纳米晶Ni-12Cr合金800℃高温氧化动力学和氧化膜结构演化

Oxidation Kinetics and Microstructure Evolution of Nanocrystalline Ni-12Cr Alloy at 800℃

利用聚焦离子束扫描电镜和扫描透射电镜研究了粗晶和纳米晶结构的Ni-12Cr合金在800℃空气中氧化的动力学行为和氧化膜的结构演化。结果表明,氧化初期25 s时,粗晶和纳米晶的氧化膜均为外层NiO和内层富Cr的双层结构。随着氧化的进行,粗晶氧化膜演化为NiO/(NiO+NiCr_(2)O_(4))/疏松Cr_(2)O_(3)/内氧化层的多层结构;而纳米晶发生了Cr的选择性氧化,形成了保护性Cr_(2)O_(3)膜,Cr_(2)O_(3)以非晶晶化的方式形核生长;纳米晶保护性氧化膜的形成早于纳米晶粒的粗化。根据有效扩散系数修正的Wagner理论,Ni-12Cr合金在800℃发生由内氧化向外氧化转变的临界晶粒尺寸约为94 nm。纳米晶的氧化动力学曲线分为两个阶段,转折点约为氧化1 h。两阶段均符合抛物线规律,抛物线速率常数k_(p)分别为1.76×10^(-13)和1.58×10^(-14) cm^(-2)·s^(-1)。

Nanocrystallization can increase the diffusion rate of alloying elements,such as Cr,Al,and then improve the high-temperature oxidation resistance of metallic alloys due to the presence of high-volume fraction of grain boundaries.Up to now,the studies on the effect of nano-structure on the oxidation behavior of alloys mainly focus on the oxidation kinetics and the structure of oxide scale at steady stage.The microstructure of oxide scale formed on nano-structured alloys during oxidation,especially at the initial stage,is lack of characterization.At the same time,nano-structured alloys are far away from thermodynamic equilibrium state,which inevitably result in the grain coarsening during oxidation process.Therefore,it is necessary to characterize the microstructure of oxide scale on nano-structured alloys to understand the relationship between grain coarsening of alloy and the formation of oxide scale on its surface.In this study,the oxidation behavior of coarse-grained(CG)and nanocrystalline(NC)Ni-12Cr alloys(mass fraction in nominal chemical composition)at 800℃in air were studied.The NC alloy was prepared by severe plastic deformation,and the average grain size is around 42 nm.Focused ion beam(FIB)microscope and scanning transmission electron microscope(STEM)were used to characterize the microstructure and composition distribution of the scale.The results demonstrate that a two-layered scale,external NiO and internal Cr-rich layer,is formed on both of CG and NC alloys after oxidation for 25 s.The scale on CG alloy develops into a multilayered structure,which includes NiO/(NiO+NiCr_(2)O_(4))/porous Cr_(2)O_(3)/internal oxidation zone after oxidation for 10 min,while the Cr-rich layer on NC alloy crystallizes and forms a protective Cr_(2)O_(3) scale after oxidation for 2 min.The oxidation kinetics of NC alloy consists of two stages,both of which follow the parabolic law.The parabolic rate constants of the two stages are 1.76×10^(-13) cm^(-2)·s^(-1)(within 1 h)and 1.58×10^(-14) cm^(-2)·s^(-1)(1-109 h)respectively,both of which are about 3-4 orders of magnitude lower than that of CG alloy.The study on grain growth kinetics of alloy indicates that the protective chromia scale forms before the significant coarsening of nano-grain.According to Wagner theory with the effective diffusion coefficient,the critical grain size for Ni-12Cr alloy to change from internal oxidation to external oxidation is about 94 nm at 800℃.