Ductile-to-Brittle Transformation

Single-crystal and poly crystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and shp systems. The fee carbides show slip upon mechanical load within the (111)plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

The main considerations of mechanical properties of metals and alloys at low temperatures taken into account for safety reasons are the transition from ductile-to-brittle behavior, certain unconventional modes of plastic deformation, and mechanical and elastic properties changes due to phase transformations in the crystalline structure. 

Significant variation of the ultimate mechanical properties of poly(hexamethylene sehacate), HMS, is possible by con-trol of thermal history without significant variation of percent crystallinity. Both banded and unbanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of r O.Ol in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

In addition to plate thickness, decreasing temperature, increasing deformation rate, or environmental aging can transform ductile-type failure (Fig. 12A) to brittle-type failure. In the case of a ball or a hemispherical dart impact test, it is commonly assumed that the resultant stress state at the center of the plate is biaxial tension. This assumption is erroneous because the normal force on the plate is significant and could lead to over optimistic values on ductile-to-brittle transition conditions, whether expressed as a critical temperature or aging time. This behavior is caused by normal stress suppressing brittle failure (15,18,20). 

Brittle Temperature n Temperature at wliich a material transforms from being ductile to being brittle, i.e., the critical normal stress for fracture is reached before the critical stress for plastic deformation. 

III.18 TIN, Sn (Ar 118-69) - Tin(II) Tin is a silver-white metal which is malleable and ductile at ordinary temperatures, but at low temperatures it becomes brittle due to transformation into a different allotropic modification. It melts at 231-8°C. The metal dissolves slowly in dilute hydrochloric and sulphuric acid with the formation of tin(II) (stannous) salts ... 

The micromechanical deformation behavior of SAN copolymers and rubber-reinforced SAN copolymers have been examined in both compression [102] and in tension [103,104]. Both modes are important, as the geometry of the part in a given application and the nature of the deformation can create either stress state. However, the tensile mode is often viewed as more critical since these materials are more brittle in tension. The tensile properties also depend on temperature as illustrated in Figure 13.6 for a typical SAN copolymer [27]. This resin transforms from a brittle to ductile material under a tensile load between 40 and 60 C. 

However, there is one very important difference between the martensitic transformation in inherently ductile materials such as iron and brittle materials such as zirconia. As for the latter the von Mises criteria for plastic deformation are not satisfied the deformation process is accommodated by a change in shape as manifest in a serrated surface akin to that shown in Figure 4.5, panel c on a microscopic... 

Tungsten has a silvery-white luster and is brittle at room temperature. At elevated temperatures (100°-500°C [212°-932°F]), it is transformed into the ductile state. Tungsten metal is stable in air only at moderate temperatures all high-temperature appfications are therefore limited to a protective atmosphere or vacuum. 

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