Ethnolinguistic structuring of sorghum genetic diversity in Africa and the role of local seed systems

Sorghum is a drought-tolerant crop with a vital role in the livelihoods of millions of people in marginal areas. We examined genetic structure in this diverse crop in Africa. On the continent-wide scale, we identified three major sorghum populations (Central, Southern, and Northern) that are associated with the distribution of ethnolinguistic groups on the continent. The codistribution of the Central sorghum population and the Nilo-Saharan language family supports a proposed hypothesis about a close and causal relationship between the distribution of sorghum and languages in the region between the Chari and the Nile rivers. The Southern sorghum population is associated with the Bantu languages of the Niger-Congo language family, in agreement with the farming-language codispersal hypothesis as it has been related to the Bantu expansion. The Northern sorghum population is distributed across early Niger-Congo and Afro-Asiatic language family areas with dry agroclimatic conditions. At a finer geographic scale, the genetic substructure within the Central sorghum population is associated with language-group expansions within the Nilo-Saharan language family. A case study of the seed system of the Pari people, a Western-Nilotic ethnolinguistic group, provides a window into the social and cultural factors involved in generating and maintaining the continent-wide diversity patterns. The age-grade system, a cultural institution important for the expansive success of this ethnolinguistic group in the past, plays a central role in the management of sorghum landraces and continues to underpin the resilience of their traditional seed system

Magnesium alloys for structural applications ; recent advances

Magnesium alloys possess unique properties, which when fully exploited can open up for major inroads into important markets for structural applications. A density of 2/3 that of Al, and only slightly higher than for fibre reinforced plastics, combined with excellent mechanical and physical properties as well as processability and recyclability, make magnesium alloys an obvious choice when designing for light weight. The paper summarizes some basic aspects of magnesium alloying practice, and describes the main commercial alloys. A selection of special developments are described with emphasis on the interaction between material properties and processing parameters

Age hardening and precipitation in a cast magnesium - Rare-earth alloy

The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped MgsMIVl β′ precipitates (MM = misch metal) and hexagonal prism-shaped MgMM β precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures

Precipitation Hardening of Mg-Zn and Mg-Zn-RE alloys

The age-hardening response of two cast Mg-Zn-RE alloys has been investigated and compared with that of a binary Mg-Zn alloy. The microstructures of the aged specimens were examined by analytical electron microscopy. Formation of a fine dispersion of rodlike β precipitates is the main cause for age hardening, while extensive precipitation of disc-shaped β coincides with the onset of overaging. Rare earth additions retard the formation of β precipitates and thus postpone overaging. Four different orientation relationships between β precipitates and matrix were found and explained in terms of the near-CSL model

Development of microstructure in cast Mg-Al-rare earth alloys

The microstructures and age hardening behaviours of a series of Mg-Al-rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, AlMM and AlMg, were found in the as cast alloys and no pseudoternary Mg-Al-RE phases were present. The AlMM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas AlMg partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of AlMg precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and AlMg formed preferentially in regions of α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of AlMg also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions. MST/338

The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys

The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new "ternary" interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified