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Relationship between the hardness of titanium alloy and its tensile strength

There is a certain correlation between the hardness of titanium alloy and its tensile strength, but this relationship is not very close, as shown below:
1. General correlation
The hardness of titanium alloy (such as Vickers hardness HV) and tensile strength (σ) are usually positively correlated, and in some studies, empirical formulas can be used for approximate conversion, such as HV ≈ 3 × σ (MPa), that is, the hardness value is about one-third of the tensile strength. This means that an increase in hardness is often accompanied by an increase in tensile strength.

2. Diverse influencing factors
The hardness and tensile strength of titanium alloys are affected by multiple factors such as alloy composition (such as Al, V, Fe, etc.), heat treatment process, phase structure (α phase, β phase ratio), and microstructure. Different titanium alloy grades (such as TC4, TC21, TB8, etc.) have different hardness and strength ranges.

3. Strength improvement does not necessarily lead to a significant change in hardness
The strength of titanium alloys can be significantly improved through heat treatment processes such as solution treatment and aging, and even reach ultra-high strength (tensile strength exceeds 1250 MPa), but the hardness improvement is relatively limited. This shows that the improvement mechanisms of hardness and strength are different. Hardness focuses more on the ability of the material surface to resist indentation and wear, while strength reflects the overall ability to resist plastic deformation.

4. Relationship in practical applications
In actual engineering, although the hardness of titanium alloys is related to tensile strength, hardness cannot fully represent the strength performance of the material. It is necessary to combine other mechanical indicators (such as elongation after fracture and fracture toughness) to comprehensively evaluate the material performance. For example, the hardness of some high-strength titanium alloys has limited improvement, but its toughness and fatigue properties have been significantly improved.

5. Research examples
• The tensile strength of TC4 titanium alloy is generally 825~900 MPa, and the hardness is about HV280-340.
• By regulating heat treatment, the tensile strength of some β titanium alloys can reach 1.8 GPa while maintaining good plasticity, but the hardness improvement is not as large as the strength.
• When using hardness to convert tensile strength in composite materials, the theoretical value is usually higher than the actual measured value, indicating that the relationship between hardness and strength is affected by the material microstructure and crack behavior.

Summary: There is a certain positive correlation between the hardness and tensile strength of titanium alloys. The change in hardness can reflect the trend of some strength changes, but the relationship between the two is not very close and is affected by multiple factors such as alloy composition, heat treatment, and microstructure. When selecting materials and evaluating performance, hardness, strength and other mechanical performance indicators should be considered comprehensively.