What is a mixed crystal? What are the causes of its formation?

Amorphous crystals, as a special type of crystal structure phenomenon, hold significant importance in numerous fields such as materials science, geology, and chemical engineering. This article delves deeply into the basic concepts of amorphous crystals, and thoroughly analyzes the various reasons for their formation, including thermodynamic factors, kinetic factors, external condition interference, and intrinsic properties of substances. Through a comprehensive understanding of amorphous crystals, it is possible to better comprehend the properties of related materials, geological evolution processes, and chemical reaction mechanisms, thereby providing a solid theoretical foundation for research and application in these fields. 

Definition of mixed crystals

Mixed crystals refer to the phenomenon where two or more different crystal grain sizes exist in the microstructure of metals or alloys. Normally, the grain size of materials should be relatively uniform. However, in the mixed crystal state, fine grains and coarse grains are mixed and distributed. This organizational inhomogeneity can have adverse effects on the mechanical properties of the material (such as strength, toughness, plasticity, etc.), potentially leading to unstable material performance or the occurrence of local defects.

The causes of inhomogeneous crystals

The formation of inhomogeneous crystals is usually related to improper control of process parameters during material preparation, processing and heat treatment. The specific reasons can be summarized into the following categories: 

I. Raw Materials and Casting Process Factors 

Uneven chemical composition: The segregation of solute elements in the alloy (such as dendritic segregation) can lead to differences in melting points in local areas. During solidification, the growth rates of crystal grains are not consistent, resulting in the coexistence of coarse and fine grains in the microstructure. 

Uneven cooling conditions during casting: During the casting process, significant differences in local cooling rates (such as uneven mold temperature, excessively high or low pouring temperatures) can cause variations in the nucleation rate and grain growth rate in different areas, thereby resulting in mixed crystals. 

II. Factors of Heat Processing Techniques 

Insufficient or uneven deformation:

During metal processing (such as forging and rolling), if the deformation amount does not reach the "critical deformation amount" (usually 5% to 15%), local areas of the crystal grains will undergo abnormal growth due to insufficient deformation, forming coarse grains, which will mix with the fine-grained areas that have not been fully deformed.

Uneven deformation of the workpiece cross-section (such as a large difference in deformation amount at the edge and center of the forging) can also lead to inconsistent crystal grain sizes.

Improper control of the processing temperature:

If the processing temperature is too high, approaching or exceeding the "re-crystallization temperature" of the material, the crystal grains are prone to grow rapidly; if the temperature fluctuates greatly, the growth degrees of the crystal grains in different areas will be different, resulting in mixed crystals.

Excessive holding time during the processing also exacerbates the abnormal growth of crystal grains. 

III. Heat Treatment Process Factors 

Inappropriate annealing or normalizing processes:

If the annealing temperature is too low or the holding time is insufficient, it will result in some grains not undergoing complete recrystallization, and the original coarse grains will remain; while if the temperature is too high or the holding time is too long, the already recrystallized fine grains will regrow, forming mixed crystals.

Uneven cooling speed (such as due to the different positions of the workpiece in the furnace causing differences in heat dissipation) may also cause inconsistent grain sizes.

Improper pre-treatment before quenching:

If the pre-treatment before quenching (such as annealing or normalizing) fails to eliminate the unevenness of the microstructure of the raw material (such as banded structure, carbide segregation), during quenching, due to the non-uniform austenitization, mixed crystals will form. 

IV. Other Factors 

Original material influence: The raw material itself has a mixed crystal structure. If the subsequent processing does not thoroughly improve this (such as not achieving sufficient deformation or through insufficient heat treatment for homogenization), the mixed crystals will remain.

Stress effect: During processing or use, the material is subjected to local stress concentration (such as mechanical processing stress, welding stress), which may cause local grain deformation or recrystallization, resulting in differences in grain size. 

The Hazards and Improvement Measures of Hybrid Crystals 

Harm: Inhomogeneous crystals will lead to an increase in the anisotropy of material mechanical properties, a decrease in toughness and fatigue strength, and even the initiation of cracks due to local stress concentration during service.

Improvement measures: By optimizing the hot processing technology (such as controlling the deformation amount and temperature), adjusting the heat treatment parameters (such as using homogenization annealing and appropriate recrystallization annealing temperature), and improving the casting or smelting process (such as reducing composition segregation), the phenomenon of inhomogeneous crystals can be eliminated or alleviated, and the grain size can be made uniform. 

In summary, amorphous crystals are a manifestation of the uneven microstructure of materials. Their formation is closely related to the multi-stage process parameters, and it is necessary to conduct full-process control from raw materials to the processing procedure to avoid or improve the situation.