Chemical and Mechanical Properties of Titanium and Its Alloys

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Introduction to Titanium

Titanium stands as an important transition element in modern materials science, featuring a composition of several isotopes with atomic weights ranging from 46 to 50. Through precise spectrographic analysis and mathematical calculations, scientists have determined titanium's mean atomic weight to be 47.88. This fundamental characteristic underpins the many remarkable properties that make titanium valuable across numerous industries.

Chemical Properties of Titanium

Isotopic Composition and Nuclear Characteristics

Titanium naturally consists of multiple isotopes with atomic weights between 46 and 50. Beyond these stable isotopes, researchers have identified five additional titanium isotopes. Titanium-43 emits beta positive particles with a brief half-life of 0.58 seconds. Titanium-45 exists in two forms: one emitting beta positive and gamma radiation with a 3.08-hour half-life, and another with a 21-day half-life. Titanium-51 has a 72-day half-life, emitting beta negative and gamma radiation, with a meta-stable form lasting 6 minutes while similarly emitting gamma and beta negative radiation.

Valence Properties

As a characteristic transition element, titanium exhibits variable valence, commonly appearing in bi-, tri-, and tetra-valent states. Although scientific literature has reported valences of five and higher, these claims remain unsubstantiated by experimental evidence.

Reactivity with Gases

Titanium's chemical reactivity significantly increases with temperature. Its interactions with atmospheric gases become particularly pronounced at elevated temperatures, necessitating inert atmospheres during hot working and surface protection for high-temperature applications. Above 950°F (510°C), titanium rapidly combines with reactive atmospheric gases, forming surface scale. With extended exposure and higher temperatures, these gases diffuse into the metal's lattice structure.

When titanium combines with oxygen, it forms a series of oxides ranging from TiO to Ti7O12, each producing different colors that create a rainbow-like surface film during brief exposures. While surface oxidation begins at 950°F, significant lattice diffusion requires temperatures above 1300°F (704°C). The metal ignites in air at 2200°F (1204°C), with this ignition temperature dropping to 1130°F (610°C) in pure oxygen environments.

Titanium's reaction with nitrogen resembles its oxygen interaction, forming a yellow-brown nitride scale on the surface. Nitrogen penetrates the lattice to a limited depth, a property utilized in nitride casing processes.

The hydrogen-titanium reaction demonstrates unique characteristics, occurring at temperatures slightly above room temperature. One gram of titanium can absorb up to 400 cc of hydrogen. At low concentrations, hydrogen occupies interstitial positions, while higher concentrations form titanium hydride (TiH). However, this hydrogen addition remains stable only below 680°F (360°C); above this temperature, hydrogen is released and combusts.

All gas-titanium reactions accelerate under decreasing vapor pressures, requiring complete atmospheric protection. At temperatures exceeding 1500°F (816°C), titanium decomposes water vapor to form oxide and release hydrogen. At higher temperatures, the metal absorbs carbon dioxide, potentially forming oxide and carbide compounds.

Reactivity with Acids

Titanium reacts vigorously with halide acids, particularly with fluorides. This reaction serves multiple purposes: dissolving the metal and its alloys for chemical analysis, providing a general etchant for metallographic work at both macro and micro scales, and functioning as a descaling agent.

Hydrochloric and sulfuric acids react slowly with titanium at room temperature. Moderate heating accelerates these reactions, forming lower chlorides and mono-sulfate. These reactions offer similar applications to hydrofluoric acid but with reduced toxicity and corrosiveness, gradually becoming preferred alternatives in industrial settings.

Interactions with Organic Materials

The metal industry has only minimally exploited titanium's reactivity with organic materials. Organic acid-titanium reactions create colored films on the metal's surface, utilized by metallographers for stain-etching microspecimens.

Reactions with Solids

In its molten state, titanium readily combines with various metals, metalloids, and carbonaceous materials, forming important compound systems. In oxide form, it reacts with alkali, alkaline earth, and heavy base metals to create titanates, some of which are being investigated for more economical production methods.

Titanium's reactivity with metalloids, particularly metal oxides, presents significant challenges in foundry applications. Molten titanium severely attacks most standard refractories, including silicon dioxide and aluminum oxide, making their use hazardous. Among all metalloids, only beryllium oxide and thorium oxide demonstrate appreciable resistance to liquid titanium.

The carbon-titanium reaction holds particular importance. Molten titanium exhibits strong affinity for carbon, which can severely compromise the properties of the finished metal. Therefore, manufacturers must exercise extreme care to minimize carbon contamination in titanium products.

Electrochemical Properties

While titanium can be electrodeposited through various complex methods, none currently produce industrially viable films. Electrolytic techniques have successfully reduced the metal from its tetravalent state to bi- and trivalent forms using acid electrolytes with electrodes made of lead, copper, platinum, or mercury jet.

Safety Considerations

Titanium's chemical reactivity generally presents minimal hazards. With two exceptions—finely divided particles and metal exposed to fuming nitric acid for extended periods—titanium has not demonstrated explosive or flammable properties.

Mechanical Properties of Titanium

Tensile Strength and Ductility

Unalloyed titanium exhibits a wide range of tensile strengths: from 250 MPa (35,000 psi) for high-purity metal produced via iodide reduction to 690 MPa (100,000 psi) for material produced from high-hardness sponge titanium. Arc-melted commercially pure titanium products typically demonstrate ductility ranging from 20% to 40% elongation and 45% to 65% reduction in area, depending on interstitial content. Higher purity iodide process titanium achieves 55% elongation with 80% reduction in area.

Like steel, titanium's strength increases through alloying with metals such as aluminum, vanadium, chromium, iron, manganese, and tin, used either as binary additions or in complex systems. While these additions enhance strength, they typically reduce ductility.

Elastic Properties

Unalloyed titanium has an elastic modulus of approximately 15×10^6 psi, which can increase to about 18×10^6 psi through alloying. This modulus compares favorably with aluminum (10.4×10^6 psi) and magnesium (6.4×10^6 psi) but falls below steel (29×10^6 psi). Similarly, titanium's shear modulus (modulus of rigidity) positions it between aluminum and steel.

Hardness Characteristics

Titanium significantly exceeds aluminum in hardness and approaches the high hardness values of some heat-treated alloy steels. High-purity iodide titanium registers a hardness of 90 VHN (Vickers Hardness Number), while unalloyed commercial titanium measures approximately 160 VHN. When alloyed and heat-treated, titanium can achieve hardness values between 250 and 500 VHN. A typical commercial alloy with 130,000 psi yield strength might exhibit a hardness of about 320 VHN or 34 Rockwell C.

Impact Resistance and Toughness

Toughness represents a critical engineering property beyond tensile strength and ductility. Titanium belongs to a select group of metals capable of simultaneously exhibiting good toughness, high strength, and ductility.

Titanium's impact strength varies widely: exceeding 100 foot-pounds Charpy for higher-purity iodide products, reaching 30 foot-pounds for commercial unalloyed material, and dropping to 1-2 foot-pounds for certain high-strength but brittle alloys.