Microstructure And Mechanical Property Of High Nb-TiAl Alloy Fabricated By Laser Additive Manufacturing

TiAl alloys feature low density and good high-temperature properties,becoming the most promising lightweight high-temperature structural materials.As a typical new-generation TiAl alloy,high Nb-TiAl alloy owns better hightemperature oxidation resistance and strength than conventional TiAl alloy.However,the inherent room-temperature brittleness and difficulty in forming complex components by traditional hot processing for TiAl alloy restrict its practical applications.Additive manufacturing(AM),realizing the stereoscopic forming of components by stacking layer-by-layer,becomes the research hotspot of complex structural parts formed from TiAl alloys.Meanwhile,the micro molten pool enables a higher cooling rate for the AM process,which greatly refines the microstructure of TiAl alloys and thus improves their mechanical properties.Typically,direct laser deposition(DLD),with a large forming chamber,endows unique forming advantages in large structural components or subparts based on large components.Moreover,the DLD process only requires a small amount of alloy powder to form samples,becoming an effective strategy for the development of novel TiAl alloys.Microstructural degradation always occurs in TiAl alloys during long-term exposure at high temperatures,deteriorating their mechanical properties.Therefore,developing a new type of TiAl alloy that features a stable high-temperature microstructure via DLD-AM is urgent.The deep mechanism of microstructural evolution and mechanical properties along the thermal cycling direction is analyzed in the TiAl alloy.Firstly,the Ti-48Al-8Nb alloy was innovatively prepared via preprinting about 10 layers of TC4 alloy and then printing TiAl alloy,addressing the cracking problem of TiAl alloy during the DLD.The influence of power change on forming,microstructure,and mechanical properties was further studied.The analysis shows that the best laser power is 500 W to prepare TiAl alloy.A lower laser power exhibits insufficient energy input and forms depression defects;conversely,a high laser power melts much pre-alloy powder and generates bulking and liquid phase collapse.The gradually increased power induces an increased heat flow gradient,generating the microstructure transforming from equiaxed lamellar to columnar lamellar along the building direction,and the γ-TiAl phase grows along {111}γcrystal plane.At room temperature,the tensile strength of high Nb-TiAl alloy additively manufactured at 500 W is 880 MPa,which is 1.71 times higher than that of the same as-cast alloy and much higher than that of other reported TiAl alloy from AM,while,exhibiting a good elongation of 0.7%.Subsequently,combining the research of the deformation mechanism during the tensile process in DLD-prepared TiAl alloy with density functional theory(DFT)calculations to alloying design and attempts to significantly improve the mechanical properties of TiAl alloy.The results deliver that numerous twins form in the DLDprinted TiAl alloy performed tensile deformation,meanwhile,the y-twins segmentγ-TiAl lamellae and then form high-density nano-twins.Furthermore,the calculated results of strengthening γ-TiAl(111)//α2-Ti3Al(0001)interface and twins interface by 11 common transition metals exhibit that,the Mn element features the optimal interface strengthening effect.A two-step heat-treatment strategy of 650℃ for 6 h and 850℃ for 2 h was carried out to anneal the additively manufactured TiAl alloy that includes 0.5%Mn element,ultimately,the TiAl-based alloy receives a high strength of 913 MPa and elongation of 1.15%at room temperature.Furthermore,increasing the Al element content in Ti-48Al-7.5Nb to 55%attempts to obtain a single-phase y-based TiAl alloy.The basic law of the effect of the interstitial elements(B,N,C)on γ-TiAl and α2-Ti3Al phases as well as(111)γ//(0001)α2 interfacial properties were verified by combining experiments with theory.The results demonstrate that the N element features the strongest interaction with the matrix and easily enters into γ-TiAl and α2-Ti3Al phases than that of B and C elements,then occupying the octahedral interstice.The solid solubility of interstitial elements in TiAl alloy follows the order of B<C<N.The N element enhances the separation energy of(111)γ//(0001)α2 interface and strengthens interfaces;but the C element hardly affects the interfacial separation energy;and the B element weakens interface strength to a certain extent.Meanwhile,the N element exhibits a better toughness effect in γ-TiAl and α2-Ti3Al phases than that of B and C elements.Moreover,the Nb element in TiAl alloy can change the composition of the Al-containing octahedral interstice,resulting in enhanced solid solubility of interstitial elements and strengthening its binding to the matrix.Finally,two alloying elements were introduced into Ti-55Al-7.5Nb by taking Si3N4 as elemental sources,and a new type of single-phase γ-based Ti-55Al7.5Nb/Ti2AlN-Ti5Si3 metal-matrix composition(TiAl MMC)was successfully prepared via the AM-DLD process.The related microstructure,texture evolution,and mechanical properties were studied.The TiAl MMC grain size increases the parabola with the increase of laser power.Compared with DLD-printed Ti-55Al7.5Nb alloy,the TiAl MMC features an equiaxed crystal structure and good hightemperature microstructural stability.The formed Ti5Si3 and Ti2AlN precipitated phases can inhibit the grain crystal anisotropy in the TiAl MMC,and exhibit a coherent interface with the TiAl matrix.Moreover,a strong electronic interaction generates between the TiAl matrix and Ti5Si3 as well as Ti2AlN precipitated phases,inducing a stronger interface cohesion and effectively restraining intergranular crack.At last,the TiAl MMC delivers a high-temperature(900℃)tensile strength of 401±23 MPa.