晶须增强Ti(C, N)基金属陶瓷的正梯度环芯结构设计与形成机制研究

In order to realize the low interfacial composition gradient and strengthened core-rim interface structure of Ti(C, N)-based cermets, a innovative design of inverse rim phases was proposed based on the good chemical stability and excellent metallurgical interface of TiC whiskers in cermets. Whiskers, hard phase (Ti(C, N), WC and Mo2C) and Ni-Ti-C pre-alloyed powders with various morphology and physicochemical properties were prepared by carbothermal reduction, high energy activated annealing or pre-sintering methods, respectively. Satisfied conditions for good distribution of whiskers with suitable rim structures were set up via building the relationship between the properties of ceramic grains and the final morphology and structure of the rim phases. Effects of Ni-Ti-C pre-alloyed powders on the content and morphology evolution of core-rim phase were investigated under different sintering shcedules. Rapid diffusion behavior of Ti atoms during solid state reaction and formation mechanism of Ti-rich inner rims were discussed. Moreover, the toughening mechanisms of the inverse rim structure and synergic relationship with mechanical performance were clarified. This study aims to provide theoretical and experimental proof for understanding the morphology evolution of hard phase and strengthening behavior of micro-interface structure of TiC whisker reinforced Ti(C, N)-based cermets.

针对Ti(C, N)基金属陶瓷环芯相结构的高梯度成份引起的内应力大、界面强度低的问题，本项目利用TiC晶须优异冶金界面结合的优势，提出构筑一种正梯度环芯相结构的设想。以碳热还原、高能活化-预烧结法制备多形态TiC晶须、不同物化特质的超细粉末(TiCN、WC和Mo2C)，采用传统粉末冶金工艺制备金属陶瓷。通过硬质相原料粉体对环芯相组织演变规律的研究，建立晶须形态、硬质相颗粒物化特质与环芯相界面形态和结构的对应关系，构建满足TiC晶须完整形态及适宜环相组织的条件。采用多阶段烧结保温制度，研究纳米级富Ti预合金粉对金属陶瓷环芯相成分和组织的影响，揭示固相反应阶段Ti的快速扩散传质行为及超薄富Ti内环相的形成机制；探明环相反转结构的自调控技术及其与金属陶瓷力学性能的协同关系，为晶须增强超细晶粒Ti(C, N)基金属陶瓷的硬质相形态演变规律和微界面结构的强韧化提供理论和实验依据。

针对Ti(C, N)基金属陶瓷环芯相结构的高梯度成份引起的内应力大、界面强度低的问题，本项目依据环芯相的冶金反应规律，提出构筑一种正梯度均质环芯结构的设想；同时，结合缺碳相在Ni-Ti-W-C体系下的碳化转变机制，拟制备板状晶/晶须原位增韧全固溶体金属陶瓷；通过研究硬质相粉体、烧结制度和方式对环芯相组织演变规律的影响，建立硬质相颗粒物化特质、烧结条件与环芯相界面形态和结构的对应关系，构建满足均质富Ti均质内环相的自调控条件；研究二次碳化反应过程中，反应物配比对碳化反应的物相组成及形态的影响规律，揭示板状晶的原位析出机制及其对金属陶瓷力学性能的影响，为Ti(C, N)基金属陶瓷的微界面结构的强韧化和(Ti, W)C-Ni的板状晶增韧提供理论和实验依据。.本项目的重要结果和数据如下：.1.探明了金属陶瓷在烧结过程种的组织演变：发现固相烧结会促进内环相的形成，亮芯-灰环相增多；延长冷却时间有利于金属相中原位析出亚微米级固溶颗粒。高活性粉体加速界面扩散反应，环相组织粗大；真空热处理可提高球磨粉末的烧结性能，均质内环的球形晶粒有效提高超细晶金属陶瓷的强韧性。.2.诱导均质内环相的形成：研究发现纳米富Ti金属相可诱导均质内环相的析出，阻碍固相阶段Ti(C, N)颗粒的合并长大。Ni中适量的TiC含量可细化晶粒，降低环芯界面错配度，获得高共格的粘结相-环形相-芯相的界面；过量的Ti-C将加速溶解-析出反应，使环相粗化，晶粒长大。NiTiC粘结相可诱导低应力高共格界面的形成；断裂时，通过裂纹偏转、桥连等机制，显著提高材料的强度。.3.板状晶/晶须增强金属陶瓷：以缺碳相原位碳化诱导板状晶固溶体的析出，发现NiTiW相经碳化反应后，可获得Ni6(W,Ti)6C为基体相的混合缺碳相粉。二次补碳碳化为M12C先与C反应转变为M6C，固相阶段M6C将碳化为板状WC和Ni；液相阶段M6C将与溶解的(Ti, W)C共析出，原位析出板状(Ti, W)C和Ni。板状(Ti, W)C的形成依赖孪晶{111}界面的吸附生长，其所需驱动力较二维形核生长模式低，生长速率快。