Study of the microstructure of Nb3Al based alloys.

Nb3Al base alloys were produced in ingot or ribbon form using cold hearth non-consumable tungsten arc melting and chill block casting by melt overflow on a Mo wheel. The following alloys were studied: Nb-25Al, Nb-18Al, Nb-18Al-xMo (x=20,30,40) and Nb-18Al-9Cr-5Ti (at%). Low levels of interstitials (< 200 wppm O2, < 10 wppm H2) were achieved in all alloys. The effects of alloy chemistry and method of solidification processing on phase selection, solidification microstructures and solid state phase transformations were studied. Room temperature microhardness and hardness of the alloys were evaluated. The A2 structure (disordered beta Nbss) transformed to the B2 structure (ordered beta Nbss) by solid state transformation in the Nb-18Al and Nb-18Al-xMo (x=20,30,40) alloys. The experimental results showed that Al promoted A2&rarr;B2 ordering in Nb rich binary Nb-Al alloys and confirmed the predictions of thermodynamic modelling of A2&rarr;B2 ordering in the Nb-Al system. Mo addition inhibited the A2&rarr;B2 ordering whereas Cr+Ti additions favoured ordering. Formation of an o-phase was observed for the first time in the binary Nb-Al system after prolonged annealing of the Nb-18Al ribbons (973 K / 500 h). It is suggested that the o-phase forms as a transition phase in the transformation B2&rarr;o&rarr;Al5. While o-phase formation has been reported in alloys of the Nb-Al-Ti system, it is shown that Mo addition does not favour the o-phase in Nb-18Al-xMo alloys (x=20,30,40). Formation of o diffuse in the beta phase supports a previous hypothesis that addition of Al to transition metal elements such as Nb,Mo,V can enhance formation of the o phase in the beta phase. Mo addition in the Nb-Al solid solution increases the electron density of A2 or B2 phases and consequently the deviation | Delta | of the o diffuse from crystalline o. A new ternary o type phase, o*, was formed as a transition phase in the B2?Al5 transformation in heat treated Nb-18Al-xMo (x=20,30,40) alloys (973 K / 500 h). The orientation relationship of o* is [111]B2//[0001]o* and (110)B2//(0110)o* and the lattice parameters are ao* = 61/2aB2 and = c o* = 31/2aB2. Rapid solidification refined the alloy microstructures by two orders of magnitude. The room temperature solid solubility of Al in Nb was extended from 6 at% to 25 at% Al. In Nb-18Al-9Cr-5Ti up to 5at%Ti+8.4at%Cr were retained in solid solution. Suppression of Nb3Al and Nb2Al formation was achieved in the ribbons of Nb-25Al. Nb3Al formation was suppressed in the ribbons of Nb-18Al, Nb-18Al- xMo (x=20,30,40) and Nb-18Al-9Cr-5Ti. The A2-*B2 ordering was suppressed in the ribbons of all Nb-18Al-xMo alloys and in the as cast ingots of Nb-18Al-40Mo. Aluminium additions in Nbss increased its microhardness by 16.6 kgmm-2/at%Al and Mo addition hardened the Nbss (16.1at%Al) by 4.2 kgmm-2/at%Mo. Regarding the oxidation behaviour, the Nb-18Al-20Mo and Nb-18Al-9Cr-5Ti alloys exhibited lower weight gain rates as compared to Nb3Al. No protective scale was formed on any of the alloys studied in this work