The preferred material for {HEAD ~ layers} 4 and 4′ are Ti alloys or {TAIL ~ Ni alloys} 
The preferred material for { HEAD ~ layers } 4 and 4′ are Ti alloys or { TAIL ~ Ni alloys }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL