ISO 28401:2024

International
Standard
ISO 28401
Second edition
Light metals and their alloys —
2024-08
Titanium and titanium alloys —
Vocabulary
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Material .1
3.2 Classification of phases and related terms .2
3.3 Classification of microstructure by morphology and related terms.2
3.4 Unwrought products .3
3.5 Wrought products.4
3.6 Castings .6
3.7 Methods of processing and treatment .7
3.8 Surface condition .9
3.9 Applications.10
3.10 Types of titanium materials and related terms.10
3.11 Titanium grades and compounds .11
Annex A (normative) Dividing line between unalloyed titanium and titanium alloys .12
Bibliography . 14
Index .15

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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This document was prepared by Technical Committee ISO/TC 79, Light metals and their alloys, Subcommittee
SC 11, Titanium.
This second edition cancels and replaces the first edition (ISO 28401:2010), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the title was changed from “Classification and terminology” to “Vocabulary”;
— new terms were added;
— some sentences were revised for clarity;
— the notations alpha and beta were changed from English to Greek;
— some abbreviated terms were removed;
— Annex A was revised and tables were added as additional normative text;
— Annex B was removed.
A list of all parts in the ISO 28401 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
There are many technical terms related to titanium used in national standards.
Unifying and interpreting these technical terms worldwide, so that specifications can be understood
accurately around the world, is essential for international trade in common titanium products.
There is a need to classify technical terms related to titanium and establish a common interpretation of
each term.
v
International Standard ISO 28401:2024(en)
Light metals and their alloys — Titanium and titanium alloys
— Vocabulary
1 Scope
This document defines terms and definitions related to titanium and titanium alloys.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Material
3.1.1
titanium sponge
products of metallic titanium in a porous and sponge-like form, which are applied as titanium metal
melting stock
Note 1 to entry: To produce titanium sponge, oxidized titanium ore is chlorinated to tetrachloride and is condensed
and purified. Then the product is reduced with magnesium or sodium under an inert atmosphere.
3.1.2
alloy
metallic substance consisting of a mixture of the basic metallic element and other elements, such as alloying
elements and impurities
Note 1 to entry: In this document, the most predominant element by mass fraction is titanium.
3.1.3
alloying element
metallic or non-metallic elements intentionally added to, or retained by, base metal to give special properties
3.1.4
impurity
metallic or non-metallic elements which are present but not intentionally added to, or retained by, a metal
3.1.5
wrought alloy
alloy primarily intended for the production of wrought products by hot and/or cold plastic forming
3.1.6
casting alloy
alloy primarily intended for the production of castings

3.1.7
master alloy
alloy intended for alloying elements added to molten or compacted titanium by controlling physical
properties such as melting point, densities and dissolvability
3.1.8
heat-treatable alloy
alloy capable of being strengthened by a suitable thermal treatment
3.1.9
non-heat-treatable alloy
alloy capable of being strengthened only by cold working rather than by thermal treatment
3.2 Classification of phases and related terms
3.2.1
α phase
solid solution at low temperature with a hexagonal closed packed crystal structure
3.2.2
α phase stabilisers
alloy elements such as aluminium, oxygen, nitrogen and carbon that expand the α phase, which is the low
temperature phase of titanium alloy, to high temperature and enhance the stability of the α structure
3.2.3
β phase
solid solution at high temperature with a body centred cubic crystal structure
3.2.4
β phase stabilisers
alloy elements such as iron, manganese, molybdenum and vanadium that expand the β phase, which is the
high temperature phase of titanium alloy, to low temperature and enhance the stability of the β structure
3.2.5
α plus β phase
mixture of the α and β phases
3.2.6
β-transus temperature
temperature above which the crystal structure turns to the β phase
3.3 Classification of microstructure by morphology and related terms
3.3.1
microstructure by morphology
microstructure observed by an optical and/or a scanning microscope characterized by composition,
processing and heat treating
Note 1 to entry: See Reference [2] for detailed classifications of titanium and titanium alloy microstructures, including
microstructure photographs.
3.3.2
equiaxed α structure
polygonal or globular α structure with approximately equal dimensions in all directions
Note 1 to entry: In some types of α-β titanium alloys, most α structures are observed in a globular rather than equiaxed
form. Therefore, this structure is sometimes called the globular α structure.

3.3.3
acicular α structure
microstructure in which β transforms the selected crystal planes by nucleation and growth or martensitically
Note 1 to entry: This microstructure is also called the Widmansttäten α structure.
Note 2 to entry: Depending on the aspect ratio and the existence state in the colonies, other names for this
microstructure are platelet α structure, blocky α structure, basketweave α structure and lamellar α structure.
3.3.4
grain boundary α structure
α structure formed at prior β grain boundaries during cooling from the single β phase region
3.3.5
elongated α structure
grain microstructure in which the length of one of the three axial directions of the crystal grains is
remarkably elongated
Note 1 to entry: The structure shows that the influence of plastic working remains strong or that it has failed to
become equiaxed and remained elongated.
Note 2 to entry: The string-like elongated structure is sometimes called stringy α structure.
3.3.6
bimodal structure
microstructure composed of equiaxed or elongated α structure embedded in transformed β matrix
3.4 Unwrought products
3.4.1
unwrought product
general term for products obtained by either melting, casting or powder metallurgy processes or a
combination thereof
EXAMPLE Ingots for rolling, ingots for extruding, ingots for forging and ingots for remelting.
3.4.2
electrode for remelting
cylindrical or rectangular semi-product formed from titanium sponge and alloying elements or consolidated
from recycled and processed scrap, to be remelted once or multiple cycles in a vacuum or in inert gas for at
least the final melting
3.4.3
ingot for rolling, extruding and forging
titanium and titanium alloy ingot melted mostly using the vacuum arc remelting (VAR) method applied for
at least the final cycle in a vacuum or in inert gas, suitable for rolling, extruding and forging
Note 1 to entry: The melting method is not necessarily limited to VAR. It also includes cases that involve electron beam
melting (EBM), plasma arc melting (PAM) and others.
Note 2 to entry: The VAR method is the typical method used for melting titanium and titanium alloys. In this method,
the titanium or titanium alloy electrode is dissolved while being consumed itself by an arc generated between a
consumable electrode and a water-cooled copper crucible in a vacuum or in an inert gas environment.
Note 3 to entry: The EBM method is also called electron beam c
...