Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800–1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)5Si3 transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)3Si5Al2 phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)2Al layer formed in the coating between the (Nb,X)5Si3 layer and the substrate, but when aluminizing was performed at 900–1000 °C, the new layer formed was (Nb,Ti)Al3. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000–1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)3Si5Al2, (Nb,X)Si2 and (Nb,Ti)Al3 was obtained by co-depositing Si and Al at 1050 °C.