Beryllium copper alloys, often referred to as beryllium-copper or BeCu, are widely used in various industries due to their unique combination of strength, electrical conductivity, and corrosion resistance. These properties are achieved through a series of heat treatment processes that enhance the alloy's mechanical and electrical characteristics.
The main steps for the solubilization treatment are:
Heating : the alloy is heated up to the solution temperature T s , where the equilibrium conditions allow the solid solution of the two phases.
Maintenance above T s : the treatment must be carried out for a sufficient time in order to obtain the complete dissolution of phase q and to achieve chemical homogenization.
Rapid cooling : rapid cooling is then carried out to obtain a supersaturated solution thus avoiding the precipitation of phase q (see img.1).
Img. 1 : Main stages of the solubilization treatment
The heating phase is very important because there must be no temperature gradients inside the material, as a result of which tensions would arise which could cause distortions in the piece. The characteristic parameters of the heat treatment (T s , T eq , treatment duration and heating rate) for each type of alloy are established by the standard.
In the heat treatment, the nucleation of the q phase , or of any other heterogeneous phase at the grain boundary, must not be favoured, it is for this reason that rapid cooling is carried out; in this way the composition of the solution is "frozen", and a supersaturated solution is obtained.
From an energy point of view, a supersaturated solution does not correspond to a minimum of free energy, in this way the condition that has been reached is a metastable condition, which corresponds to a value of D G greater than the D G min and this will be the driving force which will allow the precipitation of phase q
SOLUBILIZATION OF THE CUBE ALLOY
Solubilization is achieved by bringing the beryllium copper alloy to a temperature slightly below the solidus temperature (img 1), this allows the maximum quantity of beryllium to be dissolved, without melting the piece. Rapid cooling to room temperature is then carried out in order to obtain a supersaturated solution in beryllium.
The solubilization temperature depends on the type of alloy; in the case of high-strength it is 760-800°C, for high-conductivity it is 900-955 °C.
Temperatures below these ranges can lead to incomplete recrystallization and dissolution of insufficient beryllium to meet the required cure. Temperatures above, as well as too long treatment times at appropriate temperatures, can lead to an enlargement of the grain, or even to melting of the treated piece, but this only in the case of temperatures that are too high
The duration of the treatment depends on the size of the piece; a uniform temperature must be obtained to ensure homogeneous solubilisation.
The treatment lasts approximately one hour per inch of thickness, this guarantees that the pre-set temperature is reached. Prolonging the solubilization time does not increase the amount of beryllium solubilized, but may lead to an undesirable secondary effect of grain enlargement.
Too slow or interrupted coolings should be avoided because they allow the precipitation of the beryllium which causes too high a hardness and a poor result of any subsequent aging treatments.
The solubilization treatment to which these alloys are subjected is in contrast to those used for other copper alloys, normally conducted at low temperatures and for long times, this because the quantity of beryllium and its dispersion affects the electrical conductivity.
Img 1.1: State diagram of the CuBe alloy .
The aging treatment is the one which allows a homogeneous precipitation of phase q .
The rapid cooling, which leads to a supersaturated solution, induces a very high stress state in the material due to the temperature gradients that are established during phase C (cooling), see -img 1.
Another phenomenon linked to fast cooling is the freezing of the number of vacancies which are in thermodynamic equilibrium at T s ; and thus are found at room temperature. In this way the material is homogeneously tensioned thus favoring a homogeneous precipitation.
The temperature at which aging is carried out must be such as to allow the phenomenon to occur in not too long times, ie the kinetics of the process must be reasonably fast.
The aging treatment therefore has the aim of increasing the hardness of the material, which occurs by nucleation and growth of the precipitates.
Img 2.1 shows the effect of this precipitation on hardness.
However, the graph has a completely general trend and there will be variations depending on the material.
Img 2.1: Hardness-time diagram
In the hardening process we can identify some characteristic areas:
GPI - clusters of solute : small aggregations of solute called GPI zones. Initially they are few and cause a modest increase in hardness.
GPII - Over time, these clusters increase both in number and in size with a consequent increase in hardness.
q I - coherent precipitate .: a precipitate of phase q is formed whose stoichiometric composition is the correct one but the crystalline structure is still that of the matrix in which it was formed. Hardness continues to increase
q II - incoherent precipitate : in this stage of the treatment the incoherent precipitate q I changes its structure becoming that of phase q , which precisely causes hardening.
Overaging - Overaging : at this point the only thing the material can do is to enlarge the grain, with a consequent decrease in the mechanical properties.
What is the driving force that causes precipitation and accretion from phase q to occur ?
The process must be thermodynamically favored, and, as already mentioned, the temperature at which the aging is carried out must be high enough for the phenomenon to be observable but always lower than the solubilization temperature.
When solute clusters precipitate, these being defects, the lattice of the matrix is distorted, thus "latticular" tensions arise which increase proportionally with the size of the precipitate, this means that associated with the cluster there is always a considerable stress field, due to the elastic deformation of the lattice of the matrix, and therefore in order for incoherent precipitation to occur, a certain amount of energy called DG(strain) must be supplied .
Associated with the phase transformation a - a + q there is instead a driving force due to the fact that the solute concentration is supersaturated and tends to make it precipitate, DG > DG min , which can be considered as volume energy DG vol . , and a dissipated energy , which is used to create the new surface of the precipitate when it becomes incoherent DG sup .
In img 2.2 shows the trend of the free energy as a function of the temperature and of the monophasic or biphasic structure and the related DG .
The complete energy balance is written as
Initially D G(sup.) = 0 since the precipitate is coherent and I don't have to expend energy to create new surfaces. In this first phase DG (volume) supplies the energy necessary for the precipitates to grow by opposing DG (strain).
At some point the driving force would run out, but now the transition from consistent to inconsistent precipitate occurs with D G (strain).= 0 and the driving force DG is expended to create new surfaces.
Img 2.2: Trend of G with respect to T
AGING OF THE CUBE ALLOY
The aging treatment of the alloy involves heating up to a temperature just below T eq , to generate the nucleation and growth of the beryllium precipitates which are responsible for the hardening. Cooling takes place in the air.
The aging temperature is between 260-400°C and the time varies between 0.1 – 4 h in the case of high-strength and between 425-565°C for 0.5-8h for high-conductivity.
Img 2.2 shows the hardness trend as a function of aging time.
The aim of our experience will be to verify the trend of the curve in img 2.3, in particular to arrive at the aging time for which the hardness is maximum and to verify its subsequent decrease.
The solubility of beryllium in copper decreases with temperature and, therefore, these alloys are precipitation hardenable. For example, the precipitation sequence of a CuBe alloy is as follows:
Homogeneous nucleation of GP
Precipitation of g ¢ ¢
Precipitation of g ¢
The cast alloys have dendrites of Cu a and particles of intermetallic Be of about 10 -6 m (in the largest dimension).
The morphologies that occur are different according to the solidification interval:
Primary Be, which is formed during the first solidification, has a "Chinese script" morphology
Secondary Be, which is formed after solidification of the primary phase, has a columnar morphology
The b phase, which forms at the peritectic, has an interdendritic network around the Cu-rich a phase. This phase decomposes into phases a and g during the eutectic transformation on cooling to a temperature .
Subsequent thermomechanical treatments reduce the primary Be to a dispersion of small nearly spherical particles and usually dissolve the b-phase.
Coherent precipitates in both high-strength and high-conductivity have dimensions such that they cannot be resolved by optical methods, therefore they must be searched for by electronic transmission methods.
Img 2.3 : Trend of the properties of the CuBe alloy, in particular the hardness.
The cryogenic process consists in lowering the temperature of an object to about 88K (-185°C), this operation serves to eliminate the tensions that have arisen during the casting phase of the piece. The cryogenic treatment produces an alignment effect in the molecular structure and makes the structure almost isotropic.
The cryogenic treatment lowers the kinetic energy of the molecules, this is responsible for the interatomic distance which, once the temperature at which the treatment is carried out is reached, is almost zero and allows the molecules to approach each other.
Cryogenic treatment increases wear resistance, creates a denser molecular structure that provides a larger contact surface area to reduce friction. The treatment not only changes the external surface of the piece, but the entire structure, increases the life of the piece and decreases its fragility.