It follows that metals and alloys can be several times stronger than their constituent phases. In this case, metallic precipitates are strong obstacles as well as nonmetallic compound precipitates. When the slip plane in precipitates is not parallel to that in the matrix, dislocations gliding in the matrix are unable to cut through the precipitates regardless of the shear modulus. Recent experimental studies demonstrated that crystallography of precipitate particles is another factor dominating their obstacle strength. In general, this condition is fulfilled by a combination of metallic matrix and nonmetallic compound precipitates such as oxides and carbides whose strength is typically a few GPa, which is ā¼10 times greater than the yield strength of metals. dispersion strengthening) established in the 1950sā1960s, the obstacle strength is assumed to be determined by the shear modulus those which are harder than the matrix are strong obstacles. In the classical theory of precipitation hardening (a.k.a. They are strong obstacles in the case where dislocations are unable to cut through them ( Figure 4). Such intense hardening is achieved when precipitate particles are strong obstacles against the motion of dislocations gliding on a slip plane in the matrix. On the other hand, metals and alloys containing second-phase precipitate particles, say, 2% in volume fraction, can exhibit a strength several times greater than the matrix phase ( Figures 1ā 3). In other words, the strength of nonmetallic composites is expected not to exceed that of constituent phases. In the case of nonmetallic composite materials, their strength is determined by the volume fraction ratio of constituent phases. The strength of metals and alloys is highly affected by a minor amount of precipitates such as a few percent. The crystallography of precipitates is of interest not only for fundamental materials science but also for engineering, in particular, structural materials engineering. This chapter is a supplement to the previous chapter on crystallography of precipitate particles in metals and alloys, for the purpose of describing how the crystallography of precipitates practically affects the physical properties of entire the material. This chapter also reviews the classical theory of precipitation hardening established in the 1950sā1960s, in order to sort out open questions to be resolved. In the case where the slip plane of dislocations in precipitates is not parallel to that in the matrix, dislocations gliding in the matrix are unable to cut through the precipitates, resulting in intense hardening regardless of the shear modulus. The most recent major update in this research field is a discovery that crystallography of precipitates is another factor controlling the magnitude of strengthening. The magnitude of strengthening (hardening) due to precipitates is, in traditional understanding, controlled by the shear modulus, whether or not the precipitates are harder than the matrix. Unlike nonmetallic composite materials whose strength is determined by the volume fraction ratio of constituent phases, the strength of metals and alloys can be several times greater by introducing a minor amount of precipitate particles such as 2%. Following the previous chapter, this chapter describes crystallography of second-phase precipitate particles in metals and alloys the focus of this chapter is placed on the effect of crystallography of precipitates on precipitation hardening.
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