Co-Al-W基高温合金发展概述

Co-Al-W-based Superalloys: a Summary of Current Knowledge

传统Co基高温合金的强化机制为固溶强化与碳化物强化,强化效果弱于Ni基高温合金中的有序相γ′强化,从而使得Co基高温合金的应用受到限制。直到2006年,在Co-Al-W三元相图中发现稳定的L12相——Co3(Al,W),这种新型γ′相强化的Co-Al-W基高温合金有以下特点:(1)含Ta合金熔点高于Waspaloy合金;(2)硬度与屈服强度不低于Ni基高温合金;(3)γ/γ′两相之间的晶格错配度与Ni基高温合金在数值上接近,符号上相反,而正的晶格错配度更有利于蠕变性能。综上所述,Co3(Al,W)相的发现为Co基高温合金的发展开辟了新道路。自2006年以来,针对Co-Al-W基高温合金的组织与性能进行了大量研究。Co-Al-W基高温合金的微观组织为γ/γ′两相,此外还会存在一些二次相,其中包括富集Al和Ti元素的B2-CoAl相、富集难熔元素的拓扑密堆相m-Co7 W6以及易在时效过程析出的c-Co3 W相。这些二次相通常在晶界析出,容易成为裂纹的发源地,同时会弱化固溶强化效果,对合金的高温性能不利。虽然Co-Al-W基高温合金得到了立方形态的γ′相共格析出,但由于γ′-Co3(Al,W)相高温稳定性差,需要对其进行合金化,因此,这种γ′相强化的Co基高温合金正在由简单的Co-Al-W三元合金发展成为复杂的多元合金。综合来看,主要添加的合金化元素有Ta、Ti、Nb、V、Mo、Ni和Cr。其中,γ′相形成元素包括Ta、Ti、Nb、V、Mo,这些元素的分配系数均大于1,且能有效提高γ′相固溶温度与体积分数;Cr、Fe、Re的分配系数小于1,是γ相形成元素,添加后均降低γ′相固溶温度,其中Cr会提高γ′相的体积分数。众多合金元素中,Cr、Mo和Ni元素的过量添加会降低γ/γ′两相间的晶格错配度,从而改变γ′相形态甚至破坏γ/γ′两相组织。合金的组织与性能密切相关,γ/γ′两相、γ′相为立方形态且γ′相高温稳定性高的合金具有优异的性能。Co-Al-W基高温合金的流变应力随温度变化分为三个阶段:首先随温度升高而降低;然后随温度升高而异常升高;最后再次随温度升高而降低。故而存在峰值温度与峰值强度,Co基高温合金多应用在峰值温度下,以便获得最高的屈服强度。此外,由于Co-Al-W基高温合金中γ/γ′两相晶格错配度为正,在蠕变过程中会出现平行于拉应力的筏化,对合金的高温性能有利。除了Ta、Ti等元素能强化合金外,少量B元素的添加有晶界强化作用,可以提高合金的力学性能。添加Cr元素的Co-Al-W基高温合金在高温氧化过程中会形成三层氧化层,分别是最外层的Al2 CoO4、富Cr并含有Cr2 O和Cr2 O3的中间层以及最内层的Al2 O3。其中Cr2 O3和Al2 O3氧化层均致密且具有保护作用,可显著提高合金的抗氧化能力。本文简单介绍了Co-Al-W基高温合金的发现与发展,综述了近年来Co-Al-W基高温合金的研究现状,并指明了未来Co基高温合金的发展方向。

The strengthening mechanism of conventional Co-based superalloy usually include solid-solution strengthening and carbide strengthening,the reinforcement effect of which are weaker than the ordered phaseγ′-strengthened of Ni-base superalloys,and this making its application limited.Stable Co 3(Al,W)phase with L12 structure of Co-Al-W alloy was firstly discovered in 2006,and has many characteristics:(1)Ta-containing Co-Al-W alloys possess higher melting point than Waspaloy;(2)hardness and yield strength of Co-Al-W alloys are no less than those of Ni-base superalloys;(3)lattice mismatch betweenγandγ′is similar to that of Ni-base superalloys in numerical value and opposite in symbol,which is benefit to creep-resistance properties.Therefore the discovery of Co 3(Al,W)phase supports potential for the further development of Cobase superalloys.The microstructure and properties of Co-Al-W-based superalloys have been widely investigated since 2006.In addition toγandγ′two phases,there are also some second phases in Co-Al-W-base alloys including B2-CoAl phase enriching in Al and Ti,topologically close-packed phaseμCo 7W 6 enriching in refractory elements,andχ-Co 3W phase likely precipitated during aging.These second phases tend to precipitate at grain boundary,which is easy to become the source of cracks.At the same time,they weaken the effect of solid solution strengthening,and deteriorate the high temperature properties of alloys.The cuboidal coherentγ′phases are obtained in Co-Al-W-based alloys,however,they need to be alloying because of unstability ofγ′phase.As a result,this kind ofγ′-strengthening Co-based superalloys are developed from simple Co-Al-W ternary alloys to complicated multi-component alloys.The alloying elements being added to Co-Al-W-based alloys include Ta,Ti,Nb,V,Mo,Ni and Cr.Among these elements,Ta,Ti,Nb,V and Mo areγ′-forming elements,which have partitioning coefficient greater than 1 and attribute to improve solvus temperature and volume fraction ofγ′phase;Cr,Fe and Re areγ-forming elements with partitioning coefficient less than 1 and reducingγ′solvus temperature,especially,Cr can improve volume friction ofγ′phase.Besides,the excessive addition of Cr,Ni and Mo will reduce lattice misfit betweenγ′andγ,resulting in transformation of shape ofγ′phase and destruction of microstructure ofγ/γ′.It is noted that properties are closely related to microstructure,therein,the cuboidal and stableγ′phase precipitated inγphase usually make alloys display excellent properties.The temperature dependence of the flow stress of Co-Al-W-based alloys can be divided into three stages:firstly,flow stress decreases with the increase of temperature;secondly,there is an anomalous positive temperature variation where the flow stress substantially increase with temperature;finally,it backs to a negative temperature variation.So there are peak temperature and peak stress,alloys applied in peak temperature will have high strength.Besides,because of positive lattice misfit of Co-Al-W-based alloys.Moreover,during creep process,rafting parallel to tensile stress is beneficial to performance under high temperature owing to positive lattice misfit of Co-Al-W-based alloys.As for as elements,except Ta and Ti,a small amount of B element will improve mechanical properties by grain boundary strengthening.Cr-containing Co-Al-W-based alloys will form three oxide layers during oxidation under high temperature:outermost layer includes Al 2 CoO 4,the middle layer includes Cr 2O and Cr 2O 3 and innermost layer consisting of Al 2O 3.It is reported that compact and productive Cr 2O 3 and Al 2O 3 can improve oxidation resistance.This paper presents discovery and development of Co-Al-W-based superalloys.Then,the research status of Co-Al-W-based superalloys is summarized.Finally,the future progress of Co-based superalloy are proposed.