Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 52061038, 51661032, 51301150), the Special Program of Youth New-star of Science and Technology of Shaanxi Province, China (Grant No. 2013KJXX-11), and the Key Industrial Research Program of Yan’an Science and Technology Department, China (Grant No. 2016KG-02)
Received Date:30 August 2020
Accepted Date:27 October 2020
Available Online:24 February 2021
Published Online:05 March 2021
Abstract:The Ni-Al intermetallic compounds, as important high-temperature structural materials, have clear target requirements in a number of fields. Powder metallurgy is an important candidate for preparing the Ni-Al intermetallic compounds. Clarifying the formation and transformation process of Ni-Al intermetallic compounds in sintering process and determining the solid diffusion reaction temperature and types of intermetallic compounds are greatly important for tailoring sintering process and optimizing product quality. In this paper, the internal friction behaviors of Ni-Al powder mixture compacts in the sintering process are systematically investigated by the internal friction technique. A typical internal friction peak is observed in the internal friction-temperature spectrum. The peak height decreases with the measuring frequency increasing, but the peak temperature is independent of frequency. Moreover, the internal friction peak shifts toward higher temperature and the peak height increases as the heating rate increases. It is reasonable that the internal friction peak belongs to the typical phase transformation internal friction peak which is associated with the formation of intermetallic compounds NiAl3 and Ni2Al3 in the heating process. Furthermore, the microstructure of the Ni-Al powder mixture can be tailored by mechanical ball-milling. The internal friction peak shifts toward lower temperature and the peak height decreases with the ball-milling time increasing, which indicates that the solid diffusion reaction can be activated at lower temperature with a slower reaction rate. This decrease is related to the refinement of powder particles, the lamellar formation of powder mixture, the enhancement of solid solution degree and surface energy, and the shortened atomic diffusion distance due to the mechanical ball-milling. It is also indicated that the mechanical ball-milling can effectively reduce the initial temperature of solid diffusion reaction, thus lowering sintering temperature. Keywords:Ni-Al intermetallic compounds/ internal friction/ mechanical ball-milling/ solid diffusion reaction
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3.1.Ni-Al粉末压坯典型内耗特征及其微观机制
图1给出了球磨1 h后的Ni-Al粉末压坯升温过程中原始内耗和相对动力学模量随温度的变化关系. 从图1中可以发现, 在测量频率为1 Hz时, 556 ℃附近, 内耗-温度谱上出现一个典型内耗峰, 同时, 伴随内耗峰的出现, 相对动力学模量发生明显软化出现一个极小值, 极小值出现后, 相对动力学模量又迅速升高. 表明在内耗峰所在温区附近, Ni-Al粉末压坯微观结构发生了显著改变. 此外, 通过图1还可发现, 相对动力学模量极小值和内耗峰出现的温度并不完全一致, 低于内耗峰峰温约15 ℃, 这应与Ni-Al粉末压坯在内耗峰低温区域发生相变, 生成金属间化合物有关, 由于金属间化合物模量远高于Ni/Al单质模量, 当金属间化合物达到一定量时, 相对动力学模量提前迅速增大, 进而导致相对动力学模量极小值提前出现[22]. 图 1 球磨1 h后的Ni-Al粉末压坯IF和RDM与温度的关系 Figure1. IF and RDM as a function of temperature for the Ni-Al powder compact milled for 1 h.
图2给出了球磨1 h Ni-Al粉末压坯内耗峰与测量频率之间的依赖关系. 从图2可以发现, 内耗峰具有明显的频率依赖性, 随测量频率的增大, 内耗峰显著降低, 但峰温无明显频率依赖性. 此外, 图3表明, 内耗峰还具有明显的升温速率依赖性, 峰高随升温速率的增大而升高且峰位向高温方向移动, 存在明显的升温热滞后特征. 内耗峰的这些特征与连续变温过程中相变内耗峰一致[19,23,24], 因此, 可以判定内耗峰的产生应与Ni-Al粉末压坯升温过程中发生的相变有关. 图 2 球磨1 h后的Ni-Al粉末压坯内耗峰与测量频率的依赖关系 Figure2. Dependence of internal friction peak on measuring frequency for the Ni-Al powder compact milled for 1 h.
图 3 球磨1 h后的Ni-Al粉末压坯内耗峰与升温速率的依赖关系 Figure3. Dependence of internal friction peak on heating rate for the Ni-Al powder compact milled for 1 h.
为进一步阐明内耗峰形成机制, 图4给出了球磨1 h的Ni-Al粉末压坯热处理至不同温度的XRD图谱. 样品在492, 556和675 ℃下分别真空保温1 h并冷却至室温, 三个温度的选择分别对应于内耗峰的起始温度、峰值温度和结束温度. 通过MDI Jade 6分析和标定了热处理后样品的物相, 可以发现, 起始温度下热处理的样品中仍为单质Ni和Al, 无金属间化合物出现. 在峰值温度处热处理样品后, 样品中出现了NiAl3和Ni2Al3两种金属间化合物以及未反应的Ni单质. 在内耗峰结束温度处理样品, Ni-Al粉末压坯转化为Ni2Al3金属间化合物但样品中仍残留极少量Ni单质. 这也表明, 峰值温度热处理后, 出现的NiAl3在后续的热处理过程中, 发生了NiAl3 + Ni = Ni2Al3转变, 生成了单一的Ni2Al3金属间化合物. 所以, 内耗峰的出现与Ni-Al金属间化合物的生成有关, 属于典型的相变内耗峰. 升温过程中, Ni和Al颗粒之间发生固相扩散反应, 生成NiAl3和Ni2Al3两种金属间化合物, 在新相金属间化合物和Ni/Al单质母相间, 以及不同金属间化合物之间出现新的界面, 内耗测量过程中, 相界之间因相互摩擦和微观滑移, 产生能量耗散引起内耗. 由于相变内耗大小与相界面数量的多少正相关[22], 在内耗峰低温侧, 由于固相扩散反应产生了金属间化合物, 增加了相界的数量, 内耗随温度的增加而升高. 当相变产生的新界面数量与母相减少界面数量达到动态平衡时样品内部耗能最大, 进而内耗达到最大值. 在内耗峰高温侧, 尽管固相扩散反应继续进行, 但Ni-Al粉末混合物几乎全部转变为Ni2Al3金属间化合物, 母相Ni颗粒也在继续减少, 都会引起相界数量的持续减少, 此外, 温度的持续升高, 也会引起界面间黏滞阻力的减小, 进而样品耗能降低内耗减小. 图 4 不同温度热处理的Ni-Al粉末压坯XRD图谱 (a) 492 ℃; (b) 556 ℃; (c) 675 ℃ (三个温度分别对应于内耗峰起始温度、峰值温度和结束温度) Figure4. XRD patterns of Ni-Al powder compact milled for 1 h after heat treatment at different temperature: (a) 492 ℃; (b) 556 ℃; (c) 675 ℃ (three temperatures respectively corresponding to start temperature, peak temperature and end temperature of the internal friction peak).
23.2.机械球磨与内耗特征的响应规律及其微观机制 -->
3.2.机械球磨与内耗特征的响应规律及其微观机制
图5给出了Ni-Al粉末混合物机械球磨对内耗峰的影响. 由图5可以看出, 随球磨时间的增加, 内耗峰高度降低, 峰温向低温方向移动, 内耗峰宽度变大. 这些表明, 机械球磨可以降低固相扩散反应的起始温度, 同时, 降低固相扩散反应的速度, 延长反应时间, 显著区别于无球磨样品的快速爆炸反应, 动态结构变化难以跟踪. 所以, 球磨有助于Ni-A粉末压坯烧结过程中微观结构的可控. 图 5 机械球磨对Ni-Al粉末压坯内耗峰的影响 Figure5. Dependence of mechanical ball-milling on internal friction peak for the Ni-Al powder compact.