Continuo urbano-rural

of titanium, combined grain boundary TiC and precipitation

was observed, Plate 6( d ) ^ ^ . In all cases the orientation

relationships were in agreement with those observed by previous workers.(^9,75*76)

5.2*7 The Effect of Combined Aluminium and Titanium

Additions (Alloy No. 7 and 8)

Alloys

7

8

type, but with carbon contents of 0.012% and 0.12% respectively.

The ageing characteristics of both alloys were very similar

but the 0.12%C alloy showed a slightly lower increase in

hardness during ageing, because the titanium level was reduced by combination with the carbon to form undissolved TiC. The ageing characteristics were very different from the other alloys studied, the increase in hardness being very much greater and the intense ageing reaction extending to higher ageing temperatures, Figure 2+ 2. The age hardening was due

to 'f - Ni^ (Al, Ti) since the Ni/(A1+Ti) ratio was quite

high^-^o A feature of the ageing of .both the PE16 alloys

was the two stage ageing process, for which there are several possible explanations

t

(a) The first stage was due to zone or

f

by the formation of Ni2Al Ti. This is not believed to be the reason as no evidence of large plate-like Ni2Al Ti precipitates were observedin the heavily overaged

condition, Plates 7-9. A similar reason eliminates

the possibility of Ni(Al Ti) being responsible for the second stage of ageing. In addition the Al/Ti ratio was so low in both alloys as to eliminate any such possibility.

(b) Following zone or ^formation,sigma phase or chi phase may be responsible for the second stage of ageing. This

is not likely because no larger intermetallic compounds

were observed in the overaged condition, Plates 7-9*

titanium and aluminium for these phases to precipitate'

In addition a PhaGcm?^0 *1^2^ analysis of the constitution

indicated that the composition of PE16 was not within the range of electron hole vacancy numbers in which intermetallic compounds of the laves, sigma or chi type are formed*

(c) The most likely explanation for the two stage ageing

is that the first stage is due to the formation of

zones rich in nickel, aluminium and titanium, which grow is the second stage, with partial loss of coherency,

into7 precipitates, Plates 7-9* At temperatures above

8 5 0°C, the zone solvus was exceeded so that formed direct without the prior formation of zones, and only a single stage ageing reaction was observed. Also, the high activation energy for the ageing process of 297-308

KJ/mol (71-7& k cal/mol) indicated that bulk substitional

solute diffusion, as would be required by zone and 7

(77)

formation, was the rate controlling mechanism' .

The nucleation sites for 'Jf are believed to be pre­

existing lattice defects, i.e. both quenched in vacancies

and matrix dislocations. The nucleation of

zones on dislocation cores is hard to visualise, but zones could form near to the dislocation line due to the enhanced core diffusion.

Electron microscopy, Plate 7(e) and Plate 8(c), revealed the

presence of dislocations in association with Yzones, but the density of the zones was too great for nucleation to have occurred solely on or near dislocations. The relative concentration of quenched in vacancies and dislocations control the dominating mechanism, and in this case the

quenched in vacancies have a dominant role.^2^ The loss of coherency of these precipitates is probably associated with the absorption of matrix dislocations at their interface, perhaps due to an interaction between the strain field of

the dislocations and the elastic strain field of the precipitat (79) This is further supported by the fact that there were few dislocations present in the microstructures showing semi-

The enhanced nucleation and growth of Mg^Cg precipitates at the grain boundaries compared with'f precipitates within

the grains, Plates7 and 8, is due to the lower driving force

required for grain boundary diffusion compared with the bulk

diffusion' . However, there was no significant evidence

of{^depleted regions adjacent to the grain boundaries even during the initial stages of ageing. A possible explanation for this could be the generation of dislocations near the

grain boundaries due to M0-zC/* precipitate growth, which act

as nucleation sites forX zone formation.' ' Also, a

contributory factor could be the migration of quenched in dislocations towards grain boundaries during ageing, thus increasing the number of nucleation sites for'lfprecipitation.

Although, due to the presence of at grain boundaries,

the chromium concentration will be lower in the adjacent a r e a s , d e c r e a s e is not perhaps enough to cause

f depletion in these regions.

The precipitation of M2^Cg particles on undissolved TiC

particles,. Plates 9(a) and (b), can be attributed largely to

a simple nucleation effect at an interface, particularly as the orientation relationships are favourable*.. In addition, dislocations generated at the TiC/matrix interface will also contribute to the nucleation of

5.3 COLD WORK. RSCRY S TALLTZATT ON AID ERECT PTT ATT ON.

3,3 .1 Medium Carbon Base Composition (Alloy No. 2)

The higher hardness values observed after recrystallisation

compared with-the un-cold worked material, Pigs, hh and k5,

can be explained in terms of the finer overall grain size.