Before you read this, make sure you've read: "What is a metal? The basic structure".
Ductility formally describes how easily a metal can be drawn into wires. In practice, it describes how much we can deform a metal before it breaks. Heating an alloy to a high fraction of its melting temperature after melting can dramatically improve and restore its ductility after extensively working material. It is common practice amongst goldsmiths and silversmiths.
A brief summary of working
There are two main types of working (deformation) used to shape a piece of metal; Hot working and Cold Working. When we deform a metal, shape change occurs as layers of atoms slide over one another, causing a change in the shape and orientation of each grain and, ultimately a change in the macroscopic shape of the object.
Cold working is when we deform the material at low temperatures (typically below half the alloy's melting point). At this temperature, we see:
A macroscopic shape change,
A change in grain structure – grains become elongated and reoriented,
An increase in strength – metals become harder due to work hardening,
A reduction in further ductility – Grains are heavily deformed and cannot deform much further. Further working will cause the metal to fracture.
Hot working is where we deform a material at a high temperature. We are working and annealing at the same time. The materials are softer and easier to deform, and the material can be deformed more before it breaks.
Annealing a cold-worked alloy
When we heat metal to a high temperature, but below its melting point, the atoms have more thermal energy. As a result, they can slowly rearrange themselves into a more stable structure within the lattice structure.
The elongated grain structure formed during cold working is a high-energy state. When heated, the atoms will form undeformed equiaxed grains that grow in the place of the deformed ones during annealing. This is known as recrystallization.
The temperature we heat to, and the extent of cold working affect the final microstructure and properties of the alloy:
Below a critical temperature, no recrystallization can occur. As a result, there is no change in grain structure.
The hardness does not change significantly.
Above this critical temperature, recrystallization occurs, and undeformed grains grow in place of elongated ones.
There is a dramatic reduction in strength as the work hardening is "undone" and ductility is restored.
At high temperatures, the undeformed grains continue to grow. Larger and fewer grains are more energetically favorable as fewer high-energy grain boundaries exist.
Hardness continues to fall (due to increasing grain size), and ductility will also eventually begin to fall – there are fewer different grains to accommodate any shape change.
Remember – We need a minimum amount of cold work necessary for recrystallization to occur, typically about a 12-15% reduction. Otherwise, there is no driving force (the reduction in energy) for the microstructure to change.
Why do we cool the alloy in water after annealing?
Cooling in water (quenching) is much faster than cooling in air. Fast cooling helps to prevent any further undesirable changes to the microstructure that may occur if the alloy is allowed to cool slowly. Particularly to avoid losing any ductility.
Particularly for alloys with substantial alloying additions (e.g., low Karat Golds), it may be favorable to form (precipitate) second phases, such as copper-rich phases, which can dramatically change the properties. These often make the alloy harder but less ductile. The sample may crack during subsequent work.
By cooling quickly, there is not enough time for the atoms to rearrange themselves while they have enough energy; the microstructure is "frozen".
Practice: Controlling annealing to optimize the properties
Finer recrystallized grain size is desirable; it has higher strength (still lower than the cold-worked sample), ductility, and toughness. This can be achieved by cold working the alloy more before annealing.
The key factors in annealing are:
The initial amount of cold work – This leads to a smaller final grain structure as there is more driving force. A lower temperature and/or time is necessary.
The temperature of annealing – A higher temperature speeds up the process but also leads to larger final grain size.
So what are the preferred conditions? A lower temperature and annealing for longer give more control. The change in microstructure and increase in grain size is slower, so we can avoid a large grain microstructure (which is less hard and, in extreme, less ductile). We do not want to over-anneal our sample!
Practice: The effect of composition
The composition of an alloy is important. Some compositions can be work-hardened more than others, and the temperature necessary for recrystallization generally scales with the alloy's melting point and inversely with its hardness (a higher temperature is needed for harder alloys).
While we can measure the temperature, in practice, Goldsmiths and Silversmiths use the color of the alloy as a guide to its' temperature.
Alloy | Ideal Temperature / ˚C | Color |
Pure 24K Gold | 200 | Black heat |
18 Karat Gold | 550–600 | Very Dark Red |
14 Karat Gold | 650 | Dark Red |
White Gold | 650–750 | Cherry Red |
Sterling Silver | 600–650 | Dark Red |
950 Platinum | 800 | Bright Red |
950 Palladium | 770–800 | Bright Red |
What happens during Hot Working?
During hot working, recrystallization occurs while we are working. As a result, the material remains soft and ductile as any work hardening and change in microstructure that occurs during working are quickly reversed by recrystallization.
Key Points
The key points are as follows:
When we deform a metal, layers of atoms slide over one another, leading to a change in the shape and orientation of the grains and a macroscopic shape change.
Due to the change in microstructure, the alloy's hardness increases, and further ductility is reduced. This is known as work hardening.
By reheating the alloy to a high enough fraction of its melting point, the microstructure can return to large, equiaxed grains. Strength is reduced, and ductility is restored.
Comments