تقسية بالترسيب

تقسية بالترسيب Precipitation Hardening , وتعهد أيضاً باسم تقسية بالتعتيق age hardening أوتقسية بالانتشار dispersion hardening, هي طريقة معالجة حرارية تـُستخدم في تقسية المواد malleable, بما فيهم معظم السبائك الإنشائية من الألومنيوم, المغنسيوم والتيتانيوم, وبعض الأصلات غير القابلة للصدأ. وتعتمد على تغيرات في قابلية الذوبان الصلبة مع الحرارة لانتاج حبيبات دقيقة كما لوكانت طور شوائب, التي تعوق حركة الانخلاعات, أوالعيوب في عقد البلورة. ولما كانت الانخلاعات في معظم الحالات هم الحامل الرئيسي للدونة, فإن هذا يساعد على تقسية المادة. الشوائب, في الواقع, يلعبون نفس دور مواد المصفوفة في المواد المركبة. Just as the formation of ice in air can produce clouds, snow, or hail, depending upon the thermal history of a given portion of the atmosphere, precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging.

Note that two different heat treatments involving precipitates can alter the strength of a material: solution heat treating and precipitation heat treating. Solution heat treating involves formation of a single-phase solid solution via quenching and leaves a material softer. Precipitation heat treating involves the addition of impurity particles to increase a material's strength. Precipitation hardening via precipitation heat treatment is the main topic of discussion in this article.

النظرية

Furthermore, a dislocation may cut through a precipitate particle. This interaction causes an increase in the surface area of the particle. The area created is

{\displaystyle A=2rb\pi \,\!

where, r is the radius of the particle and b is the magnitude of the burgers vector. The resulting increase in surface energy is

{\displaystyle A=2rb\pi \gamma _{s \,\!

where ${\displaystyle \gamma _{s \,\!$ is the surface energy. The dislocation can also bow around a precipitate particle.

المعادلات الحاكمة

There are two equations to describe the two mechanisms for precipitation hardening:

Dislocations cutting through particles:

{\displaystyle \tau ={\frac {r\gamma \pi {bL \,\!

where "tau" is material strength, "r" is the second phase particle radius, "gamma" is the surface energy, "b" is the magnitude of the Burgers vector, and "L" is the spacing between pinning points. This governing equation shows that the strength is proportional to r, the radius of the precipitate particles. This means that it is easier for dislocations to cut through a material with smaller second phase particles (small r). As the size of the second phase particles increases, the particles impede dislocation movement and it becomes increasingly difficult for the particles to cut through the material. In other words, the strength of a material increases with increasing r.

Dislocations bowing around particle:

{\displaystyle \tau ={\frac {Gb {L-2r \,\!

where "tau" is the material strength, "G" is the shear modulus, "b" is the magnitude of the Burgers vector, "L" is the distance between pinning points, and "r" is the second phase particle radius. This governing equation shows that for dislocation bowing the strength is inversely proportional to the second phase particle radius r. Dislocation bowing is more likely to occur when there are large particles present in the material.

These governing equations show that the precipitation hardening mechanism depends on the size of the precipitate particles. At small r, cutting will be the dominant strengthening mechanism, while at large r, bowing will be the dominant strengthening mechanism.

Looking at the plot of both equations, it is clear that there is a critical radius at which max strengthening occurs. This critical radius is typically 5-30 nm.