Simple metallographic detection of diffusional creep from marker line offsets
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Fig. 1--Surface of Magnox ZR55 (Mg + 0.55 pct Zr) tested at 450~ to 4.9 pct strain under an initial s t r e s s of 6.55 MPa. Magnification approximately 94 times. Three sets of marker lines were placed on the surface prior to creep. The s t r e s s axis is in the vertical direction.
:/
M
S M/
J (a)
(b)
y/M M
/ J
M (c)
M (d)
Fig. 2--Schematic grain-boundary marker-line configurations: (a) before creep for both DC and RC and after creep for RC (no s component), (b) after creep for RC (s component present), (c) after creep for DC (no s component) and (d) after creep for DC (s component present). GB, M and S denote grain boundary, marker line and sliding component, respectively. The stress axis is in the vertical direction,
c o n f i g u r a t i o n at 0 to 10 deg b o u n d a r i e s a f t e r c r e e p will a p p e a r as i l l u s t r a t e d in Fig. 2(c) b e c a u s e of the l o s s of m a t e r i a l at t h e s e b o u n d a r i e s . Note that the offsets a r e of opposite s e n s e . If an s c o m p o n e n t is p r e s e n t in this c a s e the offset m a g n i t u d e s will be u n equal, even when s is l a r g e enough so that the offsets have the s a m e s e n s e , as i l l u s t r a t e d in Fig. 2(d). The METALLURGICAL TRANSACTIONS A
Fig. 3--(a) Donor-boundary marker-line configuration. Magnification 205 times. Magnox ZR55 tested at 450~ to 6.57 pct strain under an initial stress of 4.57 MPa. (b) Boundary exhibiting no change. Magnification 205 times. Same sample as that shown in Fig. i. The boundaries under consideration here are the vertical boundaries lying closest to the centers of the photomicrographs. The stress axis is vertical in both cases.
(a) configurations illustrated in Figs. 2(c) and (d) are r e ferred to as donor-boundary marker-line configurations. Thus, in order to establish the presence of a DC component during creep it is only necessary to note the appearance of marker line configurations for grain boundaries in the 0 to i0 deg range of 0. A significant percentage of donor-boundary marker-line configurations indicates the presence of a DC component. Experimental Results. A magnesium alloy, Magnox ZR55 (Mg + 0.6 wt pct Zr) was tested under DC and RC conditions to about 5 pct strain (details of the experimental procedure and results are presented elsewhere"2). For the DC sample 95 pct of the 0 to I0 deg boundaries had donor-boundary marker-line configurations (Fig. 3(a)) while, for the RC sample 85 pct exhibited no change (Fig. 3(b)) and the remaining 15 pct had donor-boundary marker-line configurations. This 15 pct in the RC case is an apparent effect produced by v offsets, ~ i.e. it can be shown that diffusion of material away from these boundaries does not actually occur. A significant v component, such as would give r i s e to this effect, can readily be detected because of the need to refocus the microscope on crossing the stepped grain boundaries.
METALLURGICAL TRANSACTIONS A
(b)
Discussion a n d C o n c l u s i o n s . T h e m a r k e d d i f f e r e n c e in behavi
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