Why are the processes called “Precipitation Hardening” and “Solid Solution Strenghtening”?

Question

Does precipitation hardening really only make the material harder while Solid Solution strengthening makes it stronger altogether (thus increasing the area under the stress-strain-curve edit: excuse the mistake here: the area under the stress-strain curve denotes the toughness, not strength)? If so, then what is the reason behind this? Don't both mechanisms work by impeding dislocation gliding (one through precipitates and the other ones through introduced strains)?

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In this link, it is mentioned that "The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process [...]", which suggests that hardening indeed also increases strength. Is there a specific reason for why the names were chosen in this manner?

Answer 1

For most metal alloys tensile strength and hardness are effectively different expressions of the same property. Although this does depend on exactly how hardness is measured and some related properties such as abrasion resistance can diverge from this especially in things like stainless steel where you have hard carbide grains embedded in a relatively soft matrix.

There are also cases where components can have a hard outer layer and a more ductile core (eg white/grey iron castings) or case hardened steel.

Similarly where an alloy can undergo changes in hardness/strength during the manufacturing process it is conventional to talk about hardening. Often this sort of terminology dates back to a period when the empirical properties of materials were well understood but not necessarily in a rigorous scientific way.

Considering the specific terms in the question, Solid Solution Strengthening is the general term for the phenomenon the at solid solution of two elements will be stronger/harder than the pure base metal because the alloying element disrupts slipping of dislocations. Precipitation hardening is a specific process used to achieve this.

I suspect the choice of strengthening vs hardening is just down to slightly different context as in manufacturing 'strengthening' can be ambiguous (as it could relate to structural design as well) whereas 'hardening' clearly relates to a material property.

However in a materials science context 'strengthening' is a bit more precise, as tensile strength is a better defined property than hardness.

It is also reasonable to suppose that precipitation hardening is named by analogy to older processes like quench hardening of steel and, work hardening where the increase in hardness is the most immediately obvious outcome of the process.

Answer 2

This started as a comment but got too long. I think you have some good answers already regarding the naming of precipitation hardening, so my answer attempts to address some of the other semantic issues it appears you are struggling with.

Area under the curve is term which refers to a stress-strain curve, which (as it sounds) is a graphical representation of a materials behavior under stress. This is a measure of the materials strength, and does not necessarily indicate toughness, or hardness.

Strength is a materials ability to resist a load (known as stress). When a load is imparted to a material, it will undergo a deformation. The measured amount of deformation is called strain.

A stress-strain curve therefore is simply a plot of the amount of strain (deformation of the material) corresponding to various levels of stress (load). Typically this is plotted with strain along the horizontal axis and stress along the vertical. There are different ways these curves may be expressed e.g. engineering strain vs true strain, but I won't go into that here. I mention it as something to be aware of if you're going to be reading and comparing curves.

Now, I mentioned this is a measure of material's strength, and I'll attempt to give a brief explanation of this here. For metallic materials, a stress-strain curve is typically generated from a uniaxial tensile test, which measures the materials ability to withstand an ever increasing load in one direction under tension. The standard test consists of rigging a (typically cylindrical) sample of material between two clamps, one clamp is stationary, and the other is attached to a platform which moves away from the stationary jaw. This movement apart applies tension to the sample. This tensile load is increased until the material breaks, and the highest load achieved before rupture is known as the ultimate tensile strength of the material. (So UTS refers to the maximum amount of tensile stress a material can withstand before failure)

Aside from ultimate tensile strength (UTS) the other common measure of strength you'll see is yield strength (YS).

With metallic materials there are typically two distinct regions in the stress-strain curve; elastic and plastic. As load is increased from 0 up until the yield point the strain will increase proportionally at a linear rate relative to stress. At loads (stress) of this intensity the material behaves elastically, which is to say that when the load is removed, the material will return back to its original shape (stress=0, strain=0). At some point, the stress will become great enough to overcome the material's elastic limit, meaning permanent deformation occurs (stress=0, strain>0). This permanent deformation is known as plastic deformation, and so the area under the curve in this region is known as the plastic region.

The point at which this transition occurs is known as the material's yield point. Typically, it's not distinct enough to measure the exact point at which yield occurs, so an approximation known as offset yield is typically used. You'll commonly see material specifications refer to 0.2% offset yield (or some other percent offset) which refers to the approximated yield point. I won't get into the methods for offsetting to determine approximate yield, but it's easy enough to google if you're interested.

One more thing regarding material strength; so far we've only discussed tensile strength, which is the most common measurement of metallic materials, however other measurements of strength exist as well. For example, ceramics and cements are not commonly measured under tension but rather under compression, which is known as compressive strength. This again will be graphed as a stress-strain curve, but is a different test involving compression of the material until failure. I just thought it worth mentioning as it relates to the understanding of the word "strength" in materials science.

On to hardness. Hardness is typically measured as the materials ability to resist indentation. Various standards exist for testing hardness, such as Rockwell, Vickers, brinnell (all common with metallics) and shore (common with rubbers and plastics).

While hardness is measured as resistance to indentation, it's commonly used to judge a materials resistance to abrasion and wear, and can can be manipulated to increase a materials performance in certain applications or to improve machinability/workability of the material.

Hardness and strength commonly have a positive correlation as the mechanisms which increase a material's hardness typically also increase the material's strength.

Toughness typically has a negative correlation to hardness. Toughness is a measure of a material's ability to resist fracture under load. Brittleness is the opposite of toughness, and typically as a material becomes more hard, it also becomes more brittle. This is not the rule, just typical for many metallics.

Toughness is a little more difficult to quantify than strength, as there are many ways for a material to to be loaded to failure and for fracture to occur. Some common indicators of toughness are impact resistance (charpy is a common standard test), fracture resistance/resistance to crack growth (plain-strain fracture toughness is a common test), and ductility.

Ductility is similar to toughness, but is not necessarily related to a materials ability to resist fracture, but is rather a measurement of its ability to flow and deform under load. Ductility is commonly expressed as reduction of area or percent elongation; both are measures of plastic deformation before failure, typically established from the tensile test.

Answer 3

To answer your other question about choice of names, it appears that solid solution strengthening may have originally been referred to as solid solution hardening in the literature. See for example this article by RL Fleishcher dating to 1963 in Acta Metallurgica, 11.3, pp. 203-209, entitled "Substitutional solution hardening."

Precipitation hardening has a fairly storied history which is detailed in this article, a lecture on precipitation hardening located at core.materials.ac.uk. From the article, Wilm discovered in 1906 that an Al-Cu-Mg alloy was found to have hardened when left to sit at room temperature before testing. The as-yet-unknown process was called age hardening, due to the material increasing in hardness with age. Later work in the late 1930s determined the hardening was due to the formation of what are now known as Guinier-Preston (GP) zones, which are nano-scale arrangements (wires or plates) of solute atoms which, as you noted, impede dislocation motion. It wasn't until the advent of transmission electron microscopy in the 1950s that GP zones were actually directly observed, and the full precipitation hardening process discovered and understood.

Despite the fact that the material is strengthened in a materials engineering sense, it appears that the terms age hardening and precipitation hardening have stuck. Oddly, solid solution strengthening appears to have changed from solid solution hardening.