Microstructure evolution during alpha-beta heat treatment of Ti-6Al-4V
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8/30/04
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Microstructure Evolution during Alpha-Beta Heat Treatment of Ti-6Al-4V S.L. SEMIATIN, S.L. KNISLEY, P.N. FAGIN, F. ZHANG, and D.R. BARKER A framework for the prediction and control of microstructure evolution during heat treatment of wrought alpha/beta titanium alloys in the two-phase field was established via carefully controlled induction heating trials on Ti-6Al-4V and accompanying mathematical modeling based on diffusioncontrolled growth. Induction heat treatment consisted of heating to and soaking at a peak temperature Tp 955 °C, controlled cooling at a fixed rate of 11 °C/min, 42 °C/min, or 194 °C/min to a variety of temperatures, and final water quenching. Post-heat-treatment metallography and quantitative image analysis were used to determine the volume fraction of primary (globular) alpha and the nucleation sites/growth behavior of the secondary (platelet) alpha formed during cooling. The growth of the primary alpha during cooling was modeled using an exact solution of the diffusion equation which incorporated diffusion coefficients with a thermodynamic correction for the specific composition of the program material and which took into account the large supersaturations that developed during the heat-treatment process. Agreement between measurements and model predictions was excellent. The model was also used to establish a criterion for describing the initiation and growth of secondary alpha as a function of supersaturation, diffusivity, and cooling rate. The efficacy of the modeling approach was validated by additional heat treatment trials using a peak temperature of 982 °C.
I. INTRODUCTION
THE wrought processing of high-integrity components from conventional alpha/beta titanium alloys often involves a series of hot working steps and final heat treatment.[1] The objective of primary hot working is to produce a uniform and fine two-phase microstructure of globular hcp alpha in a transformed matrix of alpha and bcc beta phase. This is accomplished via initial hot working and heat treatment in the high-temperature, single-phase beta field followed by breakdown of the transformed, two-phase colony microstructure in the lower-temperature, two-phase (alpha beta) field. Such wrought structures are then readily shaped via forging, extrusion, rolling, etc., usually in the alpha beta field. The final heat treatment of alpha/beta titanium components is done at a temperature dependent on service requirements.[2] To optimize fracture toughness, heat treatment usually comprises beta annealing followed by cooling to produce a colony- or Widmanstätten (basketweave)-alpha microstructure. On the other hand, higher strength and ductility are developed if heat treatment is done in the alpha beta field to produce a final microstructure of globular alpha in a matrix of transformed beta. In these latter instances, the specific properties that are obtained are a function of the volume fraction and size of the primary alpha grains/particles and
S.L. SEMIATIN, Senior Scientist, Materials P
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