Fabrication, Microstructure Control And Mechanical Behavior Of Microlaminated TiB₂–NiAl Composite Sheets

In order to satisfy the requirements of gas turbine engines blades and guidevanes for high-strength low-density alloy sheets, it is of theoretical and practicalsignificance for development and manufacture of NiAl-based alloy sheets.Microlaminated TiB₂–NiAl composite sheets have been successfully fabricated byroll bonding and reaction annealing of Ni sheets and TiB₂/Al composite sheets. Onthe one hand, this technique avoids the direct deformation of the brittle NiAlcompared with the conventional rolling method, along with the decrease infabrication costs by using simplified techniques. On the other hand, the obstacle offorming dense NiAl sheets by conventional solid-solid methods can be overcome.The strenghening effect can be effectively improved by a densification processconsisting of a solid-liquid reaction.Preparation processes of microlaminated TiB₂–NiAl composite sheets wereinvestigated. Phase composition, transformation of texture and reaction mechanismof the reaction products in the process of roll bonding and reaction annealing weresystemically studied using SEM, XRD, EBSD and TEM. The mechanical propertiesof microlaminated TiB₂–NiAl composite sheets at room temperature and hightemperature were evaluated and effect of laminate structure and TiB₂onmicrostructures and properties of microlaminated TiB₂–NiAl composite sheets wereinvestigated. Moreover, the oxidation resistance of microlaminated NiAl-TiB₂sheetwas investigated.Roll bonding of multi-laminated Ni–（TiB₂/Al） sheet show the gooddeformation compatibility between Ni and TiB₂/Al sheets with no subsequentreactions. The presence of wide stacking faults is observed in the Al （Ni） layer nearthe interface in multi-laminated Ni–（TiB₂/Al） sheet after the roll bonding process.The first reactive synthesis method is called “two-steps solid-solid annealingtreatment”. NiAl₃and Ni₂Al₃phase was detected in the multi-laminated Ni–（TiB₂/Al）composite sheet during annealing at650oC. A modified effective heat of formationmodel was applied and it predicted correctly the appearance of NiAl₃as the firstphase as well as subsequent formation of Ni₂Al₃between the Ni and NiAl₃layers.The initial NiAl₃grains demonstrated parallel columnar structure because ofthe high concentration gradient at the Ni/Al interface. Most Ni₂Al₃grains showedequiaxed morphology, due to the low concentration gradients of Al and Ni at theNi/NiAl₃interface. Growth velocity of NiAl₃towards TiB₂/Al layer is much fasterthan that towards Ni layer, due to the higher nucleation rate at the Al/NiAl₃interface.The growth of NiAl₃layer is consistent with the parabolic growth law, while annealing at650℃. When the laminate is subsequently annealed at950¹000℃after annealing at650℃for50h, TiB₂still remains stable. Finally, multi-layeredTiB₂-NiAl composite sheets are obtained. The growth of NiAl layer is attributed tothe reaction diffusion process and in consistent with the parabolic growth law, whileannealing at950℃.Because there are too much holes in the TiB₂-NiAl composite sheets after“two-steps solid-solid annealing treatment”, a new reactive synthesis method named“solid-liquid annealing treatment” was investigated. The dense microlaminated（0.7vol.%，1.3vol.%，2vol.%，3.3vol.%）TiB₂–NiAl composite sheet with alternatingTiB₂-rich and NiAl layers was successfully produced by reaction diffusion frommulti-laminated roll bonded Ni–（TiB₂/Al） sheet at1200℃/3h/50MPa. NiAl phasehas strong {111}<112> and {111}<110> texture components in the microlaminatedTiB₂–NiAl composite sheet. The texture may be considered to be a transformationtexture inherited from initial rolling texture of Ni in the multi-laminatedNi–（TiB₂/Al） sheets via reaction diffusion. The texture formation of the NiAl phasecan be qualitatively explained on the basis of the orientation relationships onclose-packed planes （N–W and K–S） between Ni phase and NiAl phase in view ofcoherency with texture of Ni. The content of Al in the microlaminated TiB₂–NiAlcomposite sheet is between51at.%Al and52at.%Al. The nanohardness is between9.5GPa and9.6GPa, and the elastic modulus is between200GPa and210GPa. Theparticle content does not influence the hardness and elastic modulus of themicrolaminated TiB₂–NiAl composite sheet.Testing for fracture toughness shows that with addition of TiB₂, the fracturetoughness increases. Furthermore, the value of fracture toughness depends onloading direction, fracture toughness of micro-laminated2vol.%TiB₂-NiAlcomposite sheets with loading parallel to normal direction （ND） is7.5±0.5MPa·m^1/2,while the value with loading parallel to rolling direction （RD） is6.9±0.3MPa·m^1/2.It was found that the crack in the micro-laminated2vol.%TiB₂-NiAl compositesheets propagates along the particular crystallographic planes close to that of{110}B2and {100}B2. TiB₂-rich fine NiAl layer hinders propagation of cracks andinduces crack deflection. These factors contribute to increase in fracture toughnessof micro-laminated2vol.%TiB₂-NiAl composite sheets with loading parallel to ND.High temperature tensile testing shows that with raising temperature, strengthof microlaminated TiB₂-NiAl composite sheets increases firstly and then decreases,coupled with an increase in elongation. The brittle-to-ductile transition temperature（BDTT） for this material lies somewhere between700℃and750℃. Yield strengthat750℃reaches the highest value of435MPa and elongation reaches4.7%. Theimprovements in strength and elongation are attributed to the unique laminated structure: bimodal distribution of grain size and good interface bonding betweenboth layers.TiB₂-rich layers acts as the “reinforcement phase” in the microlaminatedcomposite, and the monolithic NiAl layer results in cracks blunting which efficientlyreleases the stress concentration. The relationship between microstructure andstrength of the microlaminated TiB₂-NiAl composite was analyzed.