Scanning Electron Microscopy and X-Ray Microanalysis

475 27

through the structure until an anomalous region was revealed
(. Fig. 27.8). As shown with SEM/BSE imaging and elemen-
tal mapping in. Fig. 27.9, in this anomalous region the alu-
minum and iron had reacted to form two distinct Al-Fe
zones (Newbury and Greenwald 1980 ; Newbury 1982 ). Fixed

beam quantitative X-ray microanalysis with NIST DTSA II

and pure element standards (analyses performed during a

revisiting of the 1980 specimens) produced the results shown

in. Fig. 27.10, where zone 1 is found to correspond closely

to the intermetallic compound FeAl 3 , while zone 2 corre-

sponds to Fe 2 Al 5. The presence of these intermetallic com-

pounds is significant because of their resistivity. FeAl 3 and

Fe 2 Al 5 have electrical resistivities of approximately 1 μΩ–m,

similar to that of the alloy nichrome (1.1 μΩ–m), which is

used for resistive heating elements and which is a factor of 38

higher than pure Al and 10.3 higher than pure Fe. The forma-

tion of these intermetallic compounds at the screw-wire con-

tact was initiated when electrical arcing occurred because the

connection became mechanically loose due to creep of the Al

wire and the poor compliance (springiness) of the wire–

screw clamp. Once the local formation of the intermetallic

compounds had been initiated by arcing followed by local

welding of the Al wire and the steel screw, the increased resis-

tivity caused localized resistive heating that stimulated the

interdiffusion of Al and Fe, leading to the further intermetal-

lic compound growth in a runaway positive feedback.

Eventually this intermetallic compound zone expanded to

dimensions of several hundred micrometers, as seen in

. Fig. 27.7, creating a resistive heating element that caused

Al wire

Steel

screw

Mounting

epoxy

Galvanized

coating

Anomalous

region

. Fig. 27.8 SEM-BSE image of an anomalous zone of contact between
the Al wire and the Fe screw

BSE 500 μm

Fe

2

1

Al

Zn

. Fig. 27.9 SEM-BSE image and elemental maps for Al, Fe, and Zn of the anomalous contact zone

27.2 · Case Study: Aluminum Wire Failures in Residential Wiring