As　heat　treated　in　the　alpha+beta　phase　region,　the　lower　toughness　value　occurs　in　the　range　region　of　650　degrees　C　similar　to　850　degrees　C,　while　the　highest　value　is　presented　adopting　heat-treated　at　915　degrees　C.　With　annealing　temperature　decreasing,　the　volume　fraction　and　the　size　of　primary　alpha(p)　phase　increase,　and　the　bi-modal　microstructure　is　alternated　by　equiaxed　microstructure,　meanwhile　the　aluminum　element　will　be　enriched　in　the　primary　alpha　phase.　Boundaries　of　primary　alpha　grains　are　observed　to　be　preferential　sites　for　microcracks　nucleation　and　provide　relatively　easy　path　for　crack　propagation.　The　fractograph　of　impact　toughness　sample　below　the　transus　temperature　is　dimple　fracture　feature.