IMECH-IR  > 非线性力学国家重点实验室
多相无序合金成分设计及力学性能研究
Alternative TitleComposition designs and mechanical properties of multiphase disordered alloys
陈峰
Thesis Advisor戴兰宏
2019-05-29
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline固体力学
Keyword无序合金 化学成分 组织结构 力学性能 强韧化
Abstract

拓扑无序的非晶合金以及化学无序的高熵合金作为两种典型的无序合金,由于复杂的化学成分和特殊的结构拥有优秀的综合性能,在新型高强韧金属材料中脱颖而出。多组元的无序合金内蕴“拓扑/化学无序”,在外加载荷的作用下,非平衡状态的无序结构呈现跨时空尺度的涌现与演化,导致其塑性流动行为复杂、多样,因而塑性变形机制有别于以一两种组元为基元的、具有晶体结构的传统合金。所以,对无序合金的强韧化探索将极大地促进无序合金作为新型先进高强韧材料实际工程中的应用,满足越来越苛刻的极端环境服役要求。

非晶合金和单相高熵合金中存在着强度和塑性的倒置关系,在拥有高强度的同时塑性变形能力较差,或因为良好的塑性在强度方面表现较差。由此,我们分别从多相的非晶复合材料以及双相的难熔高熵合金,讨论拓扑无序合金和化学无序合金这两类无序合金的强韧化途径。在本文研究工作中,我们设计并制备了多相非晶复合材料TiZr-based TZVX合金和双相难熔高熵合金WX合金,分别研究了其合金成分与组织结构的关系,分析了组织结构对力学性能的影响,探索了其强韧化途径。主要研究工作内容及结果如下:

(1)设计了一种具有一定非晶形成能力的多相非晶复合材料TZVX合金,其名义成分为Ti48Zr20VxCu5Be15Cr12-xx=3,6,9,12),并通过铜模铸造的方式制备了一系列的ϕ5mm圆棒试样,并确定其为晶相和非晶基体复合的组织结构。随着V含量逐步增加替代Cr,晶相结构从复杂的六方Cr2Ti间隙相和体心立方的β固溶相变为简单的β枝晶相,同时析出晶相形貌由混乱不规则形状变成规则枝晶状。

(2)V含量较低、Cr含量较高的TZV3和TZV6几乎没有塑性,经弹性变形后突然断裂,表现为脆性断裂,这是因为V含量较低、Cr含量较高时,在β枝晶相界处析出了脆性相金属间化合物Cr2Ti致使材料过早脆断。随着V含量增加,间隙相不在析出,TZV9与TZV6相比塑性提高。当V含量较高时,TZV12与TZV9组织形貌相近,但TZV12塑性远远超过TZV9,压缩塑性为εplastic=6.9%,强度为σmax=1767.4MPa。塑性改善的原因在于TiZr-based BMGCs中V的添加可以降低β枝晶的剪切模量G,阻碍基体中剪切带扩展、促进多重剪切带增殖,从而改善力学性能。因此,改善BMGCs室温塑性的要点在于获得塑性晶相和降低晶相剪切模量G

(3)通过高熵合金相形成的基本理论及规律,设计并制备了一种具有室温塑性的双相难熔高熵合金WX(X=40,30,25,20,15)合金,其名义成分可记做Wx(Mo10Fe45Ni45)100-x。其合金铸态组织为BCC增强相和FCC基体的简单双相固溶结构,并随着W含量降低,凝固时过冷度∆T降低,初始析出相体积分数减小,Ostwald粗化、接邻合并进行度不充分,增强相尺寸分布范围变窄,少量小尺寸的初始颗粒被保留下来,BCC相尺寸分布由单峰变为双峰,分为大尺寸~10-100μm的BCC1团簇相和小尺寸~0.1-10μm的短棒状BCC2颗粒。BCC2颗粒体积分数愈来愈高、分布越来越连续,由零星点状到近似成网状。

(4)W30和W20合金跟其它难熔高熵合金的拉伸性能对比,以极小的强度为代价获得了良好的室温拉伸塑性。其中,W30拉伸强度为σb=872MPa,最大塑性应变为εp=15.3%,而W20拉伸强度为σb=788MPa,最大塑性应变为εp=19.2%。W20与W30相比,在塑性提高的同时强度牺牲极小,这是因为双峰尺寸分布的第二相强化效应,广泛弥散分布在W20 FCC基体内的小尺寸短棒状BCC2提高了基体整体强度。双相合金WX断口形貌为内含BCC2颗粒断面的韧窝包围解理断面的花样,因此拉伸断裂方式为韧脆复合的类型;塑性变形机制由位错运动主导,FCC基体内的位错滑移提供塑性,BCC相界对位错运动的阻碍提供强度的同时也限制着WX合金的最大塑性。因此,调节增强相形状、大小、分布也是改善双相化学无序合金综合力学性能的有效手段。

Other Abstract

Topological disordered amorphous alloys and chemical disordered high-entropy alloys are two typical disordered alloys. Due to complex composition and special microstructure, they hold excellent comprehensive performance and stand out beyond advanced high strength-toughness metal materials. With intrinsic ‘topology/chemical disorder’, the non-equilibrium disordered structures of disordered alloys appear to emerge and evolve across time and space scales under the external load, resulting in various and complex plastic flow behavior. Therefore, the plastic deformation mechanism is different from conventional alloys with master elements and crystal structures. The strengthening-toughening exploration of disordered alloy will greatly promote the application of the disordered alloy as new advanced metal materials in practical engineering to meet the increasing demands of extreme environment.

There is also a tradeoff relation between strength and plasticity among almost pure BMGs and single phased HEAs. Therefore, we discuss the strengthening-toughening approach of the two disordered alloys topological disordered alloys and chemical disordered alloys, in terms of multiphase BMGCs and dual-phase RHEAs. In this paper, we designed and prepared multi-phase TZVX BMGCs and dual-phase WX RHEAs, invested the relationship between composition and microstructure, analyzed the influence of microstructure on the mechanical properties, and explored the strengthening-toughening means, respectively.

(1) A kind of multi-phase BMGCs TZVX were designed, having the nominal composition Ti48Zr20VxCu5Be15Cr12-x(x=3, 6, 9, 12). A series of ϕ5mm columnar samples were prepared by copper casting, proved to be the composite of crystalline phase and amorphous matrix. As the V contents increase instead of Cr, the microstructure of crystal phase changes from the complex HCP Cr2Ti interstitial phase and BCC β solid solution phase to a simple BCC β dendritic phase, and the morphology of the precipitated phase changes from irregular shape to regular dendrite.

(2) TZV3 and TZV6 with low V contents and high Cr contents break immediately after elastic section, exhibiting brittle fracture due to intermetallic compound Cr2Ti precipitating near the boundary of β dendritic. As the V contents increase, the intermetallic compound disappears. The plasticity of TZV9 gets improved. When the V contents are high, although the morphology of TZV12 and TZV9 is similar, the plasticity of TZV12 well exceeds TZV9. The compressive plasticity and strength of TZV12 is 6.9% and 1767.4MPa, respectively. The reason for the improvement of plasticity is that the addition of V can reduce the shear modulus G of β dendrite, hinderring the expansion of shear bands in the matrix, promoting the propagation of multiple shear bands, and improving the mechanical properties. Therefore, the point of improving the room-temperature plasticity of topology disordered BMGCs is to obtain a plastic crystal phase and lower shear modulus G.

(3) According to the basic theory and law on phase formation of HEAs, dual-phase RHEAs WX with room-temperature ductility were designed and prepared, with the composition Wx(Mo10Fe45Ni45)100-x. The microstructure of the as-cast alloys is a simple dual-phase structure with the BCC reinforced phase and the FCC matrix. With the decrease of W contents, the supercooling degree ∆T and the initial precipitate phase volume fraction decreases, resulting in the insufficient degree of Ostwald ripening and the adjacent particles coalescing, the reduction of the size range of BCC enhanced phase, and the reserve of the small size particles. Therefore, BCC phase size distribution changes from unimodality to bimodality, that is BCC1 cluster phase with the large size of ~10-100μm and BCC2 short rod particles with the small size of ~0.1-10μm. The volume fraction of BCC2 particles is getting higher, and the distribution is more continuous, from the initial dotted to the last approximate network.

(4) W30 and W20 get good room-temperature tensile ductility at little price of strength compared with other RHEAs. The tensile strength and plasticity of W30 and W20 are 872MPa and 5.3%, 788MP and 19.2%, respectively. W20 has an apparent increase of plasticity with little decrease of strength compared with W30, due to the second phase strengthening effect with the bimodal size distribution. The small sized BCC2 dispersed widely in the FCC matrix, improveing the overall strength of the matrix. The cleavage sections are surrounded by the dimples containing the BCC2 particle sections in the fracture morphology of dual-phase WX, viz the ductile-brittle composite tensile fracture mode. The plastic deformation mechanism is dominated by dislocation motion. Slipping of dislocations in the FCC matrix provides plasticity, and the resistance of BCC phase boundary to the dislocation motion provides strength while limits the maximum plasticity. Therefore, adjusting the shape, size and distribution of the enhanced phase is an effective approach to improve the comprehensive mechanical properties of the multi-phase chemical disordered HEAs.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79100
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
陈峰. 多相无序合金成分设计及力学性能研究[D]. 北京. 中国科学院大学,2019.
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