The influence of overlay coatings on high cycle fatigue of an advanced tantalum carbide strengthened eutectic composite
- PDF / 2,651,113 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 87 Downloads / 142 Views
strength, ductility, elastic modulus, ductile to brittle transition temperature, and oxidation resistance. These properties are in turn a function of the composition of the coating. It is important that these parameters be optimized so that, across a broad range of stresses and temperatures, the coating has equivalent or superior fatigue properties relative to the substrate. The other two considerations, viz. residual stresses and effects due to the diffusion zone, are not expected to be of major importance in the NiTaC/Ni-Cr-A1 systems. Coefficients of thermal expansion are nearly equivalent for the substrate and coating 8 and the diffusion zones which form between the coating and substrate are typically narrow compared to the total sample thickness. It should be noted, however, that second phase particles which can form in the diffusion zone could lead to early crack nucleation? In general, of course, the integrity of the coating in terms of its density and its adhesion to the surface will also be important in affecting the fatigue life of the system. In the current study we have looked at the effects various plasma sprayed coatings have on the high cycle fatigue response of NiTaC 3-116A2, a recently developed TaC fiber reinforced alloy. The effects of changes in coating composition, mechanical properties, and surface condition have been evaluated at room temperature, 600 and 900 ~ EXPERIMENTAL PROCEDURE Materials The chemical composition of NiTaC 3-116A2 is given in Table I; a typical microstructure for the alloy is shown in Fig. 1. Details of the processing history are given elsewhere, l~ Briefly, chill cast ingots were direc-
ISSN 0360-2133/81/0211-0329500.75 l0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, FEBRUARY 1981--329
Table I. Composition of NiTaC 3-116A2 (Wt Pet)
A1 6.5
V 3.9
[
Co 3.7
Cr 1.9
. _l_
31.75
tionally solidified at 6.4 m m / h and samples were centerless ground to the specifications of Fig. 2. The machined samples were then polished to 0.3/~ A1203; those to be coated were subsequently vapor blasted (vapor blasting is a gentle grit blast using 320 grit suspended in water) to slightly roughen the surface. The samples were plasma coated using a new procedure called the Low Pressure High Velocity (LPHV) process.* Coatings deposited by the L P H V process are
:1:0.051
f 2.54
.J\ -0.08
RAD
C 0.24
Ni Bal.
='1 "
57.15
1_12.70
Fig. 1--Microstructure of longitudinal section of NiTaC 3-116A2 showing TaC fibers in y-V'matrix.
Ta 8.0
t:0.025
JL r'-:l:O-s7i J -
i~176 T'
Re 6.3
-J_ 127
_1
-t- :[:5.67-"~_~ :1:0.05
3-
"~
Fig. 2--High cycle fatigue test specimen, dimensions in mm.
*Electro Plasma, Inc., Irvine, Calif. more dense and oxide free than coatings deposited by the conventional plasma spray process. These improvements are due to the facts that the L P H V process uses much higher plasma energies, and deposition takes place in a low pressure atmosphere of an inert gas rather than under atmospheric co
Data Loading...
