Mete or hammerfall will correct me if I'm wrong, but I think both statements are correct. Some aluminum alloys gain from cold work before aging treatments. The reason is the cold work creates more dislocations. These act as nucleation sites for the precipitates, promoting finer precipitates more evenly dispursed. So if cold worked, then aged, there is both a structural change and indirect improvement due to dislocations. This alloy sounds like it would have similar properties.
The cold work could contribute to hardness by itself, but it depends on the recrystallization temperature relative to the aging temperature. I have no idea what it is for an alloy like this. I suspect the aging temperature is below the recrystallization temperature, since it would be tricky for the dislocations to act as nucleation sites while the extra ones are being eliminated.
hi me2.
i think the first thread i posted on bf has a brif explaination on this.
well i was trying to avoid type a wall of text to explain things. from my old forum experience, many people get annoyed by that and just quit reading. but here on bf you guys seems want to know everything lol. so here INCOMING!!! WALL of TEXT!!
The hardening machenism of the alloy is general caused in 3 ways.
First, during the aging the Ni3Al phase separate out from the prime phase, causing the strain which creat a field of stress strengthen and harden the alloy.
Second, if the Ni3Al and other dispersed phase located on the path of the line of dislocation then the Line of Dislocation will be able to cut through the dispersed phase. when this occures, the line of dislocation has to overcome the strain caused by ni3al. also the dispersed phase which cut in two has its surface energy increased. Thus created th energy field in antiphase domain. Which also strengthening the metal. here this mechanism can be caculated as: τ∝f^(1/2~5/6) x r^1/2. f is mass %of percipitation phase. r is radisu of that phase. when f is a constant, greater the r is the greater strengthening potential is. when the size of phase is a constant, then more they are the greater strengthening potential is.
Third, if the dispersed phase is very hard that can not be cut through by the line of dislocation, then the line will twist and circling arround the dispersed phase. Causing a loop on each of the duspersed phase, great increasing the degree of dislocation. Causing the alloy to be hardened futher. this mechanism can be caculated as: τ(twist)=2Gb/L, τ(twist)∝αf^1/2r^-1. α here repersants 2 constant:0.093 and 0.14, it depends on the type of dislocation.
cold work deformation will cause large degree of dislocation accumulated into sub-grain boundary. it lower the recrystalization temperature into the aging range by increasing the strain energy. once recrystalized during the aging process, it will tremendously refine size of dispersion phase. the hardness increasing after coldwork is mostly due to the refinement and higer dispersion of precipitates phrase, also combine with grain refinement. and the recrystalization temperature is not a constant number. it shapes with deformation rate. when we look into the mechanism, everything turns into the very basic. and here is a simple matter of energy conversion. more elastic strain energy granted during cold deformation, less energy is needed for recrystalization to occure. thus lower recrystalization temperature.
for example, this cold roll GNiCr40Al4 alloy from 5 to 3mm lower the recrystalization temperature into aging range 630~650c. greater the elastic strain energy is, the lower the temperature for aging and recrystalization. when hit 90% deformation, age it at 610c.
btw, me2 did you get a test sample from xiachu? if you did, cut it into 10 piece. hammer it into 4.5mm, 4mm, 3.5mm, 3mm...0.5mm. ageing those samples and you will see each of them has a different recrystalization temperature. lol and do not shoot yourself when you see the electricity bills for weeks of controlled aging.
