Question

Which of these statements about alloys and intermetallic compounds is false? (a) Bronze is an example of an alloy. (b) "Alloy" is just another word for "a chemical compound of fixed composition that is made of two or more metals." (c) Intermetallics are compounds of two or more metals that have a definite composition and are not considered alloys. (d) If you mix two metals together and, at the atomic level, they separate into two or more different compositional phases, you have created a heterogeneous alloy. (e) Alloys can be formed even if the atoms that comprise them are rather different in size.

Short answer

The false statement is (b). An "alloy" is not just another word for "a chemical compound of fixed composition that is made of two or more metals." Alloys are mixtures of metallic elements and do not necessarily have a fixed stoichiometry, unlike intermetallic compounds.

Step-by-step solution

Understanding Alloys

An alloy is a mixture of two or more metallic elements, which can enhance the properties of the constituent metals. Alloys often have better strength, durability, or melting point compared to their pure metallic components.

Statement (a)

Bronze is an example of an alloy. This statement is true, as bronze is an alloy made primarily of copper and tin.

Understanding Intermetallic Compounds

Intermetallic compounds are a type of solid-state compound formed between two or more metals with a definite composition and crystallographic structure. They may have metallic or non-metallic properties, and they can exhibit specific phase behavior.

Statement (b)

"Alloy" is just another word for "a chemical compound of fixed composition that is made of two or more metals." This statement is false. An alloy is a mixture of metallic elements, whereas intermetallic compounds are compounds of two or more metals with a definite composition. Alloys do not necessarily have a fixed stoichiometry, unlike intermetallic compounds.

Statement (c)

Intermetallics are compounds of two or more metals that have a definite composition and are not considered alloys. This statement is true. Intermetallic compounds have specific compositions, while alloys can have a range of compositions.

Statement (d)

If you mix two metals together and, at the atomic level, they separate into two or more different compositional phases, you have created a heterogeneous alloy. This statement is true. Heterogeneous alloys display multiple phases or regions within the material, making it inhomogeneous in composition on a microscopic scale.

Statement (e)

Alloys can be formed even if the atoms that comprise them are rather different in size. This statement is true, as alloys can often combine metals with different atomic sizes. This combination might lead to solid solution hardening or the formation of distinct phases that can enhance the mechanical properties of the alloy. The false statement among the given options is (b). An alloy is not simply another word for "a chemical compound of fixed composition that is made of two or more metals." Alloys are mixtures of metallic elements, and their composition may vary, while intermetallic compounds have a definite composition and crystallographic structure.

Key concepts

Intermetallic Compounds

Intermetallic compounds are fascinating structures formed when two or more metals combine to create a new compound with distinct properties. Unlike regular mixtures, these compounds have a specific, definite composition and a well-ordered crystallographic structure. This means their atomic arrangement is highly organized and does not change, giving them unique mechanical and chemical characteristics.
Intermetallics can exhibit both metallic and non-metallic properties, depending on their composition. For example, some might conduct electricity well, like a typical metal, while others might resemble ceramics in terms of brittleness. This makes intermetallic compounds very versatile in engineering.
While they share some similarities with alloys, the key is their fixed stoichiometry and distinct phase behavior. They do not vary in composition like alloys, making them less flexible but highly specialized for specific applications, such as in advanced aerospace materials.

Heterogeneous Alloy

When different metals are combined to form a heterogeneous alloy, the resulting mixture features multiple distinct regions, or phases. These phases can vary in composition and structure, visible even under a microscope. This is why they are termed 'heterogeneous,' meaning they are not uniform throughout.
Heterogeneous alloys are different from homogeneous alloys, where only one phase is present and the components are uniformly distributed. The multiple phases in a heterogeneous alloy allow it to exhibit diverse properties, which can be leveraged to meet specific engineering needs.
These characteristics often result from the separation of metals at the atomic level. Controlling phase distribution is essential in metallurgy to ensure that the alloy performs as required in its application. This makes heterogeneous alloys suitable for uses where varied mechanical properties are beneficial, such as in construction materials or specialized industrial equipment.

Solid Solution Hardening

Solid solution hardening is a technique used to strengthen materials by adding alloying elements. When a solute metal is dissolved within a solvent metal, the resulting mixture forms a solid solution. This process disrupts the orderly arrangement of the solvent metal's atoms, adding more resistance to deformation.
The added atoms cause lattice distortion because they either larger or smaller than the host metal atoms. This distortion makes it difficult for dislocations, which are defects in the crystal structure, to move through the metal. As a result, the material becomes harder and stronger.
There are two types of solid solutions: substitutional and interstitial. In substitutional solid solutions, solute atoms replace some of the solvent atoms. In interstitial solid solutions, smaller solute atoms fit into the spaces between the solvent atoms. Both types contribute to the hardening of the material, enhancing the mechanical properties of alloys used in various applications such as automotive parts, machinery, and even jewelry.