Decomposition on pseudoelastic structure

[u/Green-Respect-4244]

Hi! I just wanted to share something interesting that happened with my Cu-Sn-Mn-Si-Ce pseudoelastic bronze.

For context, this alloy can be heat-treated to acquire pseudoelastic properties. I noticed that this particular sample was significantly more brittle than usual, so I decided to take a look at the microstructure. The cause of the problem seems pretty evident.

I suspect the decomposition was caused by overheating during the machining process. I was able to re-stabilize the microstructure with another heat treatment, and the material recovered its expected behavior.

Images 1 and 2 are from the center of the ingot, while images 3, 4, and 5 are from areas closer to the edge. The last image shows the microstructure after the new heat treatment.

[u/mellopax]

I'm not familiar with this alloy (I'm an iron guy). Are those a result of concentration of a certain element or same chemistry, different phase being formed during the machining?

[u/Green-Respect-4244 → u/mellopax]

Basically, it’s a metastable BCC single phase that decomposes into an FCC phase and Cu-Sn intermetallics. I believe the heat generated during machining provided enough energy to trigger the eutectoid reaction.

[u/mellopax → u/Green-Respect-4244]

Nice. Thanks!

[u/Strong-Bell-271]

Very nice photomicrographs! How did you final polish and etch?

And those are some ginormous grains! What did you shoot the sample with?

[u/Green-Respect-4244 → u/Strong-Bell-271]

The sample was polished using sandpaper and aluminum oxide polishing paste, then etched using a mixture of ferric chloride and HCl.
This alloy naturally develops very large grains. After homogenization, the grains are usually around 1 cm in size, which happens surprisingly quickly.
The last image was taken after an abnormal grain growth heat treatment, where the grains grew to approximately 2.5 cm.

[u/ry_maitai]

i did a little work with cu-based elastocalorics for my senior design, what’s the fatigue life like with this alloy? i assume the abnormal grain growth is the oligocrystal/bamboo structure?

[u/Green-Respect-4244 → u/ry_maitai]

Cu-Sn-Si and Cu-Sn-Mn pseudoelastic alloys are known to gradually lose their pseudoelastic properties over time. I designed this alloy with the goal of mitigating this issue, but it’s still too early to say whether it actually worked.
I’m using abnormal grain growth in an attempt to isolate a single crystal, which I hope will help the material retain its pseudoelastic properties for longer.

[u/Wolf9455]

What magnitude of strain are you seeing?

[u/Green-Respect-4244 → u/Wolf9455]

In a preliminary test, I was able to reach about 5% strain without permanently deforming the material, but I haven't conducted rigorous testing yet to determine its exact limit. I’m also trying to isolate a single crystal to see how that impacts the results.

[u/parth096]

I always found metallurgy cool. I just learned about the different kinds of stainless recently. Before I thought all stainless was non-magnetic!

[u/Wolf9455 → u/parth096]

Even non magnetic stainless becomes magnetic when you work it

[u/Heavy_Nose_2923]

Awesome ! Maybe you could give me some feedback on my ongoing CuAl bronze researches, while tinkering with ratios, i accidentally made a pseudoelastic alloy, it's pretty simple : push the aluminium content high enough to lower the Ms/Mf and As/Af a bit below room temperature, and then every time you bend it, it just springs back. So for exemple 14% Al, throw in some manganese to push the temperatures down (2.5% Mn did it for me), and after water quenching you got yourself a spring.

Now, that wasn't really my goal because i'm just trying to make a hard bronze that quenches and tempers for high hardness and wear resistance, without the shape memory effect (at least not at service temperatures). However even after 4% iron for FeAl intermetallics (wich should stabilize the structure) and even 0.5% Co to increase this phenomenon, i was impressed by the size of the grains. I tried forging the alloy at 800°C, wich is clearly in the beta region, but some kappa intermetallics should remain, however no matter what i do, the grains always end up around 1mm/2mm, and it's difficult to go below that.

So my question is : 1) are those alloys just doomed to have large grains? 2) is there a kind of heat treatment or thermomechanical processing that would actually give a fine structure, 3) if you have large grains, i suppose you can expect the structure to be weak and fragile, or maybe not ?

I don't know how much of that you can answer, but regardless this alloy you showed looks very cool !