What effects will mixed crystals have on the properties of materials?

Analysis of the Impact of Mixed Crystals on Material Properties

Mixed crystals, as an uneven phenomenon in the microstructure of materials, can negatively affect mechanical properties, physical properties, and service reliability from multiple dimensions. The specific impacts are as follows: 

I. Deterioration of Mechanical Properties

Inhomogeneity of Strength and Plasticity

Due to the fewer grain boundaries and lower dislocation movement resistance in the coarse-grained regions, the strength is lower than that in the fine-grained regions. Meanwhile, the fine-grained regions have higher strength due to the grain boundary strengthening effect (Hall-Petch effect), resulting in the overall material strength showing a "locally weak" characteristic.

In tensile tests, the yield strength of mixed-grain materials can fluctuate by 10% to 20% compared to materials with normal microstructure (for example, if the yield strength of uniform fine-grained steel is 450 MPa, that of mixed-grain materials may vary between 400 and 500 MPa).

Plasticity indicators (elongation, reduction of area) decrease, and the deformation capabilities in different regions vary significantly, making necking or fracture more likely to occur first in the coarse-grained regions. For instance, the elongation of a certain mixed-grain aluminum alloy may be 15% to 20% lower than that of a uniform microstructure.

Reduction in Toughness and Fatigue Resistance

The coarse-grained regions have lower crack propagation resistance, leading to a significant decrease in impact absorption energy (AKV). For example, the low-temperature impact energy of mixed-grain steel may be 30% or more lower than that of uniform microstructure, increasing the risk of brittle fracture.

Fatigue cracks are prone to initiate at the interface between coarse and fine grains (stress concentration effect), resulting in a decrease in fatigue strength limit (for instance, the fatigue strength of mixed-grain titanium alloys may be 10% to 15% lower than that of uniform microstructure), and a shortened service life.

Increased Anisotropy

The significant difference in grain size among grains with different orientations in mixed-grain structures leads to a marked anisotropy in mechanical properties. For example, the difference in strength between the transverse and longitudinal directions of rolled plates can reach 20%, affecting the uniformity of load-bearing capacity of components. 

II. Impact on Physical and Technological Properties

Fluctuations in Thermal and Electrical Conductivity

The scattering effect of grain boundaries on phonons and electrons is weaker in the coarse-grained area and stronger in the fine-grained area, resulting in local differences in the thermal conductivity and electrical conductivity of the mixed-grain material (for example, the electrical conductivity of the mixed-grain copper alloy may fluctuate by ±5%), which affects the performance of precision components.

Reduced Adaptability to Processing Techniques

During hot working, the mixed-grain structure, due to uneven deformation, is prone to surface cracking or internal cracking of the workpiece (for instance, when forging mixed-grain steel, the coarse-grained area has low deformation resistance and is prone to folding defects).

In cutting processes, the uneven hardness of mixed-grain materials leads to inconsistent tool wear rates, affecting the surface roughness of the machined surface (Ra value may increase by 50%) and dimensional accuracy. 

III. Service Reliability and Safety Risks

Stress Concentration Leading to Early Failure

The elastic modulus and thermal expansion coefficient of coarse and fine grains in mixed crystal structures differ. During service, internal stress is easily generated under load or temperature changes, forming stress concentration at grain boundaries and accelerating crack initiation. For instance, in mixed crystal stainless steel under alternating loads, coarse grain regions are more prone to stress corrosion cracking.

Uneven Corrosion Resistance

Grain boundary composition segregation may be more severe in coarse grain regions (such as uneven distribution of precipitated phases at grain boundaries in mixed crystal aluminum alloys), leading to a decrease in local corrosion resistance and the formation of preferential areas for pitting or intergranular corrosion. 

IV. Typical Cases and Data Comparison

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Summary of the Influence Mechanism

The core hazard of mixed crystals stems from "microstructural inhomogeneity":

Imbalance of grain boundary strengthening effect: The strength difference between fine-grained and coarse-grained areas leads to uneven load distribution, making the coarse-grained area a weak link in load-bearing.

Poor deformation coordination: The different plastic deformation capabilities of fine and coarse grains easily cause microcracks at the interface during deformation.

Increased defect sensitivity: Whether it is metallurgical defects (such as inclusions) or processing defects (such as microcracks), they are more likely to propagate in mixed crystal structures.

Therefore, in high-reliability fields such as aerospace and nuclear power, mixed crystal structures are typically classified as "unacceptable microstructures" and must be eliminated through strict process control to ensure the safety of components in service.