Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at%and 0.20 at%Ce)exposed at 800℃were compared.The 0.01 Ce and 0.05 Ce alloys were c...Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at%and 0.20 at%Ce)exposed at 800℃were compared.The 0.01 Ce and 0.05 Ce alloys were consisted ofγ/γ’coherent micro structure,while theκ-Co_(3)W compound precipitated at the grain boundary of the 0.1 OCe and 0.20 Ce alloys in addition to theγ/γ’microstructure.The oxidation kinetics curves of the Cedoped alloys exhibited a parabolic time dependence on the weight gain.With an increasing nominal Ce content,the weight gain of the Co-Al-W-Mo-Ta-B alloys monotonically decreased.An oxide scale composed of a dense and uniform outer Co_(3)O_(4)+CoO layer,a middle CoAl2 O4 and CoWO4 compound layer and an inner Al_(2)O_(3)layer.The excellent oxidation resistance of 0.2 Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al_(2)O_(3)layer and the thickest Al_(2)O_(3)layer during entire oxidation process.展开更多
It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few system...It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp rnartensitic transformation and the SM effect were investigated in Co-A1 binary alloys(mole fraction of Al=0-16%). The γ/ε rnartensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60℃ at x(Al)=0 to 150℃ at x(Al) = 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4% (mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcpfee → reverse transformation above 200℃. The SM effect was significantly improved by precipitation of β (I32) phase and the max[real shape recovery strain of 2.2 % was obtained, which can be explained by precipitation hardening. The crystallographic orientations between the β, εand γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690℃ and 120 emu/g, respectively. It is concluded that the Co-A1 alloys hold promise as new high-temperature and ferromagnetic SM alloys.展开更多
Information on phase equilibria in the Co-Al based systems which are related to some magnetic and heat resistance materials is important for their microstructural control. Recently, it was proposed with a theoretical ...Information on phase equilibria in the Co-Al based systems which are related to some magnetic and heat resistance materials is important for their microstructural control. Recently, it was proposed with a theoretical calculation on electronic band structure that some Heusler-type alloys Co2XAl （X: Cr and Mn） should be a new type of spinelectronic materials so-called half-metallic ferromagnet. In the case of the Co2CrAl, however, magnetic properties expected from the theoretical work can not been experimentally obtained and the reason has been still unknown. On the other hand, a tunneling magnetoresistance （TMR） effect due to the half-metallic properties was reported in Co2（Cr<sup>0.6 Fe<sup>0.4 ）Al alloy, but not the Co2CrAl alloy.In the present paper, it is reported that this discrepancy with the theoretical work in the Co2CrAl alloy is bought about by phase separation between A2 and B2 phases, and that the substitution of Fe for Cr can suppress the precipitation of A2 phase in the B2 phase. Such a phase separation is originally due to the miscibility gap between CoAl and Cr formed in the Co-Al-Cr ternary system as well as that reported by Hao et al. in the Ni-Co-Al-Fe system.展开更多
1.IntroductionMechanical alloying is one of the effec-tive methods to prepare amorphous alloys[1].This method was first used by Koch etal.to prepare the Ni;Nb;amorphous alloy[2].Since then many other amorphousbinary a...1.IntroductionMechanical alloying is one of the effec-tive methods to prepare amorphous alloys[1].This method was first used by Koch etal.to prepare the Ni;Nb;amorphous alloy[2].Since then many other amorphousbinary alloys have been prepared by the展开更多
基金financially supported by the National Key Research and Development Program of China(No.2017YFB0702902)the National Natural Science Foundation of China(No.51471014)。
文摘Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at%and 0.20 at%Ce)exposed at 800℃were compared.The 0.01 Ce and 0.05 Ce alloys were consisted ofγ/γ’coherent micro structure,while theκ-Co_(3)W compound precipitated at the grain boundary of the 0.1 OCe and 0.20 Ce alloys in addition to theγ/γ’microstructure.The oxidation kinetics curves of the Cedoped alloys exhibited a parabolic time dependence on the weight gain.With an increasing nominal Ce content,the weight gain of the Co-Al-W-Mo-Ta-B alloys monotonically decreased.An oxide scale composed of a dense and uniform outer Co_(3)O_(4)+CoO layer,a middle CoAl2 O4 and CoWO4 compound layer and an inner Al_(2)O_(3)layer.The excellent oxidation resistance of 0.2 Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al_(2)O_(3)layer and the thickest Al_(2)O_(3)layer during entire oxidation process.
文摘It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp rnartensitic transformation and the SM effect were investigated in Co-A1 binary alloys(mole fraction of Al=0-16%). The γ/ε rnartensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60℃ at x(Al)=0 to 150℃ at x(Al) = 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4% (mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcpfee → reverse transformation above 200℃. The SM effect was significantly improved by precipitation of β (I32) phase and the max[real shape recovery strain of 2.2 % was obtained, which can be explained by precipitation hardening. The crystallographic orientations between the β, εand γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690℃ and 120 emu/g, respectively. It is concluded that the Co-A1 alloys hold promise as new high-temperature and ferromagnetic SM alloys.
文摘Information on phase equilibria in the Co-Al based systems which are related to some magnetic and heat resistance materials is important for their microstructural control. Recently, it was proposed with a theoretical calculation on electronic band structure that some Heusler-type alloys Co2XAl （X: Cr and Mn） should be a new type of spinelectronic materials so-called half-metallic ferromagnet. In the case of the Co2CrAl, however, magnetic properties expected from the theoretical work can not been experimentally obtained and the reason has been still unknown. On the other hand, a tunneling magnetoresistance （TMR） effect due to the half-metallic properties was reported in Co2（Cr<sup>0.6 Fe<sup>0.4 ）Al alloy, but not the Co2CrAl alloy.In the present paper, it is reported that this discrepancy with the theoretical work in the Co2CrAl alloy is bought about by phase separation between A2 and B2 phases, and that the substitution of Fe for Cr can suppress the precipitation of A2 phase in the B2 phase. Such a phase separation is originally due to the miscibility gap between CoAl and Cr formed in the Co-Al-Cr ternary system as well as that reported by Hao et al. in the Ni-Co-Al-Fe system.
文摘1.IntroductionMechanical alloying is one of the effec-tive methods to prepare amorphous alloys[1].This method was first used by Koch etal.to prepare the Ni;Nb;amorphous alloy[2].Since then many other amorphousbinary alloys have been prepared by the
