The hydrogen-induced microstructure evolution and phase transformations in Ti-6Al-4V alloy during heating and cooling were studied.The specimens were heated to 1273 K and subsequently cooled to room temperature.The hydrogen content is up to 0.8%（mass fraction）.The hydrogen-induced dynamic phase transformations and the corresponding mechanisms were analyzed.When the hydrogen content increases,the β transus temperature significantly decreases and the magnitude decreases,and the volume fraction of β phase increases.During heating,the phase transformations in hydrogenated Ti-6Al-4V alloys can be divided into three stages,and the phase transformation order is δ→α＋H2↑？δ＋α′→βH？α′→αH＋βH？αH→α＋H2↑？α→β？βH→β＋H2↑.In addition,the relationship among hydrogenation and Ms and Mf of α′ martensite were determined.
The influence of Mo content on the microstructure and mechanical properties of the Ti?45Al?5Nb?xMo?0.3Y(x=0.6,0.8,1.0,1.2)alloys was studied using small ingots produced by non-consumable electrode argon arc melting.The results show that smallquantities ofβphase are distributed alongγ/α2lamellar colony boundaries as discontinuous network in the TiAl alloys owing to thesegregation of Mo element.Theγphase forms in the interdentritic microsegregation area when the Mo addition exceeds0.8%.Theβandγphases can be eliminated effectively by subsequent homogenization heat treatment at the temperature above Tα.The evolutionof the strength,microhardness and ductility at different Mo contents under as-cast and as-homogenization treated conditions wasanalyzed,indicating that excessive Mo addition is prone to cause the microsegregation,thus decreasing the strength andmicrohardness obviously,which can be improved effectively by subsequent homogenization heat treatment.
