Strength assessment of austenitic steel grain boundaries by impact bending tests for miniature specimens

The paper proposes methods for assessing the strength of grain boundaries according to the results of testing miniature specimens by impact bending. Results of bending at low temperature are given to assess the strength of grain boundaries in austenitic chromium-nickel steels. The test temperature was determined when the proportion of brittle intergranular fracture of embrittled chromium-nickel steel 10Kh18N9 is at least 90%. Three types of miniature specimens of different geometric shapes have been developed, providing approximately the same absorbed energy when tested for impact bending. It is shown when it is necessary to use such miniature specimens.

Effect of Liquid Hot Isostatic Pressing on Structure and Mechanical Properties of an Aluminum Alloy

The effect of liquid hot isostatic pressing (LHIP) on microstructure and mechanical properties of a high-strength cast Al-6Zn-2Mg-0.5Fe-0.7Ni alloy was examined. LHIP eliminates shrinkage porosity that highly improves strength and fatigue limit. Yield stress (YS) and ultimate tensile strength (UTS) in T6 condition increased from 135 to 470 MPa and from 410 to 510 MPa, respectively. Endurance limit on the base of 107 cycles increased from 95 to 140 MPa. However, a small number of gas pores with an average size less than 2 μm retains. LHIP suppresses the crack initiation on coarse cavities. However, brittle intergranular fracture occurs in the hipped alloy through the breaking of eutectic phase Al9FeNi. As a result, elongation-to-failure was of 1.2% and the fatigue strength is equal to one of AA356.02 alloy subjected to LHIP.

Effect of Grain Size on Cryogenic Mechanical Properties of an Al-Mg-Sc Alloy

An aluminum alloy with a chemical composition of Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.) and an initial grain size of ∼22 μm was subjected to equal-channel angular pressing (ECAP) at 593 K up to a total strain of ~12. Extensive grain refinement provided the formation of fully recrystallized structure with an average grain size of ∼0.6 μm. The mechanical properties of the alloy in two different structural conditions were examined at temperatures ranging from 77 to 293 K. It was shown that ECAP highly enhanced the strength, ductility and fracture toughness of the material over the wide temperature interval. Positive effect of grain refinement tends to increase with decreasing temperature due to suppression of brittle intergranular fracture. At ambient temperature, the extensive grain refinement provides +65% increase in yield stress (YS) and ductility, concurrently. At 77 K, YS increase is + 77%, and the ductility increase is +113% owing to grain refinement. Effect of the grain size on fracture toughness at cryogenic temperatures is discussed.

Study on Mechanical Properties of Cu-Bi Bearing Materials

Cu-matrix bearing materials were prepared by powder metallurgy method and their mechanical properties were studied in present work. The continuity of copper alloy matrix was destroyed with the increase of Bi element. Because Bi is brittle phase and distributed in the copper alloy matrix grain boundaries as thin ribbon network. The hardness, crushing strength and impact toughness of copper-bearing materials were reduced with increase of the content of Bi. The mainly of Copper-bearing materials containing bismuth fracture are brittle intergranular fracture. When the Bi content is over 4wt%, the Bi has a relatively full distribution in the rich Bi phase of copper alloy matrix grain boundary like a chip mesh belt. As the further increase of Bi, the reduced trends of the mechanical properties of Cu-matrix bearing materials containing bismuth are slow down. because the role of fragmented Copper alloy matrix will not significantly increase with further increase of Bi.

Ductilization of a Ni3(Si,Ti) Intermetallic Alloy by Addition of Interstitial Type Elements

The effect of a concomitant doping of interstitial type elements boron (B) and carbon (C), and boron (B) and nitrogen (N) on tensile properties of a Ni3(Si,Ti) intermetallic alloy was investigated in the temperature range between room temperature and 973 K. It was found that the concomitant doping of (C/B) and (N/B) remarkably improved the intermediate-temperature tensile elongation of the Ni3(Si,Ti) alloy compared with the simple doping of B or C. It was also shown that the fracture surface of the alloy doped with (C/B) and (N/B) exhibited the ductile transgranular fracture mode while that of the alloy doped with only B showed a brittle intergranular fracture mode at 773 K. These results clearly indicate that the concomitant doping of the interstitial type elements are useful for improving the intermediate-temperature tensile ductility of the Ni3(Si,Ti) alloy.

Effects of Heat Treatment on Microstructures and Properties of Ti811 Alloys

Mechanical properties and microstructures of Ti811 alloys under different heat treatment conditions were studied. The results show that the microstructures of Ti811 alloy consist of α, massive β and grain-boundary β after normal annealing. The intensity and hardness decreased, while the ductility increased. After double annealing, the primary phase α reduced and needle-like secondary α〞 was formed. The massive β and grain-boundary β changed into intergranular β gradually. The microstructure was relatively uniform. Then the intensity and hardness increased, the ductility decreased when compared to general annealing. The needle-like secondary α〞and metastable β would break down into dispersed α and β after solution hardening and aging. The combination properties of alloys would be improved. According to fractography of titanium alloy, the fracture mechanism of the Ti811 alloy was dissociation - brittle intergranular fracture, which was a type of fracture between the intergranular fracture of brittle and cleavage fracture.