Unrealistic SCI-FI metals

Connor MacLeod · Post by **Connor MacLeod** » 2003-01-02 04:03am

Darth Wong wrote: There are certain exotic metals which are used for applications such as jet turbine blades (didn't you ever wonder what they made those out of?) which require a set of characteristics impossible to meet with other materials.

A jet turbine blade requires high temperature strength and good creep resistance, but it also must have excellent tensile strength, because there's a fuckload of centrifugal force involved here. So ceramics are out, and that means you need something made of metal, but which won't creep at the high temps. Enter superalloys: alloys with lots of alloying elements designed for elevated-temperature strength.

Nickel-based superalloys have good creep resistance up to beyond 1000 degrees C, and good resistance to corrosion as well. Cobalt-based superalloys are even better, but very expensive. Haynes 25 (no, it's not a fucking underwear) is 50% Cobalt, 20% Chromium, 15% Tungsten, 10% Nickel, and a few other sundries. Tensile strength is 135 kpsi with excellent ductility (60% elongation before failure). It's used for jet engines. Stellite 6B is 60% Cobalt, 30% Chromium, 4.5% Tungsten, and a few other sundries. Tensile strength is 177 kpsi. It's used for abrasion resistance (and the M60 gun barrel, IIRC).

MAR-M246 is also used for jet engines, and it's 10% Nickel, 9% Cobalt, 10% Chromium, and a bunch of Tungsten, Titanium, Aluminum, and Tantalum (real smorgasbord). It's good for 140 kpsi, but it's not as ductile as Haynes 25.

Ah.. so are refractory metals also a superalloy, or is that something else? (what exactly does the refractory bit have to do with them, for that matter as well?)

Post by **Darth Wong** » 2003-01-02 04:07am

Connor MacLeod wrote:Ah.. so are refractory metals also a superalloy, or is that something else? (what exactly does the refractory bit have to do with them, for that matter as well?)

No, refractory metals are metals whose melting points are naturally higher than steel in their natural, unalloyed state. Similarly, refractory ceramics are materials whose melting points are higher than steel. They are preferable to refractory metals in many situations because of cost. On the Space Shuttle, they are preferable because of insulation and ablation, since the heat load of a re-entry is a somewhat unusual condition (spread out over the entire surface, but vapourized material is carried away almost instantly).

Obviously, you will tend to see refractory metals in superalloys, but they aren't the same thing as a superalloy.

Connor MacLeod · Post by **Connor MacLeod** » 2003-01-02 04:13am

Darth Wong wrote: Obviously, you will tend to see refractory metals in superalloys, but they aren't the same thing as a superalloy.

Yes, since a superalloy is basically composed of a number of elements to fit the special requirements you described, which included temperature. A refractory metal would be an obvious choice.

Why do they call such metals "refractory?" Do they involve refraction in some fashion?

Pu-239 · Post by **Pu-239** » 2003-01-02 06:42pm

Vympel wrote:
Darth Wong wrote: Or it was a matter of cost.
Both cost and weight I think.

In the case of the M2 Bradley, they thought they'd better add on some armor because it was too easily killed.

The M2A1 Bradley IFV weighs 22 tons. The M2A2 which added on applique steel armor plating jacked the weight up to 33 tons, IIRC.

In the end, you're going to have to spend some major cash to make an IFV that can withstand modern weapons- even the 33 ton applique armor Bradley couldn't take a hit from a modern RPG-7 round and survive.

Isn't Al more costly than steel?

Zoink · Post by **Zoink** » 2003-01-02 07:30pm

Pu-239 wrote: Isn't Al more costly than steel?

In WW2 they needed steel for pretty much everything. Building something out of an alternate metal was of great advantage. Sometimes quantity was more important than quality.

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As a side note, for anyone interested in the material properties of metals, a good source is Military Handbook 5: Metallic Materials and Elements for Aerospace Vehicles. Its an internationally recognized source for material properties, and it comes in handy if you ever want to design an aircraft and have it certified by the FAA... if you're into that kind of thing.

It can be downloaded at the Department of Defense's document site:

http://assist2.daps.dla.mil/quicksearch ... _query.cfm

The document ID is MIL-HDBK-5H. There's a bunch of other documents of interest to anyone doing design. I really wish I had found this in University, because it would have made a number of design project much easier... Alternately, you can download the packaging requirements of a 16-inch naval gun if thats more your style

Post by **Beowulf** » 2003-01-02 09:47pm

Zoink wrote:
Pu-239 wrote: Isn't Al more costly than steel?
In WW2 they needed steel for pretty much everything. Building something out of an alternate metal was of great advantage. Sometimes quantity was more important than quality.

Um... WW2? The Bradley is currently in service.

phongn · Post by **phongn** » 2003-01-02 09:48pm

Connor MacLeod wrote:
Darth Wong wrote: Obviously, you will tend to see refractory metals in superalloys, but they aren't the same thing as a superalloy.
Yes, since a superalloy is basically composed of a number of elements to fit the special requirements you described, which included temperature. A refractory metal would be an obvious choice.

Why do they call such metals "refractory?" Do they involve refraction in some fashion?

Well, Merriam-Webster has a definition as "difficult to fuse, corrode, or draw out; especially : capable of enduring high temperature" (Third definition)

Also:

"Etymology: alteration of refractary, from Latin refractarius, irregular from refragari to oppose, from re- + -fragari (as in suffragari to support with one's vote)"

Post by **Darth Wong** » 2003-01-02 09:49pm

High-grade steel is fairly expensive. Low-grade stuff is cheap and plentiful, but they don't use that for armour.

Wicked Pilot · Post by **Wicked Pilot** » 2003-01-02 11:48pm

Darth Wong wrote:High-grade steel is fairly expensive. Low-grade stuff is cheap and plentiful, but they don't use that for armour.

I hope this doesn't sound too ignorant, but what makes steel high or low grade? Is it purity, or something else?

Sokar · Post by **Sokar** » 2003-01-03 01:25am

Not to sound like a total whore or kiss ass, but I could listen to this all day , I always find science and enginering discussion fascinating even though I lack the math skills to understand it all

Einhander Sn0m4n · Post by **Einhander Sn0m4n** » 2003-01-03 04:56am

Sokar wrote:Not to sound like a total whore or kiss ass, but I could listen to this all day , I always find science and enginering discussion fascinating even though I lack the math skills to understand it all

LOL! So can I.

Warspite · Post by **Warspite** » 2003-01-03 07:02am

Wicked Pilot wrote:
Darth Wong wrote:High-grade steel is fairly expensive. Low-grade stuff is cheap and plentiful, but they don't use that for armour.
I hope this doesn't sound too ignorant, but what makes steel high or low grade? Is it purity, or something else?

Purity in steel isn't what diferentiates high from low grade.
You know steel is a mix of several components, mainly Carbon, but also, Chrome, Copper, Sulfur, Manganese, Phosphor, Silicium, Nickel and flavouring.
On a quick analisys, high grade steel posesses less carbon and more of the other components than low grade, mainly chrome, manganese, and silicium, generally in order to improve certain properties, be that welding capability, yield strength or hardness.
It all comes down to how we mix the components. Do a google search on steel works, usually they provide tables with element concentrations and properties.

Zoink · Post by **Zoink** » 2003-01-03 08:38am

Beowulf wrote: Um... WW2? The Bradley is currently in service.

My bad.

aerius · Post by **aerius** » 2003-01-03 02:58pm

Warspite wrote: Purity in steel isn't what diferentiates high from low grade.
You know steel is a mix of several components, mainly Carbon, but also, Chrome, Copper, Sulfur, Manganese, Phosphor, Silicium, Nickel and flavouring.
On a quick analisys, high grade steel posesses less carbon and more of the other components than low grade, mainly chrome, manganese, and silicium, generally in order to improve certain properties, be that welding capability, yield strength or hardness.

Actually, it does when we're talking about unwanted impurities. Low grade steels generally contain a good amount of contaminants such as sulphur or phosphorous, which screws up the grain and results in weaker brittle steel. They also pour these steels into larger ingots which take much longer to cool, giving the grains more time to grow which is not good. This stuff is poured right out of the basic oxygen furnace where the excess carbon is burned off and doesn't go through any other processing steps.

The high grade steels go through additional steps where the steel is remelted and other metals are alloyed in and the impurities burned off or removed. The really good stuff gets remelted a couple times in a vacuum to prevent contamination and get rid of all unwanted impurities. These steels are generally made in smaller batches and poured into smaller ingots for faster cooling and smaller grain size and to prevent alloyed elements from coming out of solution and screwing up the composition of the steel. All this is greatly simplified of course, the books that cover this stuff are really big and heavy.

Warspite · Post by **Warspite** » 2003-01-03 03:12pm

aerius wrote: Actually, it does when we're talking about unwanted impurities. Low grade steels generally contain a good amount of contaminants such as sulphur or phosphorous, which screws up the grain and results in weaker brittle steel. They also pour these steels into larger ingots which take much longer to cool, giving the grains more time to grow which is not good. This stuff is poured right out of the basic oxygen furnace where the excess carbon is burned off and doesn't go through any other processing steps.

The high grade steels go through additional steps where the steel is remelted and other metals are alloyed in and the impurities burned off or removed. The really good stuff gets remelted a couple times in a vacuum to prevent contamination and get rid of all unwanted impurities. These steels are generally made in smaller batches and poured into smaller ingots for faster cooling and smaller grain size and to prevent alloyed elements from coming out of solution and screwing up the composition of the steel. All this is greatly simplified of course, the books that cover this stuff are really big and heavy.

You're rigth, of course, I was thinking more along the final properties, for example, yield strength, and the influence of the "desired" elements.

Heck, I have one of those books rigth here with me, I just didn't feel going through it... Too many pages to read.

Wicked Pilot · Post by **Wicked Pilot** » 2003-01-03 04:51pm

Well. you've all be helpful. If you ever need any info on how to fly an airplane, don't hesisate to ask.

XPViking · Post by **XPViking** » 2003-01-08 06:44pm

Has anyone done a study on some of the more common sci-fi materials? That is, categorizing them according to the various criteria (melting point, strength, fatigue cycling limit, etc...) and then discussing its applications? I don't know what criteria would be needed to categorize the different materials.

Something like:

Name: Durasteel
Melting point: X
Strength: Y
Fatigue cycling limit: Z
Properties: looks like...

Commentary: Durasteel is found in a number of application in the Empire...

Something like that.

XPViking