阻燃钛合金Ti40的热加工与力学性能研究

Hot Processing and Mechanical Properties of Chinese Ti40 Burn-Proof Titanium Alloy

研究了 Ti40 ( Ti- 2 5 V- 1 5 Cr- 0 .4Si)阻燃钛合金的热加工与力学性能的关系 ,运用扫描电镜( SEM)及能量散射谱仪 ( EDS)对其热压、室温及高温拉伸、热稳定及蠕变断口进行了形貌观察与微区成分定量分析。结果表明 ,多向大变形加工有利于 Ti40合金组织均匀性及力学性能的提高 ;Ti40合金具有一定的耐热性 ,且在不同试验条件下呈现不同的断裂特征 ;断口上存在的 Fe、C、O、S等杂质元素是导致合金过早断裂的内在原因 。

Conventional commercial titanium alloys, if used in the manufacture of gas turbine engines, are very likely to cause “titanium fire” due to elevated temperature environment. Burn proof titanium alloys “Alloy C” and “BTT 3” emerged in U.S.A. and Russia respectively. In recent years, Northwest Institute for Nonferrous Metals Research in Xi′an developed a new burn proof titanium alloy, Ti 25V 15Cr 0.4Si alloy, and named it Ti40 alloy; Ti40 alloy is based on Ti V Cr Si alloy system. Unlike other commercial titanium alloys, burn proof titanium alloys, including Ti40 alloy, often suffer from hot deformation cracking. In this paper, we report on investigations on relationships among hot working, mechanical properties and fracture modes of Ti40 alloy. We carried out micro zone chemical composition analyses on scanning electron microscope (SEM), which was also equipped with energy dispersion spectrum (EDS) instrument. Results show that boundary brittleness of Ti40 alloy induces hot working difficulty. Good quality Ti40 alloy can be obtained only by selecting appropriate working parameters. In subsection 2.1, we provide two formulas, eqs. (1) and (2), for computing deformation rate and point out the conditions under which they can be used. Ti40 alloy cakes were processed by axial direction press and total deformation rates ranged from 65% to 75%. Ti40 bars were processed by multi direction forging technology and total deformation rates were all above 95%. Ti40 alloy bars exhibit better mechanical properties than Ti40 alloy cakes because total deformation rates of bars during processing are higher than that of cakes. When compared with room temperature properties, at 540℃ tensile strength decreases, plasticity of bars improved, and plasticity of cakes remains unchanged. Thermal exposure makes strength increase somewhat but makes plasticity decline notably. Brittle oxide scale makes crack easier to occur and causes Ti40 alloy to fracture earlier. Nonuniformity of hot working causes creep property of cake to be unstable. We made fractographic study of Ti40 alloy. Lamellar characteristic of rupture implies microstructure anisotropy of cakes and isotropy characteristic of fracture of bars is determined by its equiaxial micro grains. As a kind of heat resistant and burn proof alloy, the fracture mode varies with test conditions. Brittle cracking modes of Ti40 alloy include transgranular cleavage, transverse intergranular cracking, mixture cracking (mainly intergranular) and second intergranular cracking. Ductile cracking modes of Ti40 alloy include transgranular dimple, mixture cracking (mainly dimple) and creep slipping. Fractography micro zone analysis shows that microstructure abnormality, composition segregation and metallurgical inclusions such as Fe, C, O, S etc are the intrinsic causes of earlier rupture of the alloy. Improvement of the quality of metallurgy and multi direction upset with large amount of deformation are two keys to improving properties of Ti40 alloy.