To be a versatile engineering material, a metal must offer a wide range in its mechanical
properties to meet industrial requirements. Such a variation is accomplished in the
mechanical properties of titanium by alloying and heat-treatment. Since neither one
alloy nor one heat-treatment alone is capable of perfecting a metal with properties
meeting all the demands of industry, alloying and heat-treatment have become major
tools in the production of titanium materials.
Alloying
Research and development have shown that alloying can raise the tensile strength of
titanium metal to more than 200000 psi (1380 MPa) while still maintaining useful
ductility. The presence of interstitial elements, mainly carbon and the reactive gases
of the atmosphere, also add strength to the metal but at the expense of severe loss in
ductility. For simplicity the interstitial elements carbon, nitrogen, oxygen, boron,
and hydrogen will be referred to as contaminants, and the substitutional elements,
intentionally added, will be referred to as alloying elements.
Contaminants. Contaminants remain in titanium metal from incomplete
purification in the reduction process or are absorbed in the melting practices employed.
Iron in small qualities, 0.5 to 1%, appears to have a contaminating effect on ductility,
but in larger quantities acts to influence the ductility no worse than other good
substitutional alloying elements. The interstitials and iron are introduced in the
sponge production reactions; in the melting practice, carbon, nitrogen, and oxygen
contents may be further increased.
One major problem confronting both the sponge and wrought producer is to eliminate these
impurities or at least keep them at a minimum. Present production of titanium metal has
kept nitrogen and hydrogen contents, in general, below the critical combining quantity
where reduction in ductility becomes marked. Hydrogen, carbon, oxygen, and, in some cases,
iron, however, are still found frequently in titanium in proportions which are intolerable
to the user.
Carbon and oxygen appear to have a combined effect on ductility and toughness. A low
quantity of one will allow a greater toleration of the other. Large variations
(0.03%-0.20%) have been noted in oxygen contents of commercially produced metal,
and carbon contents up to 0.2% still result, although most titanium metal currently
arc-melted is below 0.1% in carbon.
Nitrogen has been maintained generally below 0.05% in commercial production, and this
quantity does not influence severely tile strength or ductility. With nitrogen contents
above this, strength rises sharply and ductility falls off as severely as occurs with
oxygen. The effect of boron, which is only slightly soluble in titanium, has not been
thoroughly investigated.
Recently the titanium industry has become aware that hydrogen is a major factor in
embrittling titanium. Most alloys cannot tolerate more than 200 parts per million of
hydrogen, particularly if the material is to be subjected to fatigue or creep loading.
Hydrogen can be substantially reduced by vacuum annealing, but this process does not lend
itself to production economics.
Carbon, oxygen, nitrogen, and hydrogen, although they increase the strength of titanium,
adversely affect the ductility and toughness so severely that these elements are kept at
a minimum and are rarely employed as alloy additives.
Alloy Additives. To increase the strength of titanium metal and still
maintain useful ductility, substitutional elements are employed. These elements replace
titanium atoms in the lattice structure rather than situate in the voids between them,
as do the interstitials.
By the utilization of basic physical metallurgical studies such as equilibrium diagrams
of various alloy systems and by practical alloy development work, several substitutional
elements have emerged as promising alloy additions. Manganese, aluminum, chromium, tin,
iron, vanadium, and molybdenum in various combinations have been shown to lend versatility
to tile mechanical properties of titanium metal.
These alloying elements increase the strength of titanium with an accompanying loss
in ductility and toughness. The ductility and toughness, however, are far less influenced
by these elements than by the contaminants, where increased strength is gained at a great
sacrifice of ductility and toughness.
Heat-treatment
The mechanical properties of titanium are more dependent on the phases present than
they are on the actual composition of the alloy. Substitutional elements partially replace
the titanium atoms in the lattice and in this manner alter the properties. In actuality,
the amount of any and all phases present is better governed by the heating and cooling
cycles than by this atom alteration.
Most alloy additives stabilize the body centered beta phase and lower the temperature
of transformation to such an extent that at room temperature the alloys are a mixture
of both the alpha and beta. The hexagonal alpha is relatively soft, tough, and ductile;
whereas the beta is harder, stronger, but less ductile.
From this it can be seen that by changing the proportions of these phases, the mechanical
properties can be varied. Many methods have been employed to produce the desired phase
proportions, and from these have emerged five basic methods of heat-treatment: quenching,
tempering, continuous cooling, isothermal transformation, and solutionizing and aging.
Quenching. If alloys are rapidly cooled by water quenching from the all
beta region, the tendency of the alpha phase to form is suppressed, and the beta phase is
retained. Certain alloy compositions, however, exhibit a peculiar transformation on
quenching. This mechanism of martensitic or shear-like transformation is not completely
understood. The formation of this structure, the so-called alpha prime, causes some
distortion of the lattice. This distortion and the resulting strain produce a material,
which is hard and tough, and possesses better fatigue properties than alpha. This
quenching process is also the initial point for tempering.
Tempering. When titanium is quenched from an elevated temperature,
reheated to a temperature below the beta transus, held for a length of time and again
quenched, it is said to have been tempered. Three variables exist in tempering: the
phases present, the time held, and the tempering temperature.
When the initial structure contains alpha prime, two changes occur: the alpha prime
transforms to alpha, and at longer times the alpha becomes serrated. The result is a
loss of hardness and strength and an increase in ductility and impact. Alpha-beta
structures, however, do not follow this pattern. The alpha primarily remains unchanged;
the beta decomposes to form more alpha at the expense of the beta phase. At low
temperatures more alpha, will be formed; thus, low tempering temperatures result in a
greater decrease in strength and hardness and a larger increase in ductility than the
high temperature tempering over identical time intervals.
Solutionizing and Aging. If a titanium alloy is held in the beta or
high in the alpha-beta region, quenched, and then reheated again to the alpha-beta region,
it is said to have been solution-treated and aged. This treatment on titanium alloys
produces much the same effect as tempering, with the exception that the initial structure
is, for the most part, beta. Maximum hardness can he achieved in short-time aging, which
is associated with the formation of a phase, referred to as beta prime. With longer times
this beta prime is dissipated and alpha-precipitated, decreasing the hardness and
resulting in better ductility.
Isothermal Transformation. On hot-quenching an alloy from the all beta
region to temperatures in the alpha-beta field and holding for a period of time and then
further quenching to room temperature, the material is transformed isothermally. Treatment
in this way causes precipitation of the alpha phase from the beta. At high temperatures
the alpha precipitates first at grain boundaries and later within the beta grains
themselves.
This treatment, when holding at temperatures just below the transformation temperature,
at first gives a very hard material due to formation of beta prime. If the time of holding
is extended, the hardness and strength decrease with an accompanying increase in ductility
and toughness. At lower temperatures a gradual rise in hardness and brittleness takes
place, and at prolonged times a higher hardness may be obtained than by short time high
temperature treatments.
Continuous Cooling. Continuous cooling is the lowering of the temperature
of an alloy from the all beta field at any rate without interruption or subsequent
reheating. Quenching, already discussed, is a specialized form of continuous cooling.
The cooling rate, although not associated with one temperature, governs the interval of
the transformation period. Rapid cooling rates suppress alpha formation and result in the
beta phase being at least partially retained, which gives a moderately hard material.
Slightly slower cooling rates result in a much harder and brittle metal of the same type
previously referred to as beta prime. Slower rates give alpha-beta structures. The slower
the rate, the greater the amount of alpha formed. As alpha increases, the ductility and
toughness increase and hardness falls off.
When high hardness is the ultimate need, the material must be treated in such a way
that the peak of the curve is reached. High hardness throughout the piece is best
obtained by quenching a rich alloy, which falls to the left of the peak, and then
tempering at low temperatures until the peak is reached.
When toughness is the prime factor, it is best obtained by quenching the lean alloys,
which fall far to the right, from just below the beta transus. Such treatment gives low
yield strength, but high impact strength. Some increase in yield strength can be obtained
if these alloys are hot-worked in the alpha-beta region prior to quenching.
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