Hydride formation under cathodic charging of titanium and TiAl-based alloys in alkaline solutions

The electrochemical methods and the data of X-ray diffraction analysis are used to determine the parameters of cathodic polarization for the hydrogenation of α-Ti and alloys based on the TiAl intermetallic phase without formation of the hydride phase or with formation of hydrides. In α-Ti, the increasing cathodic polarization in a 0.1 M NaOH solution leads to the dissolution of hydrogen in the metal lattice and its modification and to the increase in the amount of hydrides. The hydride phase is not recorded TiAl-based alloys even for much higher levels of absorption of hydrogen as compared with pure titanium. However, hydrogen affects the phase composition of alloys and the lattice parameters of the phases. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 103–106, May–June, 2008.     

High temperature compressive behaviour of as-cast Al-Cu alloys of varying composition

Al-Cu alloys with different copper contents ranging from 10 to 45 wt% were deformed in compression in the as-cast condition. Measurements of the strain-rate sensitivity parameterm using the crosshead speed jump technique show that alloys of composition close to the eutectic become superplastic after about 25% strain, withm increasing from about 0.2 at the beginning of deformation to about 0.5 or more, depending on composition. This transition to the superplastic state is accompanied by an important decrease of the flow stress during compression and it is associated by the breakdown of the initially lamellar structure of the eutectic phase in the highly deformed regions of the specimen. Moreover for the alloy with 37 wt% copper, the transition corresponds also to the degeneration of the dendritic primary phase and this alloy shows a particularly high value ofm and a low steady state flow stress in spite of the large volume fraction of the hard CuAl₂ compound. Alloys of composition far away from the eutectic do not exhibit superplastic behaviour during compression, at least in the range of strain rate investigated and this is due to the large grain size and the important grain growth that occurs during deformation.     

High-temperature deformation of Al-Li-Cu-Mg-Zr alloys 8090 and 8091

Specimens were prestretched in the range 0 to 7% plastic deformation prior to artificial ageing, or were duplex aged, to investigate the effect of dislocation substructure and of S(Al₂CuMg) particles on plastic flow during room and elevated temperature tensile tests. The yield stresses increased in Al-Li-Cu-Mg-Zr alloys 8090 and 8091 after stretching, due to the dislocation cells introduced by the stretch and also to the nucleation and growth of the S-phase particles on these dislocations. Up to 400 K the modulus-normalized proof stresses for the variously treated materials were constant, but they fell at higher temperatures, as dislocation climb mechanisms operated. The proof stresses of alloy 8090 fell at a temperature 50 K lower than alloy 8091, and this difference is considered to arise from the differing volume fraction of S-phase in the two alloys. The work hardening rate (ϑ) at a strain of 0.2% was measured between 300 and 500 K. In alloy 8090, the temperature at which dynamic recovery occurs was influenced by the degree of stretch, but this was not so in alloy 8091. In alloy 8090, the substructure introduced by stretching was able to act as a dislocation sink during dynamic recovery, whereas in alloy 8091 the higher copper content brought about more S-phase precipitation which was sufficient to inhibit this effect. The athermal hardening rate (ϑ_o) was obtained by extrapolation of the ϑ-σ_t curves, giving ϑ_o=10000 GPa, for alloy 8090 and ϑ_o=8500 G Pa, for alloy 8091. This difference may reflect a higher mobile dislocation density in alloy 8090 due to the lower volume fraction of S-phase particles in that material.     

Influence of microstructure on the hydrogen embrittlement of Al-Li-Cu-Mg-Zr alloys

The effects of microstructure and hydrogen charge time on the delayed fracture of the AA 8090 Al-Li alloy have been studied using fracture tests and TEM techniques. The results obtained from the fracture tests showed that the solubilization heat treatment confers the lowest hydrogen sensitivity on the alloy which increases with the hydrogen discharging time, and the hydrogen embrittlement sensitivity increases with the ageing time and reaches a maximum at 90 h. This behaviour was attributed to the interaction of the mobile dislocations, which are responsible for the hydrogen transportation, with phases precipitated during the ageing treatment.     

The hydrogen content of austenite after cathodic charging

We have made direct measurements of deuterium concentration profiles and have shown that the concentration of hydrogen (deuterium) in the near-surface layers of 310 austenitic steel following room-temperature cathodic charging in poisoned electrolytes is 0.5 to 0.8 hydrogen atoms per metal atom. There is also considerable microstructural damage, including the formation of an unstable martensite. With no arsenic poison in the electrolyte the hydrogen ratio is about 0.15, and there is low associated damage structure. These hydrogen (deuterium) concentrations are many orders of magnitude greater than those required to embrittle ferritic steels, implying a different embrittlement mechanism in the austenite. The observations of a hydrogen-induced martensite phase in this supposedly phase-stable alloy indicates that hydrogen embrittlement mechanisms involving martensite are no longer excluded.     

The influence of cathodic hydrogen charging on the mechanical behaviour of Al-4Zn-1Mg alloy

The effect of cathodic charging on the mechanical behaviour of Al-4Zn-1Mg alloy was studied. Hardening of the Al-4Zn-1Mg alloy surface, due to the hydrogen absorption, was observed. The ultimate tensile stress of the charged aluminium alloy was noted to be a non-linear function of the charging current density. The cathodically charged aluminium alloy exhibited brittle transgranular fracture at the surface layer, whereas ductile intergranular fracture was observed at the deeper layers of the same alloy.     

Deformation and recrystallization in cross-rolled Al-Cu precipitation alloys

The deformation and recrystallization behaviour of single-phase f.c.c. metals have been studied mostly after conventional, uniaxial rolling. In this paper, the effect of both the presence of a second phase and the deformation path (i.e. straight rolling versus cross-rolling) on the evolution of microstructures and textures of an Al-3% Cu alloy are presented. The changes in the deformation and recrystallization texture are found to be complex, and it is not the deformation path nor solely the type of precipitate alone which produces the changes.     

Joining Joining of silicon nitride with Al-Cu alloys

The sessiie drop technique was used to evaluate the equilibrium contact angle and work of adhesion of molten Al-Cu alloys on Si₃N₄ at 1373 K under vacuum. The wettability of Al-Cu alloys on Si₃N₄ is improved by an addition of copper content up to 20 at%. The joining of Si₃N₄ to Si₃Ni₄ was also conducted using Al-Cu filler metal at a brazing condition of 1373 K for 3.8 ksec. The dependence of strength of the Si₃N₄ joint against the copper content in the filler corresponds to the copper content dependence of work of adhesion for molten Al-Cu alloy on Si₃N₄. The superior wettability and mechanical property of filler provide the superior strength of Si₃N₄ brazed with the filler. In particular, the Si₃N₄ joint brazed with Al-1.7 at% Cu filler exhibits the maximum fracture shear strength of 188.3 MPa at room temperature. This superior strength of Si₃N₄ brazed with Al-1.7 at% Cu filler is maintained at elevated temperatures up to 850 K.     

Effects of ageing after cathodic charging in austenitic stainless steels

Ageing after cathodic charging resulted in a number of significant structural changes as the hydrogen diffused out of the specimen. The -phase is formed during hydrogen charging of the 316 steel. The weight fraction of the -martensite decreased with increasing ageing time. It is suggested that the transformation might be taking place during hydrogen release. Techniques for quantitative phase distribution in the stainless steel after various ageing times were used.     

Dendrite coarsening during solidification of hypo- and hyper-eutectic Al-Cu alloys

Dendrite coarsening during cooling at a constant rate was compared at various stages of solidification with that during isothermal holding for Al-Cu alloys of hypo- and hypereutectic compositions. For each specimen, the undercooling for the initial dendrite formation and the time elapsed after it were measured directly. The dendrite arm spacing was shown to be determined solely by the latter, and the dendrite structure was therefore coarsening-controlled from the early stage of solidification. The rate of coarsening in terms of the dendrite arm spacing during solidification at a constant cooling rate was same as that during isothermal holding in all the alloys tested. Numerical values of the fractional rate of solidification were evaluated for the hypo-eutectic compositions and the results show that the rate of dendrite coarsening does not depend on the fractional rate of solidification. Aluminium dendrites show structural coarsening with progressive solidification in the same way as during isothermal holding. CuAl₂ dendrites show curved boundaries after isothermal holding whereas those cooled at a constant rate are faceted.     

Joining and wetting of CaO-stabilized ZrO2 with Al-Cu alloys

The contact angles of molten Al-Cu alloys on CaO-stabilized ZrO₂ have been measured using a sessile drop technique at 1373 K under a vacuum. The work of adhesion, W _ad, of an alloy against ZrO₂ was evaluated from the equilibrium contact angle of the alloy. The W _ad values Al-Cu alloys with copper contents up to 10 at% are the same or slightly higher than the value of 1.25 J m^–2 for pure aluminium. On further increase in copper content, W _ad gradually decreases to 0.8 J m^–2 for pure copper at 1373 K. The general trend in the work of adhesion against copper content of Al-Cu alloys is in accordance with the copper-content dependence of the joining strength of ZrO₂ joints brazed with the Al-Cu alloys. A ZrO₂ joint brazed with Al-1.7 at% Cu filler provides the maximum fracture strength of 105 MPa at room temperature, and this improved strength of ZrO₂ is maintained at elevated temperatures up to 773 K. The joining strength of a ZrO₂ joint brazed with an Al-Cu alloy is dominated by the mechanical properties of the alloy in addition to the wettability of the alloy against ZrO₂.     

Simulation of clusters formation in Al-Cu based and Al-Zn based alloys

A Monte Carlo computer simulation is adopted to investigate the role of micro-alloying elements Mg and Ag in Al-Cu and Al-Zn alloys. Small amount additions of Mg to the Al-Cu alloy markedly retard the formation of Cu clusters due to the preferential trapping of free-vacancies available for Cu diffusion. On the other hand, additions of Mg to the Al-Zn alloy promote the formation of Zn clusters due to the preferential Mg-Zn interaction. As for the effect of Ag, it is found that, in both Al-Cu-Mg and Al-Zn-Mg alloys, Ag atoms are preferentially bounded to Mg-Cu-vacancy or Mg-Zn-vacancy complexes. However, in Al-Cu-Mg alloy Ag atoms interact with Mg, while in Al-Zn-Mg alloy they interact with both Mg and Zn.     

Ageing behaviour and toughness of silicon carbide particulate reinforced Al-Li-Cu-Mg-Zr metal matrix composites

The effects of lithium content on the ageing characteristic and notched tensile properties of particulate reinforced Al-Li-Cu-Mg-Zr based metal matrix composites (MMCs) have been investigated. MMC sheet containing 20 wt% silicon carbide particulate produced by a conventional powder metallurgy route aged at a similar rate as unreinforced sheet, and the highest strengths were achieved in samples containing 2–2.5 wt% Li. A proprietary processed 8090 Al-Li alloy MMC sheet aged more rapidly, however, and gave considerably higher strengths. The toughness of Al-Li-Cu-Mg-Zr MMC sheet, as indicated by the notched tensile behaviour, can be improved by reducing the lithium content albeit at the expense of strength.