Suppression Of Intergranular Fracture Research Articles

Tensile Properties of Directionally Solidified AZ91 Mg Alloy

AZ91 Mg alloy consisting of elongated grains was processed by a directional solidification method and mechanical properties of the directionally solidified Mg alloy were compared with those of the non-directionally solidified Mg alloy by tensile tests at room temperature and at 473 K. The directionally solidified Mg alloy exhibited higher strength at 473 K than the non-directionally solidified Mg alloy because grain boundary sliding was suppressed. Also, the directionally solidified alloy exhibited higher strength and larger elongation at room temperature than the non-directionally solidified alloy. Suppression of intergranular fracture was responsible for high ductility for the directionally solidified alloy.

Development of Tough and Strong Cubic Titanium Trialuminides

ABSTRACTIn this work, the recent breakthroughs in the understanding of the fracture behavior and fracture toughness of L12-ordered titanium trialuminides are described and discussed. First, it is shown that, as opposed to many other intermetallics and specifically those with an L12 crystal structure, the fracture toughness of L12 titanium trialuminides is insensitive to testing in various environments such as air, water, argon, oxygen and vacuum (∼1.3×10–5 Pa). Second, it is reported here that by increasing the concentration of Ti combined with boron (B) doping, the room temperature fracture toughness of a Mn-stabilized titanium trialuminide can be improved by 100% from ∼4 MPam1/2 to ∼8 MPam1/2 and by 150–250% at 1000°C to ∼(10–12) MPam1/2 with a simultaneous suppression of intergranular fracture (IGF) to ∼(40–50%). Almost three fold increase in yield strength to ∼550 MPa is attained at room temperature for high Ti, boron-doped trialuminides. Both Vickers microhardness and strength increase linearly with increasing concentration of (Ti+B) indicating a classical solid solution strengthening response.

An atom probe field ion microscope analysis of grain boundary chemistry in boron and carbon doped NiAl

The low temperature brittleness of nickel aluminides has been a serious impediment to their technological applications. A commonly employed technique to ductilize these materials involves the addition of suitable microalloying elements and correlating grain boundary chemistry with fracture mode. In the well documented case of Ni3Al, boron segregation to grain boundaries is accompanied by suppression of intergranular fracture and a significant increase in ductility. The high resolution microanalytical technique of atom probe field ion microscopy (APFIM) has been used in this study to analyze grain boundaries in order to characterize similar attempts to ductilize NiAl. APFIM specimens were prepared from tensile specimens of stoichiometric NiAl doped with either 0.04 or 0.12 at. % boron or 0.1 at % carbon, respectively. A field ion image of a grain boundary in a B-doped NiAl specimen is shown in Fig. 1. The brightly-imaging spots decorating the boundary were determined by atom probe analysis to be boron atoms. The boron enrichment factor at the boundary depends on the assumed thickness of the segregation as shown in Fig. 2 with an enrichment factor of ∼850 times for a monolayer coverage (i.e. 0.2 nm).

Ａｌ‐４．４％Ｚｎ‐１．４％Ｍｇ合金の極低温における引張特性に及ぼす銅添加の影響

Tensile properties at 77 and 4.2K have been studied in Al-4.4%Zn-1.4%Mg alloys containing copper up to 1.5% both in under-aged and over-aged conditions. The ductility at a fixed strength level was significantly improved by the copper addition at both temperatures under both aging conditions. Fractographic observations revealed that the fraction of intergranular fracture decreased with increasing copper content, and that intergranular fracture surface consisted of ledges instead of micro-dimples. According to TEM and SEM observations after deformation, intense slip bands were seen in the copper-free alloy while the distribution of slip bands was relatively uniform in the copper-containing alloys. Microstructures adjacent to grain boundaries were also examined, but were considered to exert a minor effect. Thus the ductility improvement by the copper addition, connected with the suppression of intergranular fracture, was ascribed to the reduction in the stress concentration at the grain boundaries through the uniform distribution of slip bands.

An Atom Probe Field Ion Microscope Investigation Of The Role Of Boron In Precipitates And At Grain Boundaries In NiAl

ABSTRACTThe high resolution analytical technique of Atom Probe Field Ion Microscopy (APFTM) haseen used to characterize grain boundaries and the matrix of a stoichiometric NiAl alloy doped with 0.04 (100 wppm) and 0.12 at. % (300 wppm) boron. Field ion images revealed boron segregation to the grain boundaries. Atom probe elemental analysis of the grain boundaries measured a boron coverage of up to 30% of a monolayer. Extensive atom probe analyses also revealed a fine dispersion of nanoscale boride precipitates in the matrix. The boron segregation to the grain boundaries was found to correlate with the observed suppression of intergranular fracture. However, the decrease in ductility of boron-doped NiAl is attributed in part to the precipitation hardening effect of the boride phases.

High ductility and shape memory effect of rapidly solidified B-2 compounds in the NiFeAl system

This paper deals with the microstructure, ductility and shape memory of rapidly solidified B-2 (β) and martensite (β′) compounds in the NiFeAl system. The β and β′ phase fields in rapidly solidified NiFeAl alloys are extended tot he lower aluminum and higher iron concentration side as compared with the equilibriu, phase fields at room temperature. The reverse transformation temperature A ∗ of the metastable β′ to β phase decrease because of the dissolution of large amounts of iron and aluminium and lies near room temperature in the vicinity of 14 at.% Fe and 27 at.% Al. The rapidly solidified Ni14Fe27Al0.17B (at.%) wire with β′ + ( β) phases was found to exhibit a reversible shape memory effect near room temperature and high ductility with tensile elongation as large as 18% because of the extension of the β and β′ phase fields by rapid solidification, the quenching-induced structural modification and the suppression of intergranular fracture.