冷轧态Ti-35Nb-2Zr-0.3O合金的异常热膨胀行为

Anomalous Thermal Expansion Behavior of Cold-Rolled Ti-35Nb-2Zr-0.3O Alloy

采用高真空非自耗电弧熔炼炉对Ti-35Nb-2Zr-0.3O (质量分数,%)合金进行熔炼。运用OM、XRD、SEM、TEM和静态热机械分析仪对Ti-35Nb-2Zr-0.3O合金进行表征,研究冷轧形变对合金显微组织及热膨胀行为的影响。结果表明:Ti-35Nb-2Zr-0.3O合金在冷轧过程中产生应力诱发马氏体α"(stress-induced martensiticα",SIMα")相,并形成平行于轧制方向的强<110>织构。等轴晶组织的Ti-35Nb-2Zr-0.3O合金表现出正常的热膨胀行为。形变后,合金的热膨胀行为出现异常现象,轧制方向表现为负膨胀,负膨胀程度随着形变量的增加而增大,截面方向表现为大于固溶态的正膨胀。30%形变合金的轧制方向在室温到250℃具有Invar效应,这一现象归因于SIMα"相变、晶格畸变和<110>织构的形成。冷轧态Ti-35Nb-2Zr-0.3O合金在室温到110℃的异常膨胀归因于SIMα"相到β相的晶格转变,而在高于110℃的异常膨胀行为归因于ω相和α相的析出。

Thermal expansion behavior is one of the intrinsic properties of most materials, which is very difficult to control their thermal expansion behavior. Metallic material with ultra-low coefficient of thermal expansion named Invar effect was first found in Fe-Ni alloys. Recently, a multifunctional titanium alloy termed Gum metal(the typical composition is Ti-36Nb-2Ta-3Zr-0.3O, mass fraction, %;three electronic parameters: electron per atom ratio e/a≈4.24, bond order Bo≈2.87 and d electron orbital energy level Md≈2.45 e V) has been developed, and the alloy exhibits Invar effect after severe cold working. It is well known that the Invar effect of Fe-Ni alloys is related to the magnetic transition. However, titanium and its alloys are paramagnetic, and thus this mechanism cannot be used to explain Invar effect of Gum metal.In addition, the Invar effect of Gum metal is related to a dislocation-free plastic deformation mechanism.So far, there is still some controversy about this mechanism. In this study, a new β-type Ti-Nb base alloy Ti-35 Nb-2 Zr-0.3 O(mass fraction, %) was developed whose three electronic parameters are different from those of the above mentioned Gum metal. The alloy was melted under high-purity argon atmosphere in an electric arc furnace, and the effects of cold rolling on microstructures and thermal expansion behaviors were characterized by OM, XRD, SEM, TEM and thermal mechanical analyzer(TMA). Results showed that the stress-induced martensitic α "(SIM α") phase transformation occurs after cold rolling, and the dominant <110> texture forms after severe plastic deformation. The equiaxed grains of Ti-35 Nb-2 Zr-0.3 O alloy exhibit ordinary positive thermal expansion behavior and the thermal expansion rate increases with the increase of temperature. After cold deformation, negative thermal expansion occurs along rolling direction, and normal thermal expansion higher than solution treated sample occurs along transverse direction. The abnormal thermal expansion extent of the alloy increases with the increase of deformation reduction. The 30% cold deformed alloy along rolling direction possesses Invar effect between room temperature to 250 ℃, which is possibly related to SIM α " phase transformation, lattice distortion and <110>texture formation. The anomalous thermal expansion of the cold deformed samples in a temperature range from 25 ℃ to 110 ℃ is attributed to the lattice transition of SIM α" to β phase, while above 110 ℃ is attributed to the precipitation of ω and α phases.