【正文】 the grain boundaries and alpha phases were estimated about 30176。. The reasons for the easy precipitation of alpha phases at grain boundaries can be interpreted as low nucleation energy and the segregation of alloying element at grain boundaries due to the inadequate solution treatment. The large quantity of precipitates at grain boundaries was responsible for the brittleness of the alloy. Figs. 3 (a) and (b) present TEM image of the alloy aged at 450 ℃for 6 h and 24 h. With increasing ageing time, the alpha phases bee coarser. The volume fraction of the alpha phase increases simultaneously. Fig. 3 (c) present the parison of TEM image of the alloy with duplex ageing treatment. The alpha phases became coarser with a longer distance between alpha precipitates after duplex aged. 3. 4 Mechanical properties of alloy after heat treatment Fig. 4 (a) demonstrates the change of mechanical properties of the alloy aged at different temperatures for 8 h. With increasing ageing temperature , the tensile strength and yield strength decrease and elongation increases. The direct reasons for the change of mechanical properties of the alloy is the size, quantity and distribution of alpha phases in the matrix. With raising ageing temperature, the alpha phases bee coarse that leads to the more easiness for dislocations to cross the precipitation during the test process so that the strengthening action of the alpha phase lessens which will cause the lower strength and higher elongation of the alloy. When the ageing temperature is 450 ℃, σb equals to 1. 406 GPa. While the ageing temperature is 650 ℃,σb equals to 905 MPa. Compared with the strength, the elongation of the alloy has different change trends. The elongation is from 4. 5 % (450 ℃) to 14. 4 % (650 ℃) .Fig. 4 (b) demons rates the change of mechanical properties of the alloy aged at 450 ℃ for different times. With increasing ageing time, the tensile strength and yield strength increase a little with the decrease of elongation. With prolonging the ageing time, the distance between the precipitations bee nearly that leads to the more difficulty for dislocations to cross the precipitations during the test process, which cause the higher strength and lower elongation. Fig. 5 shows the fracture morphology of the alloy aged at 450 ℃and 650 ℃for 8 h. It has shown that the fracture is inter granular characterized by dimples. Although the fractures are inter granular , the relatively smaller grain sizes maybe the better explanation of the high elongation. When the alloy is duplex aged, the strength decreases with increase of elongation. σb has been cut down by 386 MPa. The elongation has been improved by 3. 5 %. The longer distance between the precipitations after duplex aging is responsible for the lower strength and higher elongation.3. 5 Effect of solidification cooling rate on mechanical properties of alloy after heat treatmentFig. 6 presents s the mechanical properties of the alloy which locate in the middle and thick section after heat treatment. The samples were solute treated at 800 ℃for 20 min and then ageing at different temperatures for 8 h. The tensile strength of the alloy from both the middle and thick section decreases with the increase of elongation when the ageing temperature increases. As a whole, the tensile strength and elongation of the alloy from middle section are higher than that from the thick section. But aged at or above 510 ℃for 8 h , the tensile strength of the alloy from middle section is lower than that of the thick section. This unexpected behavior can be explained by means of model which incorporate the contribution of the grain boundaries to and the grain interior to the tensile strength [8].The contribution of the grain boundaries to σs can be expressed by the well known Hall Petch relationship σs =σi + kL d 1/ 2 (1)Where σi is the friction stress opposing to dislocation motion , kL is a constant , and d is the grain diameter. The contribution of the grain interior to precipitates (τ) maybe expressed by the Orowan equation modified by Ashby[11] .τ= Gb/ 2π( D l) ln l/ r0 (2)Where G is the shear modulus , b is the burgers vector and r0 = 4 b , D is the distance between precipitates , l is the thickness of the precipitates. When considering the tensile yield strength it is possible to assume that for a polycrystalline material τ= 1/ bined contribution of grain boundaries and the precipitates in the grain interior is σs =σi + kL b 1/ 2 + Gb/π( D l) ln l/ r0 (3)Increasing the grain size would decrease the yield strength, but would also cause larger precipitate density in the grain interior , which result in a smaller distance between precipitates. Compared with the small grain size , the contribution of precipitates in the grain interior to the tensile strength is the dominating factor for the large grain size. When aged at or above 510 ℃for 8 h , the tensile strength of large grain size has exceeded the small one due to the relatively stronger strengthening action of the precipitates in the grain interior.4 　CONCLUSIONS1) The microstructure of the alloy after solidification is the equiaxed beta grain with a few of gas and shrinking holes in grain interior and grain boundaries as well. With increasing solidification cooling rate, the size of grain bees smaller, the tensile strength and yield strength of the alloy increase. At the same time , the elongation of the alloy is improved.2) With increasing ageing temperature and also prolonging of ageing time, the acicular alpha phase bees coarse. The volume fraction of the alpha phase increases simultaneously. The alpha phase bees coarse after duplex aged.3) With increasing ageing temperature, the tensile strength and yield strength decrease and the elongation increases. With increasing ageing time the tensile strength and yield strength increase a little and the elongation decreases. After the alloy is duplex aged, the strength decrease