Patent ID: 6589371
Filing Date: 2003-07-08
Classification: C22F

Abstract:
A method for preparing an alpha or alpha-beta titanium alloy article having a desired substantially controlled homogeneous fine grain microstructure, comprising the steps of:(1) starting with a titanium alloy article having an initial grain size (do); (2) selecting a final grain size (dK) to achieve in the titanium alloy article a homogenous fine grain size (dK); (3) plotting a curve of the relationship between a recrystallized grain size (d) for the titanium alloy on the y-axis versus a strain temperature (T) for the alloy on the x-axis, between a range of 400Â° C. and a temperature of complete polymorphous transformation (TCPT), in accordance with the relationship d=f(T); (4) locating an area T* on the strain temperature axis to divide the temperature axis into two zones comprising a first zone 400Â° C. to T*, and a second zone T* to TCPT, where the T* is located by first calculating a corresponding recrystallization grain size (d*) on the y-axis, where d* is logarithmically related to the initial grain size do by the ratio log (do/d*); (5) further locating on the curve the final grain size (dK) on the y-axis and then a corresponding strain temperature TK on the x-axis, wherein the strain temperature TK is determined according to the final grain size; (6) determining heating and deforming stage and stages to process the article based on TK, with respect to the curve and the position of TK to the curve, where for TCPT>TK>T*, there is at least one heat and deforming stage to obtain the final grain size (dK), and where TKK) is obtained, the number of stages and the temperature of each stage being defined by adding the difference in the temperatures at the nearest stages to the TK temperature until the strain temperature exceeds or is getting equal to the T* temperature, each deformation within each stage being provided with a reduction of strain where e>[a log(dn+1)]TM/Tn, where dn is the grain size before the beginning of n-stage, Tn is the strain temperature at n-stage in K degrees, TM is the melting temperature of the stock material in K degrees, and â€œaâ€  is the coefficient which is a=1 at dn>10; a=1.5 at 1 &mgr;mn<10 &mgr;m; a=2 at 0.5 &mgr;mn<1 &mgr;m; a=3 at dn<0.5 &mgr;m; (7) then heating and deforming the titanium alloy article in accordance with the determined number of heat and deforming stages to achieve (dK), where each heat and deforming stage has at least one heating and deforming step and one cooling step, where the heat and deforming stage occurs for a sufficient period of time to reduce the grain size of the titanium alloy article, and where the deformation of the titanium alloy article is in a substantially controlled manner during each heat and deforming stage at a rate of strain to achieve the desired grain size of the heat and deforming stage, where the true strain during the deformation is greater than or equal to 0.6 for each of the at least two heat and deforming stage, and where the one cooling step is conducted to temperatures below the heat and deforming stage temperature at a cooling rate for substantially maintaining the reduced grain size obtained during the heat and deforming stage; and (8) repeating paragraph (7) until the final substantially controlled homogeneous grain size (dK) is obtained in the article having substantially homogeneous mechanical properties.