24]. Alternatively, we have successfully prepared nanosized tungsten powders by a novel liquid- liquid doping method, i.e. solution combustion synthesis (SCS) [25]. Combined the advantages of wet-chemical synthesis with rapid flame combustion, SCS saves energy and time owing to the abundantly released heat during the short-time processing [26-29], compared with other liquid- liquid doping methods. Particularly, nanoscale rare-earth oxide dispersions (e.g. La₂O₃ [19],  Y₂O₃ [20,21] and Pr₂O₃ [30]) doped in the ultrafine-grained tungsten matrix are preferred to gettering impurities due to strong rare-earth- oxygen interactions, which inhibits grain growth  by reducing the surface diffusivity of tungsten skeleton [30,31].

Currently, there are three routes to dope the rare-earth oxide particles into refractory metals: (I) Solid-solid (SS). Lu₂O₃ particles could be doped into the tungsten matrix by ball milling to prepare the W-Lu₂O₃ alloy [32]; (II) Liquid-solid (LS). YAR et al [33,34] produced the W-1.0wt.%Y₂O₃ and W-0.9wt.%La₂O₃ nanoscale powders through the reactions in aqueous solutions of yttrium nitrate or lanthanum nitrate with ammonium paratungstate; (III) Liquid-liquid (LL). A sol-gel method was also used to produce W-1.0wt.%Y₂O₃ powders [20]. LIU et al [13] employed the aforementioned three doping routes to fabricate the ultrafine-grained Mo-La₂O₃. They showed that the LL method  leads to the most homogeneous and nanoscale microstructure with ultra-high strength and good ductility, since it is processed at a molecular level. However, the commonly-used doping routes are mainly focused on the fabrication of microscale tungsten alloys whose properties should be inferior as compared to the nanoscale-grained tungsten alloys [35]. Additionally, the report on fabricating nanoscale powders and ultrafine-grained tungsten alloys by pressureless sintering as a function of doping route is limited. Furthermore, the comparison of ultrafine-grained tungsten alloys using various doping routes is still lacking either on microstructure or on mechanical properties, as pointed out in a recent review by REN et al [36]. Beyond question, this is beneficial to upgrading the ductility of tungsten alloys.

In this study, we prepared the ultrafine- grained W-1.0wt.%La₂O₃ alloys from house-made nanosized powders using three different doping methods, i.e. LL, LS and SS, followed by pressureless sintering. As such, the purpose of this work is to provide a promising approach to produce high-performance ultrafine-grain tungsten alloys using the three doping routes via pressureless sintering. Besides, the study also focused on the influences of the doping route on the sintering densification, microstructure and mechanical properties.

2 Experimental

Nanoscale tungsten powders doped with 1.0 wt.% La₂O₃ (LL, termed as WL10 hereafter) were made from solution combustion synthesis (SCS) followed by hydrogen reduction, which has been successfully carried out in our previous  reports [37-39]. First, the starting materials of 0.01 mol ammonium metatungstate ((NH₄)₆H₂W₁₂O₄·nH₂O), 0.24 mol ammonium nitrate (NH₄NO₃), 0.1 mol glycine (C₂H₅O₂N) and 0.0014 mol lanthanum nitrate (La(NO₃)₃·6H₂O) were dissolved into the deionized water. A homogeneous solution was achieved by continuous stirring, and then heated until the combustion started. The foamy precursors were obtained after the short-time combustion process, which were subsequently ground into powders. The WL10 powders were fabricated by reducing the SCS-synthesized powders at 700 °C for 2 h under flowing hydrogen. The XRD pattern of the WL10 powders is shown in Fig. 1(a). The SCS-made pure tungsten (PW) powders [25] were mechanically mixed for 4 h with La₂O₃ particles (commercially available with the purity of 99.99% and the D₅₀ of ~50 nm) in an anhydrous ethanol medium, termed as WLO hereafter, which is called the SS doping process. Meanwhile, mixing the SCS-made PW powders [25] with nanoscale La₂O₃ powders via the La(NO₃)₃ solution is the LS doping route, called WLNO. Nanoscale La₂O₃ powders with the composition of 1.0 wt.% were doped in the WL10, WLNO and WLO powders, respectively. Figures 1(b) and (c) show the micro-morphologies of the SCS-made pure tungsten powders whose particle size is below 50 nm. Carbon and oxygen levels of the SCS-made tungsten powders are 0.043 wt.% and 1.74 wt.%, determined using a carbon/sulfur analyzer (LECO TC-436) and an oxygen/nitrogen analyzer (LECO CS-444), respectively.

Fig. 1 XRD pattern of WL10 powders (a), and SEM (b) and TEM (c) morphologies of SCS-made nanoscale PW powders

Subsequently, all types of powders with 4 g were pressed into a 15 mm diameter cylindrical die with a uniaxial compaction pressure of 700 MPa holding for 5 s, and then sintered at the final temperatures of 1200, 1350, 1500, 1650 and 1800 °C for 2 h under flowing hydrogen, respectively.

Densities of the as-sintered samples were measured according to the Archimedes principle as per the ASTM B962—14 standard. Microstructures and morphologies of the samples were observed by scanning electron microscopy (SEM, Zeiss LEO-1450) and transmission electron microscopy (TEM, FEI G2), combined with the analysis of selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDS). The grain size of the as-sintered samples was quantitatively determined by employing the linear intercept method [40,41] using the particle size analysis software based on SEM images of the fractured samples. The Vickers microhardness test was carried out on the polished samples using a microhardness tester (Leica MH-6) under a 2 N loading at room temperature. Twelve testing readings were used to ensure repeatability in each experiment, and average microhardness values were reported.

3 Results

3.1 Powder morphologies and sintering behavior

Figure 2 shows the TEM images and elemental distributions for all types of powders, i.e. WL10 (Fig. 2(a)), WLNO (Fig. 2(b)) and WLO (Fig. 2(c)), respectively. First, the tungsten powders doped with La₂O₃ particles seem no remarkable difference from the PW powders from the morphological viewpoint by comparing Fig. 2 with Fig. 1. Second, the elemental mapping results indicate that the La₂O₃ particles are homogeneously distributed in the tungsten powder matrix in the case of WL10 and WLNO powders as displayed in Figs. 2(a) and (b), respectively. However, the La in the WLO alloy powders exhibits accumulation (Fig. 2(c)). The results show that both LL and LS doping can realize the uniform distribution of La. The doped tungsten powders demonstrate a similar particle size below 50 nm.

The sintering behavior was investigated and compared as a function of sintering temperature for the samples of PW, WL10, WLNO and WLO. The measured relative density of the green sample with and without doping is similar, i.e. 50%. Figure 3 displays the relative density for each type of sample after sintering at different temperatures. It can be seen from Fig. 3 that the relative density increases with the sintering temperature for all samples. This is a typical phenomenon in conventional powder sintering, since the atomic diffusion is promoted with increasing the sintering temperature, thus favoring sintering densification. Interestingly, the relative density of the as-sintered PW is at least about 10% higher after sintering at 1200 °C (Fig. 3), beyond which it reaches a relatively constant value, as compared with the three types of doped sample. This is because the doped La₂O₃ particles prevent tungsten powders from sintering densification. However, with further increasing the sintering temperature, all samples after sintering at/above 1500 °C achieve a high relative density above 95.0%, attributed to the enhanced atomic diffusion at high temperatures using the nanosized powders.

Fig. 2 EDS mapping images at STEM mode for powders of WL10 (a), WLNO (b) and WLO (c)

Fig. 3 Relative densities as function of sintering temperature for samples of PW, WL10, WLNO and WLO

3.2 Microstructure

SEM images of the W-1.0wt.%La₂O₃ fabricated using three doping routes (WL10, WLNO and WLO) before and after sintering are illustrated in Fig. 4. It seems that the initial status of all powder compacts is similar in terms of particle size and porosity (Figs. 4(a)-(c)). After sintering at 1200 °C, particle sintering necking occurs (Figs. 4(d)-(f)), concomitant with densification. With continuous sintering at higher temperatures (i.e. 1500 and 1800 °C), sintering densification is enhanced, while the grain size obviously increases with increasing temperature in all samples as observed in Figs. 4(g)-(l). In particular, the average grain size approaches (0.57±0.19), (0.54±0.17) and (0.60±0.17) μm for the samples of WL10, WLNO and WLO after sintering at 1500 °C as demonstrated in Figs. 4(g)-(i), respectively. However, grain coarsening appears after sintering at 1800 °C for the WLO sintered sample, exhibiting the largest grain size (Figs. 4(j)-(l)).

The average grain sizes for the PW and W-1.0wt.%La₂O₃ samples are shown in Fig. 5(a)  as a function of sintering temperature. With the increase of sintering temperature, the average grain sizes of the PW and W-1.0wt.%La₂O₃ alloys gradually increase due to the Ostwald ripening mechanism [13]. The average grain sizes of the W-1.0wt.%La₂O₃ alloys are smaller than that of PW at an identical sintering temperature, because the La₂O₃ particles can refine grains by promoting grain nucleation and hindering its growth. When the sintering temperature is below 1650 °C, there is no difference for all the as-sintered samples in terms of average grain size. However, when the sintering temperature rises to 1800 °C, the average grain size of the as-sintered WLO sample is larger as compared to those of WL10 and WLNO samples.

Fig. 4 SEM images of W-1.0wt.%La₂O₃ alloys fabricated using three doping routes as function of sintering temperature

Fig. 5 Average grain sizes for samples of PW and doped W-1.0wt.%La₂O₃ as function of sintering temperature (a) and comparison of average grain sizes as reported in literatures (b)

The sintered morphological result is consistent with the relative density data in Fig. 3. The result implies that the SCS-made W-1.0wt.%La₂O₃ powder compacts can achieve ultrafine-grained microstructure (grain size below 600 nm) with the relative density above 95.0% after sintering at 1500 °C without assisted pressure. As compared with other reports [1,19,20,30,32-34,42-47] of the powder-sintered tungsten-based alloys as summarized in Fig. 5(b), this study demonstrates a huge improvement on sintering densification without significant grain coarsening.

Fig. 6 TEM images, intragranular and intergranular particle size distributions for samples after sintering at 1500 °C

TEM images and dispersion particle sizes of the samples (WL10, WLNO and WLO) after sintering at 1500 °C are shown in Fig. 6. The La₂O₃ particles are homogeneously distributed in the tungsten matrix either in the grain interior or at the grain boundary, as observed from the EDS mappings in Figs. 6(a)-(c). The white-colored particles are determined as La₂O₃ under the STEM- HAADF mode and EDS analysis in Figs. 6(d)-(f). The average sizes of the La₂O₃ particles at grain boundaries are (57±29.7), (111±104.6) and (128±37) nm, while the average sizes of the La₂O₃ particles in the grain interior are (27±13.1), (37±16.3) and (57±17.7) nm for the as-sintered WL10, WLNO and WLO samples as shown in Figs. 6(g₁)-(i₂), respectively. Obviously, La₂O₃ dispersions with the smallest size are observed in the sample of WL10, as compared with WLNO and WLO samples. In addition, with the increase of sintering temperature, the particle size of La₂O₃  also grows because of the Ostwald ripening mechanism [13]. The average sizes of the La₂O₃ particles at grain boundaries and in the grain interior for the WL10 are (120±74.4) and (40±18.5) nm after sintering at 1800 °C, respectively.

3.3 Microhardness

To evaluate the effect of the resultant microstructure on the mechanical properties, the microhardness of the as-sintered samples was investigated as a function of sintering temperature as shown in Fig. 7(a). The microhardness of the as-sintered samples increases with increasing the sintering temperature below/at 1500 °C, beyond which it decreases with increasing the sintering temperature (Fig. 7(a)). In other words, the microhardness values reach the maximum values of HV (684.1±14.3), (679.7±43.3) and (650.5±38.6) for the doped samples of WL10, WLNO and WLO after sintering at 1500 °C, respectively. The microhardness for the as-sintered samples of WL10 and WLNO are similar, but slightly higher than that of WLO sample. However, the maximum microhardness is HV (587.1±14.3) after sintering at 1500 °C for the PW sample without doping, much lower as compared with those of the doped samples. The ultrafine-grained tungsten-based alloys in this study using La₂O₃ doping exhibits superior properties as compared with the results in other reports [19,30,32-34,43,46,47] either using pressureless sintering or spark plasma sintering, as summarized in Fig. 7(b). The achieved micro- hardness values in this case are approximately HV 100 higher than those reported in literatures (Fig. 7(b)), which is mainly due to the ultrafine grains obtained using the SCS-made nanoscale tungsten powders. Therefore, SCS is a promising pathway to produce nanosized powders and ultrafine-grained tungsten-based alloys using pressureless sintering.

Fig. 7 Microhardness of as-sintered samples of PW and W-1.0wt.%La₂O₃ as function of sintering temperature (a) and comparison of microhardness as reported in literatures (b)

4 Discussion

4.1 Effects of doping on grain size and densification

As shown in Fig. 3, when the sintering temperature is below 1500 °C, relative densities of W-1.0wt.%La₂O₃ are lower than that of PW after sintering. This indicates that the addition of La₂O₃ particles via the three doping routes (LL, LS and SS) hinders sintering densification. This is because oxide particles, dispersed at the grain boundaries, can retard the migration of grain boundaries [16]. As the sintering temperature increases, however, the atomic diffusion increases and thus promotes densification kinetics [40]. Thus, the difference of relative density between the PW and W-1.0wt.%La₂O₃ gradually minimizes when the sintering temperature increases above 1500 °C. Further, with the increase of relative density as well as sintering temperature, the average sizes of tungsten grains and doped particles also grow because of the Ostwald ripening mechanism [13], as proved in Figs. 4 and 6. The as-sintered sample of WLO exhibits a larger grain size than the WL10 and WLNO after sintering at 1800 °C by comparing Figs. 4(j)-(l). This indicates that grain coarsening is more critical when the doped La₂O₃ particles are likely located at the grain boundaries. In this case, the WLO sample was fabricated from the mechanically-mixed PW and La₂O₃ powders, causing most of La₂O₃ powders located on the tungsten powder surface and thus most likely retained at grain boundaries after sintering. Therefore, the results are totally different from those of the as-sintered samples of WL10 and WLNO. Moreover, the coarsened La₂O₃ particles in the as-sintered sample of WLO are believed to weaken the pinning effect on the grain boundary migration during sintering [48].

4.2 Effects of doping on microhardness

It is well known that the microhardness of materials is correlated to the grain size, relative density, phase constituent, dislocation and secondary phases [13,49,50], etc. In this study, when the sintering temperature is below 1500 °C, the as-sintered PW exhibits a higher microhardness as compared with the doped samples after sintering at an identical temperature (Fig. 7(a)). Nevertheless, this scenario changes after sintering at temperatures above 1500 °C. This can be attributed to the following factors. First, the relative density is much larger for the as-sintered PW sample than that of the doped samples when the sintering temperature is below 1500 °C (Fig. 3). However, with further increasing the sintering temperature, the relative density of the doped samples increases to a  similar value with that of the PW sample. Second, the nanoscale La₂O₃ particles act as the dispersion-strengthened role in underpinning the dislocation movement and thus enhancing the microhardness/strength [48,51] when the plastic deformation is carried out. Thus, both density and La₂O₃ dispersions play a dominant role in promoting the final microhardness.

However, the grain sizes of both tungsten matrix and La₂O₃ particles increase with increasing the sintering temperature, although the relative density increases, as observed in Figs. 4 and 6. The microhardness first increases and then decreases with the increase of sintering temperature for all samples (Fig. 7(a)). This is due to the grain size effect on the mechanical properties of materials. We plotted the data of grain size and microhardness from Figs. 4, 6 and 7(a) into Fig. 8 that displays  the relationship between the grain size and microhardness of all samples. It seems that the fitting line of relationship is linear as shown in  Fig. 8, which obviously agrees well with the Hall-Petch equation of H_v=H₀+k_Hd ^-1/2 [8,52], where H_v is the microhardness of the sample, H₀ is the microhardness of single crystal, k_H is a constant, and d is the grain size.

Fig. 8 Relationship between grain size and micro- hardness of as-sintered PW and W-1.0wt.%La₂O₃ alloys

From the particle sizes of doped La₂O₃ and microhardness of the as-sintered samples in Figs. 4, 6, 7 and 8, it can be concluded that the LL (WL10) and LS (WLNO) doping routes are more beneficial to grain refinement and microhardness enhancement, as compared with the SS (WLO) route. Mechanical properties can be improved by reducing the grain-boundary La₂O₃ particles both in their size and population [13]. A large number of ultrafine La₂O₃ particles are uniformly distributed in the grain interior, while few large La₂O₃ particles are concentrated at grain boundaries for the as-sintered samples of WL10 and WLNO. However, most of the large La₂O₃ particles in the as-sintered sample of WLO are more likely located at grain boundaries, which is prone to produce cracks during plastic deformation. Consequently, the microhardnesses for the as-sintered samples of WL10 and WLNO are similar, but slightly higher as compared with that of WLO.

5 Conclusions

(1) W-1.0wt.%La₂O₃ alloy powders were prepared by SCS using the LL, LS and SS doping routes. The average particle size is below 50 nm for the as-fabricated WL10, WLNO and WLO doped powders.

(2) After sintering at 1500 °C, the relative density, microhardness, and average grain size are (95.0±0.10)%, (96.9±0.35)% and (97.4±0.59)%; HV (684.1±14.3), (679.7±43.3) and (650.5±38.6); (0.57±0.19), (0.54±0.17) and (0.60±0.17) μm for the as-sintered samples of WL10, WLNO and WLO, respectively.

(3) The as-sintered WL10 sample using the LL doping route exhibits uniformly distributed La₂O₃ particles either at grain boundaries or in the grain interior, and thus demonstrates superior microhardness as compared with the samples of PW, WLNO and WLO.

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掺杂方法对W-1.0wt.%La₂O₃合金显微组织和力学性能的影响

杨军军^1*，陈 刚^1*，陈 铮¹，母晓东¹，于 瀛^{1, 2}，章 林¹，李星宇¹，曲选辉¹，秦明礼¹

1. 北京科技大学 新材料技术研究院 北京材料基因工程高精尖创新中心，北京 100083；

2. 有研科技集团有限公司，北京100088

摘  要：对采用溶液燃烧合成法并结合不同掺杂方法(液-液(WL10)、液-固(WLNO)和固-固(WLO))制备的纳米尺度粉末及采用常压烧结法制备的超细晶W-1.0wt.%La₂O₃合金进行对比研究。与纯钨相比，W-1.0wt.%La₂O₃合金具有超细晶粒和优异的力学性能。烧结后，WLO样品的平均晶粒尺寸大于WL10和WLNO样品的平均晶粒尺寸；WL10和WLNO的显微硬度相差不大，但大于WLO的显微硬度。相对于其他样品，在1500 °C烧结后的WL10样品中La₂O₃颗粒呈现最佳分布状态，均匀分布在晶界或晶粒内部，且平均尺寸最小，晶界和晶粒内部的La₂O₃颗粒平均尺寸分别为(57±29.7)和(27±13.1) nm。与传统方法制备的钨材料相比，采用液-液掺杂方法制备的W-1.0wt.%La₂O₃钨合金呈超细的显微组织和优异的性能。

关键词：钨合金；溶液燃烧合成；掺杂方法；超细晶；显微硬度

(Edited by Wei-ping CHEN)

Foundation item: Projects (2017YFB0306000, 2017YFB0305600) supported by the National Key Research and Development Program of China; Projects (51774035, 51604025, 51574031, 51574030, 51574029, 51604240) supported by the National Natural Science Foundation of China; Project (2019JZZY010327) supported by the Shandong Key Research and Development Plan Project, China; Projects (2174079, 2162027) supported by the Natural Science Foundation Program of Beijing, China; Projects (FRF-IDRY-19-025, FRF-TP-17-034A2, FRF-TP-19-015A3, FRF-IDRY-19-003C2) supported by the Fundamental Research Funds for the Central Universities of China

Corresponding author: Ming-li QIN; Tel: +86-10-82377286; E-mail: qinml@mater.ustb.edu.cn

* Jun-jun YANG and Gang CHEN comtributed equally to this work

DOI: 10.1016/S1003-6326(20)65462-0