Not just a phase: challenges in patenting alloy inventions at the EPO

Alloy patents have long presented particular challenges under the European Patent Convention, in particular in relation to the assessment of clarity of claims to alloy inventions. Clarity is assessed in relation to Art. 84 EPC, which simply states that the claims must be “clear, concise, and supported by the description”. The provision is elegantly brief, but for alloys it has generated rigorously applied alloy-specific examination practice at the EPO that can often be challenging for applications drafted with other jurisdictions in mind.

An alloy is a mixture of elements which includes at least one metallic element. Alloys are highly sensitive to even minor compositional changes, which can dramatically alter their phases and properties. Steel is a classic example: adding just a few percentage points of chromium can transform plain carbon steel into stainless steel with radically different corrosion resistance.

Because of this compositional sensitivity, claims that use ‘open’ language such as

Alloy comprising elements X, Y, Z

and which therefore implicitly allow any number of further elements to be present in the alloy composition, might encompass alloys that fail to exhibit the properties central to the invention. 

(1) Purely open language using “comprising” is not permitted when claiming alloy inventions at the EPO

An alloy claim in the following form:

Alloy comprising elements X, Y, Z.

will almost certainly face a clarity objection. The problem is not the word “comprising” itself but the absence of any closure. Without a mechanism that indicates what the rest of the composition consists of, the claim admits unlimited further elements, in unlimited amounts. Because alloy properties are inherently linked to composition, the EPO is likely to find that such a claim omits essential technical features.

(2) The Balance-Clause Technique “Comprising/consisting of … the balance being [metal] (and impurities)” is accepted

A practical workaround employed by many European attorneys is to close the composition through a balance clause, e.g.:

Alloy comprising X, Y, the balance being Z.

EPO Boards of Appeal and examiners accept that where a balance is identified, the composition is inherently closed, irrespective of the use of “comprising” or “consisting of” in the introductory claim language.  This was confirmed in the EPO Board of Appeal case T 1563/10 “according to EPO practice, [‘comprises … the remaining mass Cu’] in alloy claims defines a ‘closed composition’ which… excludes the presence of further elements”, and was further confirmed by later cases T800/12 and T 0107/14.

It is also typically accepted to use a formulation that specifies the presence of some degree of impurity content: the EPO recognises that it is not possible in practice to manufacture alloys which do not contain impurity levels of at least some elements due to e.g. atmospheric contamination or the purity of the starting materials.  The following types of claim formulation can also be used:

Alloy comprising X, Y, the balance being Z and unavoidable impurities.

Alloy consisting of X, Y, the balance being Z and usual impurities.

Examiners typically do not require listing specific impurities or their amounts when using this type of claim formulation, unless the invention specifically revolves around ultra-purity or particular impurity limits.  However, if it is possible to define impurity limits, then this may be preferred, with the following type of claim formulation being a suitable option:

Alloy consisting of X, Y,…, other elements being <α each and <β total, and the balance being Z.

The explicit specification of “usual/unavoidable impurities” or enumerated amounts of impurity elements is strongly preferred in Europe over use of the ‘quasi-closed’ “Consisting essentially of” claim formation that is common in other jurisdictions such as the USA.  The use of “consisting essentially of” is likely to be objected to at the EPO, since it introduces borderline uncertainty about which additions are permitted.

(3) Independent claims must be self-contained 

In addition to requiring claims to use ‘closed’ claim language, the EPO also requires that independent claims are self-contained – that is, the composition (including optional elements) must be 100% defined in the independent claim itself.  

Importantly, the EPO does not allow optional alloying elements to be defined in dependent claims, since this creates a contradiction with the closed language needed for the independent claims.  Dependent claims can still refine ranges or specify sub-options (like particular impurity limits or preferred ranges for elements specified in the independent claims), but they shouldn’t introduce completely new components that weren’t in claim 1.  In other words, the following claim structure is not allowable at the EPO:

1. Alloy comprising X, Y, the balance being Z.
2. The alloy of claim 1, further comprising W.

Instead, to be allowable at the EPO, optional alloying elements should be explicitly included in the independent claims. They can either be explicitly stated as being “optional” additions, or alternatively, addition ranges can be presented with a range starting from 0% or stated as being “up to X%”.  The following types of claim formulation can be used, for example:

Alloy comprising X, Y, and optionally W, the balance being Z.

Alloy consisting of (in weight %):
X: 1–5 
Y: 0.5–3
W: up to 2
the balance being Z and usual impurities.

Alloy comprising (in weight %):
X: 1–5 
Y: 0.5–3
W: 0–2
other elements being <0.05 each and <0.2 total, the balance being Z.

This claim structure can pose some challenges for applicants since it is necessary to anticipate at the drafting stage what possible alloying elements should be included as optional components in the claims. The specification of an upper bound for “other elements” can be helpful in this regard to cover a wide range of minor alloying additions in a general manner and with a degree of flexibility.  

Care should also be taken to ensure that the description does not cast any doubt on the scope of the independent claims by suggesting that other elements can be present outside of those defined in the claimed closed composition. Where possible, testing protocols for how the alloy composition can be accurately and consistently determined should also be included.

Clarity Beyond Composition: Microstructure Matters!

Although composition is critical, the properties of an alloy may also depend heavily on its microstructure.  If mechanical, magnetic, corrosion, or other technical behaviour of the alloy depends on a specific phase distribution, grain size, or heat-treatment-induced structure, defining the alloy by its composition alone may be insufficient.  The EPO may require that such microstructural features are included in the claims to comply with the EPO requirements on defining essential features of the invention in the claim (Guidelines F‑IV, 4.5.2).

Applications should therefore include clear descriptions of relevant microstructural features and the methods used to measure them. For example:

…wherein the average grain size is in a range of from XX µm² to YY µm², measured using [technique]  

This level of detail can help ensure that any parameter-based features are objectively measurable and reliably understood, which is particularly important given the EPO’s strict approach to parameter based definitions (Guidelines F-IV 4.11).

Process claims provide underutilised potential

Although applicants often prioritise traditional format product claims, process claims and ‘product-by-process’ claims can provide  valuable additional protection in the alloy space.

The topic of product-by-process claims can be complex, so we will consider the advantages and potential pitfalls of such claim formulations in detail in a future article rather than here. However, pure process claims can have significantly utility, since in the United Kingdom, and in many other jurisdictions, a claim directed to a process also provides protection for the products directly obtained from that process.  This means that a well drafted manufacturing or processing method can secure protection for an alloy even when a fully closed composition claim is difficult to define with precision.

Process claims are particularly useful where the alloy composition is best described by the proportions of starting materials rather than by final elemental percentages.  This situation commonly arises in processes such as steelmaking, where certain elements may be partially lost during melting or refining.  In such cases a method claim that specifies the input charge materials and their relative quantities can provide a clear and enforceable definition of the invention.

Applicants should also consider process claims that capture essential processing activities such as heat treatment steps or thermomechanical processes.  If the desired microstructure or phase distribution results from a specific processing route, a process claim may be the most straightforward way of protecting the invention without giving rise to difficult-to-address clarity objections.

For many alloy inventions, including both a closed composition claim and a supporting process claim ensures a more resilient and comprehensive protection strategy.

Summary

Successfully prosecuting alloy inventions at the EPO requires careful attention to the essential features that determine the alloy’s technical performance.  For applicants drafting with multiple jurisdictions in mind, this often means that substantial adaptation of claim sets is necessary to satisfy European clarity expectations.

A central requirement is ensuring that composition claims are genuinely closed. This required careful consideration of impurity terminology, the inclusion of optional alloying additions, and the use of balance clauses.  Addressing these points at the drafting stage can prevent many of the clarity objections commonly encountered during European examination.  

Where the invention also relies on microstructural or other non-compositional parameters, these should be defined with equal precision to ensure that all features essential to the alloy’s performance are reflected in the claims.

Process claims can provide valuable, and often underutilised, additional protection, particularly where the invention cannot be fully defined by a closed composition alone.  A well-considered combination of composition and process claims can therefore deliver a more robust and commercially meaningful protection strategy.

By anticipating these requirements early, practitioners can draft applications that meet the EPO’s strict clarity standards while still delivering strong and commercially meaningful protection for alloy inventions.  However, there is still much more to discuss – in future articles we will consider other factors relevant to alloy inventions including how best to draft applications to alloy inventions which satisfy the EPO’s support requirements and provide a basis for broad and enforceable claims.