【作者】 董智；

【导师】 马宗青；

【作者基本信息】 天津大学 ， 材料科学与工程， 2022， 博士

【摘要】 与相应的钨合金相比,氧化物弥散强化（ODS）钨合金具有显著改善的强度、组织稳定性和蠕变抵抗力。但目前常用的氧化物掺杂技术易导致氧化物颗粒在钨晶界处偏聚长大,这大大削弱了其对钨合金综合力学性能的改善效果。针对该问题,本文从ODS钨合金复合先驱粉的制备入手,制备了氧化物掺杂W-Y₂O₃纳米复合粉末和W包覆Y₂O₃（Y₂O₃@W）核壳结构纳米复合粉末,实现了对晶界和晶内氧化物第二相颗粒的设计和调控。随后深入研究了上述复合粉体中氧化物分布状态对W基体烧结致密化和晶粒生长的影响机制,考察了合金组织中氧化物第二相颗粒与W基体的界面结构及其强化机制,研制出了强度和韧性匹配良好的ODS钨合金,本文主要研究内容及获得的结论如下:为了细化和分散W-Y₂O₃合金中晶界处大尺寸氧化物第二相颗粒,采用原位化学制粉工艺制备了各种氧化物掺杂的W-Y₂O₃复合纳米粉体,经低温烧结后发现La₂O₃和HfO₂掺杂对晶界处Y₂O₃颗粒的细化分散作用最为显著,这与其在W/Y₂O₃界面的偏聚分布密切相关。弹性应变能、氧化学势梯度和界面能是掺杂氧化物在W/Y₂O₃界面发生偏聚的三种内在驱动力。此外,La₂O₃掺杂还能细化W基体的晶粒组织,而HfO₂掺杂则使W基体晶粒组织发生粗化,这是因为第三组元氧化物界面偏聚后会显著改变W/Y₂O₃界面上的W-O化学键合,进而影响了W基体的烧结活性。La₂O₃掺杂同时细化了晶界处第二相颗粒和基体晶粒组织,最终显著提升了W-Y₂O₃合金的强度和韧性。为了促进更多的氧化物第二相颗粒在W晶粒内部均匀分散,研发了水热+冷冻干燥原位化学制粉工艺,制备出了钨包覆氧化物核壳结构（Y₂O₃@W）复合纳米粉体。采用该粉体为先驱粉制备的ODS钨合金中,W晶界处的氧化物颗粒完全消失,氧化物第二相纳米颗粒高密度均匀分布于W晶粒内部,并与W基体保持良好的共格关系。该合金在室温下抗拉强度达到1400 MPa,延伸率达到2.7%,打破了传统钨合金低温脆性的局限性。600℃下,该合金的抗拉强度达到721 MPa,延伸率达到12%,具有优异的强韧性匹配。该合金的高强度主要得益于细化的亚晶组织以及晶内氧化物纳米颗粒的位错剪切效应,而高延展性主要得益于合金中具有大量小角度晶界的层状晶粒组织和晶内氧化物纳米颗粒对位错的钉扎积累效应。氧化物的分布状态不同,对钨基体烧结动力学过程的影响亦明显不同。基于此,进一步研究了上述各类复合粉体中氧化物分布状态对W基体烧结致密化和晶粒生长的影响机制。发现无论将氧化物分布在晶界还是晶内,烧结过程中W基体的物质传输机制均为晶界扩散。但晶界氧化物显著提升了W晶粒的生长激活能,抑制了W晶粒的生长。此外,晶界氧化物还能降低W的晶界扩散率,抑制W基体的致密化。与之不同,晶内氧化物不会影响W晶粒的生长激活能和W的晶界扩散率,亦不会影响W晶粒的生长和W基体的致密化。本文最后部分发现了HfO₂对W基体兼具烧结促进作用和第二相强化作用。在低温烧结过程中,HfO₂能显著提高W基体的晶界扩散率和烧结应力,因此其能够同时满足促进W基体低温烧结的热力学条件和动力学条件。此外,该体系的烧结动力学行为遵循W通过HfO₂扩散的动力学机制,说明晶界处的HfO₂颗粒能够为W原子的晶界扩散提供快速通道,进而促进W基体的低温烧结快速致密化。经1480℃无压烧结2小时,W-HfO₂合金的相对密度达到97%,晶粒尺寸仅为200 nm,其抗压强度达到近3 GPa。该合金的主要强化机制为第二相强化和细晶强化。更多还原

【Abstract】 Compared with the corresponding tungsten alloys,oxide dispersion strengthened（ODS）tungsten-based alloys exhibit significantly enhanced strength,microstructural stability and creep resistance.However,the commonly used oxide doping techniques generally lead to the aggregation and growth of oxide particles at W grain boundaries,greatly weakening the improvement effect of oxide addition.Based on this situation,starting from the preparation of composite powder precursor of ODS tungsten alloy,we prepared oxide-doped W-Y₂O₃ composite nanopowder and Y₂O₃@W core-shell structural composite nanopowder.Using these nanopowders as precursors,the oxide second-phase particles at grain boundaries and within grain interior can be regulated respectively.Then,the effect mechanisms of oxide distribution state in the above composite powders on the grain growth and densification mechanisms of W matrix are investigated deeply,the interfacial structure of oxide second phase particles with W matrix and the strengthening mechanisms of oxide particles are studied.On this basis,we developed the ODS tungsten alloy with excellent match of strength and ductility.The main contents and conclusions are as following:In order to refine and disperse the large-sized oxide second-phase particles at grain boundary of W-Y₂O₃ alloy,various oxide-doped W-Y₂O₃ composite nanopowders are prepared by in-situ chemical powder preparation technique.After low-temperature sintering,it is found that La₂O₃ and HfO₂ doping have the most significant refinement effect on the intergranular Y₂O₃ particles,which is closely associated with the interfacial segregation of La₂O₃ and HfO₂.Oxide chemical potential gradient,elastic strain energy and interfacial energy are three driving forces for the segregation of doped oxides.Besides,La₂O₃ doping refines the grain microstructure,while HfO₂ doping coarsenes the grain microstructure.This is because the W-O chemical bonding at the W/Y₂O₃ interface can be significantly changed by oxide segregation,which will affect the sintering activity of W matrix.La₂O₃ doping can simultaneously refine the intergranular oxide second phase particles and matrix grain microstructure,significantly improving the strength and toughness of W-Y₂O₃alloy.In order to promote more oxide second-phase particles to disperse within W grain interior uniformly,we independently develop the in-situ chemical powder preparation technique of hydrothermal+freeze-drying method,preparing Y₂O₃@W core-shell structural composite nanopowder.Using this powder as precursor,the intergranular oxide particles disappear completely in prepared W-Y₂O₃ alloy,and the oxide second-phase particles are distributed uniformly within W grain interior and keep coherent interface with W matrix.As a result,the fabricated ODS-W alloy possesses both high strength（1390 MPa）and excellent ductility（2.6%）at room temperature,breaking the brittle feature of traditional W-based alloys at room temperature.Its strength at 600℃ reaches 721 MPa with a total elongation of 12%.Its high strength mainly originates from the refined sub-grain microstructure and dislocation shearing effect of intragranular nanoparticles,and its ductility is closely associated with the lamellar microstructure with abundant low-angle grain boundaries and the dislocation pinning and accumulation effects of intragranular oxide nanoparticles.The different oxide distribution states have different effects on the sintering kinetic of W matrix.Based on this,we further studied the influence mechanism of oxide distribution state in the above composite powders on the grain growth and densification of W matrix.It can be found that no matter the oxide is distributed at grain boundary or within grain interior,the mass transport mechanism of W matrix during sintering process does not change and remains as grain boundary diffusion.But the intergranular oxides significantly enhance the activation energy for W grain growth,inhibiting W grain growth.Besides,the intergranular oxides also reduce the grain boundary diffusivity of W,inhibiting the densification of W matrix.In contrast,if the oxides are dispersed within W grain interior,they will not affect the activation energy for W grain growth and the grain boundary diffusivity of W matrix.Thus,the intragranular oxide do not affect the grain grwoth and the densification of W matrix.In the last part of this dissertation,it is found that HfO₂ can be used as both strengthening phase and sintering accelerator for W matrix.During the low-temperature sintering process,HfO₂ can significantly increase the grain boundary diffusivity and the sintering stress of W matrix,so the kinetic and thermodynamic conditions promoting the sintering of W matrix are simultaneously satisfied.In addition,it is found that the sintering kinetics mechanism of W-HfO₂ system was consistent with the kinetic mechanism of W diffusion through HfO₂,indicating that the intergranular HfO₂ particles could provide a fast channel for the diffusion of W atoms along grain boundaries,thus promoting the rapid densification of W matrix during low-temperature sintering.After pressureless sintering at 1480℃ for 2 h,the grain size of W-HfO₂ alloy is only 200 nm,but the relative density reaches 97%.The corresponding compressive strength reaches nearly 3 GPa.Second-phase strengthening and fine-grain strengthening are the main strengthening mechanisms of W-HfO₂ alloy.更多还原