Kinetic Competition During Duplex Partitionless Solidification in Ni-V Alloys
- PDF / 1,638,640 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 1 Downloads / 216 Views
(tud) .iaouuzd amUI ocl -
0n
0
On
o'
00
o0
t-
00
C
M
I
13
R A
9 ,v
C-A ts CM
a.
T"'-•
aoa
° . .it
op.
*m
'o_
'4
40
02
a
r.
00
o ' f-
0
r-
', %6
0m \5
'A 'A
0 in C4, e4
J
E.
.+
CY 0C tU X
(3o)Bwnpwedujm
58
z
the first heating cycle at approximately 700'C, and no deviations from the baseline on the second cycle. XRD following the thermal cycling showed a structure composed of a and a with lattice constants corresponding to equilibrium values. A typical cross-sectional microstructure for a Ni-49 at.% V splat quenched foil is shown in figure 3. Reflection mode XRD analysis indicated that one side of the foil was predominantly bcc (left side in figure 3), as determined qualitatively by x-ray peak intensity, and other side of the foil was predominantly fcc (right side in figure 3). The regions near the edges of the cross section had very little response to etching; this is an observation consistent with a microstructure of uniform composition. In the final area to freeze (near the center of the foil), a dendritic morphology developed for both a and 13; the etch response indicates that this region solidified with some solute segregation. This region varied in thickness from zero to about 20 ltm along the length of the foil. TEM analysis was conducted on the SQ foils in the plan view, parallel to the plane of the splatquencher anvils. Splat-quenched foils of compositions 47, 49, and 51.7 at% V exhibited a duplex microstructure primarily composed of morphologically similar a and 1 grains about 0.1-1 ýtm (figure 4) in diameter; selected area diffraction (SAD) analysis has confirmed the presence of contiguous a and 13phases in an equiaxed, duplex structure. High angle boundaries were observed between the grains, which suggests that the grains impinged on one another at the end of solidification. The absence of any partially transformed regions coupled with the observation of high angle grain boundaries indicates that the duplex structure formed from solidification rather than a massive transformation. The massive transformation [ -> a is possible since the nucleation temperatures of the alloys of interest are likely to be below T1c/1; however, the rapid cooling inherent in the SQ process has apparently kinetically bypassed this reaction. A comparison of the SEM and TEM results indicates that multiple microstructural morphologies exist in the SQ foils; this paper will consider only the equiaxed region observed in TEM. This region is present within the featureless region observed in the cross-sectional SEM view. The microstructural variation suggests that the solidification structure is quite sensitive to local quenching conditions and supports the condition that special kinetic factors are needed to produce the duplex microstructure. Kinetics Analysis An analysis based on the nucleation and growth kinetics of the ax and 1 phases has been developed to define a range of undercooling levels and compositions over which the duplex structure may develop. An independent nucleation mechanis
Data Loading...
