Research On Mechanical Behavior Of TiAl Alloy Based On Machined Surface State

The processed surface quality is of great significance to the actual production and application ofγ-TiAl alloy.In this paper,the surface quality of the sample is processed by mechanical processing,and the bending strength,tensile fatigue life,and bending fatigue life of theγ-TiAl alloy at room temperature and high temperature are systematically studied.The mechanism of crack initiation,propagation and final fracture ofγ-TiAl alloy under different fracture forms was discussed.The change of bending strength of the fully layerγ-TiAl alloy with surface quality and temperature and the fracture mechanism of this alloy were studied.At room temperature,the flexural strength of the fully layerγ-TiAl alloy decreases with the increase of the surface roughness value.The high temperature makes its bending strength change tendency no longer obvious.As the temperature increases,the bending strength of the fully layerγ-TiAl alloy increases,and it has a good bending resistance at about 700℃.The static bending cracks of the fully layerγ-TiAl alloy mostly originate from the layer grain boundaries or among layers near the surface,and preferentially propagate between the layers,eventually brittle fracture occurs.The effects of machined surface roughness on the tensile fatigue properties of the fully layerγ-TiAl alloy and the tensile fatigue fracture mechanism were studied.As the roughness value decreases,the conditional fatigue limit of the alloy increases.For surface roughness values below 0.5μm,each time the roughness value decreases by about 0.15μm,the tensile fatigue of theγ-TiAl alloy is reduced 2%to 5%.The alloy has a tensile fatigue cycle of 10~7times from static breaking,the maximum stress that it can withstand is above 0.65σ_b,and 10~6times can be used as a criterion for judging its conditional fatigue limit.The tensile fatigue crack source of this fully layerγ-TiAl alloy is mostly generated at the edges.After the crack is generated,it spreads along the interlayer or grain boundary in a small range,eventually causing the sample to break instantly and form a full view of divergence to the fracture.The river pattern is a typical cleavage fracture.The influence of machined surface roughness on the bending fatigue properties of the fully layerγ-TiAl alloy and its bending fatigue fracture mechanism were studied.Under the maximum bending fatigue cycle loading stress of 500 MPa,the number of bending fatigue cycles of the alloy decreases as the roughness value increases.The bending fatigue performance of the fully layerγ-TiAl alloy is lower than that of tensile fatigue.The fully layerγ-TiAl alloy bending fatigue crack source is generated more on both sides and in the middle of the specimens.The crack source on the tensile stress side plays a dominant role in the instantaneous fracture of the specimen.The study analyzed the formation mechanism of machining defects in the fully layerγ-TiAl alloy during milling and its effect on the performance characterization of theγ-TiAl alloy.Common machining defects in this alloy during milling include tool path texture,surface scratches,material pullout,microcracks,grooves,pits and chipping.Among them,the tool path texture and the chipping has the biggest influence on the properties of the alloy,especially on the characterization of the fatigue properties.