Evolution Of The R-Phase Transformation Temperature Of Ti 50 Ni 50-x Fe x Shape Memory Alloys With Iron Concentration
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Mat. Res. Soc. Symp. Proc. Vol. 360 0 1995 Materials Research Society
while heating to room temperature. Later, the samples were cooled down to 2 K with the applied field on to get a complete cycle. RESULTS AND DISCUSSION Specific heat fluctuations are observed in samples SI and S3. In sample Sl, the fluctuation occurs around 245 K and the peak shifts about 2 K during the cooling and heating process. Similar results are obtained with S3, except that the fluctuation occurs around 280 K No specific heat fluctuation was observed in sample S2. The room temperature and 13 K M6ssbauer spectra (MS) of the studied alloys are shown in Figs 1, 2 and 3. As can be seen in those figures the spectra evolve from what seems to be a single line to a quadrupole doublet. No magnetic splitting is observed at any temperature, indicating that the spectral features are due solely to electrostatic interactions. However, during the fitting process with a constrained least squares program it was realized that in samples S I and S2, good fits could only be attained using two asymmetrical doublets, even at room temperature. z0
z
,,
o-
0.
0
0: (0
Co
>
~
W
-4
.
..". *....
".
" ... :',".
28 280KY
2 -2 0 VELOCITY (mm/as)
-4
4
-2 0 2 VELOCITY (mm/s)
4
z
0 Co
0 Co W
13K
w
-4
2 -2 0 VELOCITY (mmls)
4
Figure 1. M6ssbauer spectra of S1. It is worthy to recall that electrostatic interactions manifest themselves in a MS through measurable quantities, known as isomer shift 8 and quadrupole splitting AQ, which are related, respectively, with the electron density wave fimction at the nuclear site (s and P1/2 wave functions) and with the symmetry of the electric field around the nucleus: the higher the symmetry, the lower AQ values. In the case of iron, the explicit expressions for these quantities are.4.
5
•(
SJO• -[I()a -
)-
490
1
and AQ = l
QV,
(2)
where AR is the change of the nuclear radius R between the excited and the ground state, P(O))a,s are the electronic wave functions at the absorber and source nuclei, respectively, Q is the quadrupole moment of the nucleus and
V~x - Vyy IS-1 v
(3)
is the asymmetry parameter, where Vii are the components of the electric field gradient along each of its principal axes, defined in such a way as to have 0 •
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