Date of Award

7-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Advisor

FOULGER, STEPHEN H

Committee Member

SKAAR , ERIC

Committee Member

LUO , JIAN

Abstract

This investigation has examined the mechanisms controlling the precipitation of various alpha (α) phase morphologies which form during the elevated temperature beta(β)beta(β)+alpha(α) phase transformations in TIMETAL LCB (Ti-6.8Mo-4.5Fe-1.5Al, in wt.%).
Alpha (α) phase precipitation was promoted by aging the TIMETAL LCB specimens in the α+β two phase region. The temperature range considered was between 700-745oC.The specimens were isothermally aged for successively increasing times, starting at 30 seconds until the equilibrium microstructure was achieved. Solution treated and aged TIMETAL LCB specimens were investigated using x-ray diffraction (XRD), optical (OM) and scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and quantitative image analysis techniques.
EBSD analysis indicated that, the grain boundary character distribution in the single phase, solution treated TIMETAL LCB can be controlled and modified by the proper choice of solution treatment schedule. Quantitative image analysis measurements also showed that, grain boundaries within the single phase microstructure tend to reduce their overall energy by decreasing the surface area of high energy boundaries during grain growth.
During isothermal aging, the β phase decomposes into three different α morphologies. These morphologies can be classified using the system developed by Dubẻ as a) grain boundary allotriomorphs (αGRB), b) widmenstŠtten side plates (αWSP) and c) widmenstŠtten intragranular plates (αWIG).
SEM examination of the aged microstructures showed that, grain boundary alpha allotriomorphs (αGRB) are the first transformation morphology to appear, independent of the aging temperature. Precipitation of αGRB does not occur simultaneously on the entire grain boundary area during the early stages of the transformation and αGRB distribution is confined to select grain boundaries. EBSD analysis indicated that, the specific choice of a particular grain boundary is based on the orientations of the αGRB and β grains. In each instance, αGRB hold a Burger's orientation relationship (OR) with respect to one of the adjacent β grains and lower the activation energy barrier required during precipitation. Further reduction in the activation energy barrier is possible if the orientation relationship between αGRB particles and the adjacent β matrix slightly deviates (typically 7-8o) from an exact Burger's OR.
Quantitative image analysis measurements showed that, the uniformity of αGRB precipitates increases with increasing aging time and decreasing undercooling. Untransformed grain boundary area at high undercoolings involves the low angle boundaries.
Once the orientation of αGRB is established, widmenstŠtten side plate morphology (αWSP) grows into the β matrix from αGRB with the same orientation. αWSP particles also maintain a Burger's OR with one of the adjacent β grains and grow into this grain. SEM examination suggests that, evolution αWSP morphology is controlled by the formation of micron sized facet along the grain boundaries with the increasing surface area of micron sized facets at lower undercoolings being associated with higher αWSP volume fraction.
The final ββ+α transformation involves the homogenous nucleation of widmenstŠtten intragranular plates (αWIG) within the matrix grains. The driving force for the formation of αWIG is the volume free energy change and increases with decreasing undercooling. As a result, the volume fraction of αWIG particles increases with decreasing aging temperature.
Jonhson-Mehl-Avrami (JMA) analysis implied that, overall phase transformations up to 745oC can be described by two stages. At temperatures below 745oC, the transformation includes rapid lengthening of grain boundary alpha particles which consume the available heterogeneous nucleation sites early during the reaction. Further progression of the ββ+α transformation takes place by the lengthening of the side plates into the β matrix. The first stage is terminated when αGRB+αSP reaches its equilibrium state. The second stage of the transformation is controlled solely by the two dimensional thickening of the intragranular alpha plates. At 745oC, ββ+α transformation takes place at a single stage. Grain boundary alpha is the only transformation product available at this temperature and the transformation is controlled by the thickening of grain boundary alpha precipitates.

Share

COinS